KR20210042472A - Graft copolymer binder and a cathode for lithium ion secondary batteries containing the same - Google Patents

Abstract

The present invention relates to a graft copolymer binder and to a positive electrode for a lithium ion secondary battery and, more specifically, to a positive electrode for a lithium ion secondary battery comprising: a graft copolymer including an ethylene glycol oligomer as a side chain and acrylate as a main chain; and to a positive electrode active material layer including a lithium metal oxide-based positive electrode active material. The positive electrode for a lithium ion secondary battery has high charge/discharge efficiency and high capacity retention.

Description

Graft copolymer binder and a cathode for lithium ion secondary batteries containing the same

The present invention relates to a graft copolymer binder and a positive electrode for a lithium ion secondary battery, and in particular, a graft copolymer containing an ethylene glycol oligomer as a side chain and an acrylate as a main chain, and a lithium metal oxide-based positive electrode active material. It relates to a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer is formed.

Recently, as the spread of electronic devices using batteries such as mobile phones, notebook computers, and electric vehicles has increased, the demand for a secondary battery having a relatively high capacity and small size, is rapidly increasing. Among them, lithium secondary batteries are used in various portable devices because they are lightweight and have high energy density. Accordingly, research and development efforts for improving the performance of lithium secondary batteries are being actively conducted.

The positive electrode of a lithium ion secondary battery is manufactured by coating and drying an electrode composition including a positive electrode active material, a binder, and a conductive material on a current collector. Typically, the positive electrode _{may be prepared by applying a slurry in which LiCoO 2} as an active material, polyvinylidene difluoride (PVdF) as a binder, and carbon black as a conductive material is dispersed on a current collector of an aluminum thin film and dried. PVdF is most widely used as a binder for positive electrode active material, but it swells by electrolyte in high temperature and high voltage environment, weakening the binding force with the positive electrode active material, lowering durability, and increasing electrode resistance. Due to the low ionic conductivity of PVdF Additionally, there is a problem in that it must be mixed with a material having high ion conductivity such as a polymer electrolyte.

Accordingly, there is a need for further development of a binder for a positive electrode of a lithium ion secondary battery, which has high ion conductivity, high temperature cycle stability, and excellent binding power with a lithium metal oxide-based positive electrode active material even under high temperature and high voltage.

Republic of Korea Patent Publication No. 10-2019-0053120 (2019.05.17)

An object of the present invention is to provide a positive electrode for a lithium ion secondary battery having high ionic conductivity and high-temperature cycle stability compared to a fluorine-based binder, and having high charging/discharging efficiency and high capacity retention.

Another object of the present invention is a non-crosslinked lithium-ion secondary that does not contain fluorine as a binder for binding a lithium metal oxide-based positive electrode active material, and at the same time can stably bind a lithium metal oxide-based positive electrode active material without including an additional binder. It is to provide a binder for a battery positive electrode.

The present invention includes a graft copolymer including an ethylene glycol oligomer represented by the following formula (1) as a side chain on a current collector and including an acrylate as a main chain; And a positive electrode for a lithium ion secondary battery on which a positive electrode active material layer including a lithium metal oxide-based positive electrode active material is formed.

[Formula 1]

R ₁ -(O-CH ₂ -CH ₂ )n-*

(In Formula 1, R ₁ is hydrogen or a C ₁ to C ₆ alkyl group, and n is an integer of 3 or more and 20 or less.)

In one aspect of the present invention, the graft copolymer may be a positive electrode for a lithium ion secondary battery on which a positive electrode active material layer including a repeating unit derived from an alkyl acrylate-based monomer is formed.

In one aspect of the present invention, the graft copolymer comprises a repeating unit derived from an acrylate monomer including an ethylene glycol oligomer and an alkyl acrylate monomer, and an acrylate monomer including the ethylene glycol oligomer and The molar ratio of the alkyl acrylate-based monomer may be a positive electrode for a lithium ion secondary battery having a positive electrode active material layer of 1:0.1 to 1:5.

In one aspect of the present invention, the graft copolymer is for a lithium ion secondary battery having a positive electrode active material layer comprising a repeating unit derived from an acrylate monomer and methyl methacrylate including an ethylene glycol oligomer represented by Formula 2 It can be positive.

[Formula 2]

R ₂ -(O-CH ₂ -CH ₂ ) _m -OC(O)-CH(R ₃ )=CH ₂

(In Formula 1, R ₂ is a C ₁ to C ₄ alkyl group, R ₃ is a hydrogen or a methyl group, and m is an integer of 3 or more and 15 or less.)

In one aspect of the present invention, the weight average molecular weight of the graft copolymer may be a positive electrode for a lithium ion secondary battery having a positive electrode active material layer having a weight average molecular weight of 20,000 to 500,000.

In one aspect of the present invention, the positive electrode for a lithium ion secondary battery may be a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer substantially not containing a fluorine-based polymer is formed.

In one aspect of the present invention, the ionic conductivity of the graft copolymer has a range of 1X10 ^-8 to 1X10 ^-6 S/cm at 25°C, and has a range of 1X10 ^-6 to 1X10 ^-4 S/cm at 60°C, It may be a positive electrode for a lithium ion secondary battery on which a positive electrode active material layer is formed.

The present invention provides a lithium ion secondary battery including a positive electrode, a separator, and a negative electrode for a lithium ion secondary battery as described above.

In one aspect of the present invention, the negative electrode may have a structure in which a negative electrode current collector and a metal foil including lithium are laminated on the upper portion thereof.

In one aspect of the present invention, the lithium ion secondary battery may have a capacity retention rate of 90% or more at 25°C and 70% or more at 45°C of the 100th cycle.

The positive electrode for a lithium ion secondary battery according to the present invention has high ionic conductivity and excellent high-temperature cycle characteristics compared to the positive electrode containing a fluorine-based binder, and has high charging/discharging efficiency and high capacity retention.

Specifically, the positive electrode for a lithium ion secondary battery according to the present invention can stably bind a lithium metal oxide-based positive electrode active material without including a conventional fluorine-based binder. In addition, since the binder is uniformly dissolved in the solvent for manufacturing the positive electrode, even if a small amount of the binder is used, it can have excellent binding properties.As the binder has high ionic conductivity, lithium-ion secondary batteries have high charging/discharging efficiency and high It can have a capacity retention rate.

1 is a diagram showing the structure of a coin cell for measuring ion conductivity.
2 is a diagram showing the structure of a half cell for measuring the charging/discharging efficiency of the battery.
3 is a diagram showing half-cell cycle characteristics of Example 2 and Comparative Example 1. FIG.
4 is a diagram showing ionic conductivity characteristics of Example 2 and Comparative Example 1. FIG.
5 is a diagram showing the results of a binding force test of positive electrode active materials of Comparative Examples 1, 1, and 2;

Hereinafter, the present invention will be described in more detail through specific examples or examples including the accompanying drawings. However, the following specific examples or examples are only one reference for describing the present invention in detail, and the present invention is not limited thereto, and may be implemented in various forms.

In addition, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. The terms used in the description in the present invention are merely for effectively describing specific embodiments and are not intended to limit the present invention.

In addition, the singular form used in the specification and the appended claims may be intended to include the plural form unless otherwise indicated in the context.

In addition, when a part "includes" a certain component, it means that other components may be further included rather than excluding other components unless specifically stated to the contrary.

In addition, unless otherwise defined in the specification of the present invention, the molecular weight of a polymer means a weight average molecular weight.

In addition, numerical ranges used in the specification of the present invention include lower and upper limits and all values within the range, increments that are logically derived from the shape and width of the defined range, all values limited among these, and numerical ranges limited to different forms. Includes all possible combinations of upper and lower limits of.

The present invention includes a graft copolymer including an ethylene glycol oligomer represented by the following formula (1) as a side chain on a current collector and including an acrylate as a main chain; And a positive electrode for a lithium ion secondary battery on which a positive electrode active material layer including a lithium metal oxide-based positive electrode active material is formed.

[Formula 1]

R ₁ -(O-CH ₂ -CH ₂ ) _n -*

(In Formula 1, R ₁ is hydrogen or a C ₁ to C ₆ alkyl group, and n is an integer of 3 or more and 20 or less.)

_{Preferably, R 1} of the side chain of the ethylene glycol oligomer represented by Chemical Formula 1 is a C ₁ to C ₄ alkyl group, and n may be an integer of 3 or more and 18 or less.

In one aspect of the present invention, the acrylate repeating unit included in the main chain of the graft copolymer may be derived from an alkyl acrylate-based monomer.

The alkyl acrylate-based monomer may be an alkyl methacrylate having an alkyl group having 1 to 20 carbon atoms, for example, methyl methacrylate, ethyl methacrylate, n-propylene methacrylate, isopropyl methacrylate, n-butyl methacrylate, t-butyl methacrylate, isobutyl methacrylate, 2-ethylhexyl methacrylate, 2-ethylbutyl methacrylate, pentyl methacrylate, hexyl methacrylate, cyclohexyl methacrylate Rate, n-octyl methacrylate, isooctyl methacrylate, tetradecyl methacrylate, dodecyl methacrylate, tridecyl methacrylate, octadecyl methacrylate, and the like may be used, but are not limited thereto. Preferably, it may be an alkyl methacrylate having an alkyl group having 1 to 4 carbon atoms.

In one aspect of the present invention, the graft copolymer comprises a repeating unit derived from an acrylate monomer including an ethylene glycol oligomer and an alkyl acrylate monomer, and an acrylate monomer including the ethylene glycol oligomer and The molar ratio of the alkyl acrylate-based monomer may be 1:0.1 to 1:5. It may be preferably 1:0.2 to 1:4, more preferably 1:0.5 to 1:2. It has excellent ionic conductivity in the above range and at the same time can have excellent durability.

The graft copolymer may be a random copolymer, an alternating copolymer, or a block copolymer, and preferably a random copolymer. Particularly preferably, the graft copolymer has a property of being uniformly dissolved in a solvent. In the case of a copolymer that is not soluble in a solvent, a larger amount of the copolymer should be used as a binder and the binding properties may be inferior, but the graft copolymer according to the present invention has a characteristic that is soluble in a solvent, so when used as a binder, a small content Even if it is used, it has an advantageous advantage in that it can have very excellent binding properties.

In addition, as the ethylene glycol oligomer is included in the graft copolymer as a side chain (side branch), the ethylene glycol oligomer may have high flexibility and very high ionic conductivity for lithium ions. This characteristic has an advantage in ionic conductivity in that ethylene glycol oligomer or polyethylene glycol has a higher flexibility compared to the polymer or copolymer binder contained in the main chain.

In one aspect of the present invention, the graft copolymer may include a repeating unit derived from an acrylate monomer including an ethylene glycol oligomer represented by the following Formula 2 and methyl methacrylate.

[Formula 2]

R ₂ -(O-CH ₂ -CH ₂ ) _m -OC(O)-CH(R ₃ )=CH ₂

In Formula 2, R ₂ is a C ₁ to C ₄ alkyl group, R ₃ is hydrogen or a methyl group, and m is an integer of 3 or more and 15 or less. Preferably it is a C ₁ ~ C ₂ alkyl group, R ₃ is hydrogen or methyl group, m is an integer of 3 or more and 10 or less, more preferably triethylene glycol monomethyl ether methacrylate, pentaethylene glycol monomethyl ether Methacrylate, 9 mol added polyethylene glycol monomethyl ether methacrylate, and the like.

In one aspect of the present invention, the weight average molecular weight of the graft copolymer may be 20,000 to 500,000, preferably 50,000 to 300,000, more preferably 80,000 to 200,000, but is not limited thereto.

In one aspect of the present invention, the positive electrode for a lithium ion secondary battery may be one that does not substantially contain a fluorine-based polymer. The positive electrode for a lithium ion secondary battery according to the present invention may have excellent binding properties without including a fluorine-based binder. In addition, by using a graft-structured copolymer containing an ethylene glycol oligomer in the side chain, the main chain has the same or similar binding properties as the fluorine-based binder, and the ethylene glycol oligomer side chain can have high-temperature cycle characteristics and high ionic conductivity. have. Accordingly, the lithium ion secondary battery including the positive electrode for the lithium ion secondary battery may have a high charge/discharge efficiency and a high capacity retention rate.

In one aspect of the present invention, the ionic conductivity of the graft copolymer has a range of 1x10 ^-8 to 1x10 ^-6 S/cm at 25°C, and may have a range of ^{1x10 -6} to 1x10 ^-4 S/cm at 60°C. have. Although not limited to a specific lithium salt and measurement conditions, the ionic conductivity includes lithium bis(trifluoromethanesulfonyl)imide as a lithium salt, and the thickness of the graft copolymer and the lithium salt mixture is 0.02 to 0.05 It may be defined in the measurement condition of mm.

Meanwhile, the ionic conductivity of the composition mixed with the graft copolymer and the polymer electrolyte has a range of 1x10 ^-6 to 1x10 ^-4 S/cm at 25°C, and has a range of 1x10 ^-4 to 1x10 ^-3 S/cm at 60°C. I can. At this time, the polymer electrolyte may be polyethylene glycol dimethyl ether (PEGDME), the weight ratio of the graft copolymer and PEGDME is mixed at 2:1, and lithium bis(trifluoromethanesulfonyl)imide is included as a lithium salt, The thickness of the graft copolymer and the lithium salt mixture may be defined under a measurement condition of 0.02 to 0.05 mm.

The present invention also provides a lithium ion secondary battery comprising a positive electrode, a separator, and a negative electrode, wherein the positive electrode includes an ethylene glycol oligomer represented by the following formula (1) as a side chain on a current collector, and a graph comprising an acrylate as a main chain. T copolymer; And a positive electrode on which a positive electrode active material layer including a lithium metal oxide-based positive electrode active material is formed.

[Formula 1]

R ₁ -(O-CH ₂ -CH ₂ ) _n -*

(In Formula 1, R ₁ is hydrogen or a C ₁ to C ₆ alkyl group, and n is an integer of 3 or more and 20 or less.)

The positive electrode active material may be a conventional positive electrode active material used in this technical field, and specifically, lithium cobalt oxide (LiCoO ₂ ), spinel crystalline lithium manganate composite oxide (LiMn ₂ O ₄ ), lithium manganate composite oxide (LiMnO ₂ ), nickel acid lithium composite oxide (LiNiO ₂ ), lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), lithium cobalt phosphate (LiCoPO ₄ ), lithium iron pyrophosphate ; Li ₂ FeP ₂ O ₇ ), lithium niobate composite oxide (LiNbO ₂ ), lithium iron _{acid composite oxide (LiFeO 2} ), magnesium acid lithium composite oxide (LiMgO ₂ ), copper acid lithium composite oxide (LiCuO ₂ ), zinc acid Lithium composite oxide (LiZnO ₂ ), lithium molybdate oxide (LiMoO ₂ ), lithium tantalate composite oxide (LiTaO ₂ ), tungstate lithium composite oxide (LiWO ₂ ), permanganese nickel cobalt composite oxide (xLi ₂ MnO ₃ (1-x)LiMn1-y-zNiyCozO ₂ ), lithium nickel cobalt aluminum composite oxide (LiNi _0.8 Co _0.15 Al _0.05 O ₂ ), lithium nickel cobalt manganese composite oxide (LiNi _0.33 Co _0.33 Mn _0.33 O ₂ , LiNi _0.4 Co _0.2 Mn _0.4 O _2, LiNi _0.5 Co _0.2 Mn _0.3 O ₂ , LiNi _0.6 Co _0.2 Mn _0.2 O ₂ ,LiNi _0.7 Co _0.15 Mn _0.15 O ₂ , LiNi _0.8 Co _0.1 Mn _0.1 O ₂ ), Manganese nickel oxide) (LiNi _0.5 Mn _1.5 O ₄ ) and the like, but are not limited thereto.

The positive electrode may further include a conductive material, and the conductive material includes carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; A conductive material such as a polyphenylene derivative may be used, but it is not particularly limited as long as it has conductivity without causing chemical changes to the battery.

In one aspect of the present invention, the negative electrode may have a structure in which a negative electrode current collector and a metal foil including lithium are stacked on an upper portion thereof. The negative electrode may further include a negative electrode active material in addition to the lithium metal foil, and may be a compound capable of reversibly occluding and desorbing lithium. As a specific example of the negative electrode active material, a carbonaceous material such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon may be used. In addition, in addition to the carbonaceous material, a metal compound capable of alloying with lithium, or a composite including a metal compound and a carbonaceous material may be used as the negative electrode active material.

In one aspect of the present invention, the capacity retention rate at the 100th cycle of the lithium ion secondary battery may be 90% or more at 25°C and 70% or more at 45°C. More specifically, the capacity retention rate at the 100th cycle may be 94% or more at 25°C and 80% or more at 45°C. It is, but is not limited to, 99% or less at 25°C, and 90% or less at 45°C, but is not limited thereto.

Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples. However, the following Examples and Comparative Examples are only one example for describing the present invention in more detail, and the present invention is not limited by the following Examples and Comparative Examples.

[Test Example]

(1) Ion conductivity measurement

The evaluation of ionic conductivity characteristics was measured at -20°C to 100°C using an impedance meter (Zahner IM6e). The prepared coin cell was measured for resistance by temperature, and the conductivity value was calculated by dividing the thickness by resistance and area.

(2) Evaluation of charge/discharge characteristics

The evaluation of charge/discharge characteristics was measured at ^{25 o} C and 45 ^o C using a battery charge/discharge tester (WonAtech WBCS3000Le). Cycle data was obtained through an iterative process of discharging the coin cell to 3.0 V and then charging it to 4.2 V. The initial efficiency was expressed by dividing the initial discharge capacity by the charging capacity. Since the irreversible capacity of NCM811 (nickel: cobalt: manganese = 8:1:1) used as the positive electrode active material is large, the capacity retention rate at the 100th cycle was measured based on the second discharge capacity.

(3) Weight average molecular weight measurement

The prepared polymer was dissolved in tetrahydrofuran and measured using gel osmosis chromatography (GPC).

The weight average molecular weight was measured using a Gel Permeation Chromatography (GPC) equipment manufactured by Waters. The equipment consists of a mobile phase pump (M515 Pump), a column heater (ALLCOLHTRB), a detector (2414 RI Detector), and an injector (2695 EB automatic injector), and the analytical column is WATERS' Styragel HR. Late (PMMA) American polymer standard corporation STD was used. As the mobile phase solvent, HPLC grade tetrahydrofuran (THF) was used, and the measurement was performed under the conditions of a column heater temperature of 40°C and a mobile phase solvent flow rate of 1.0 mL/min. The resin prepared for sample analysis was dissolved in tetrahydrofuran (THF), a mobile phase solvent, and then injected into a GPC device to measure the weight average molecular weight.

(4) Peel strength measurement

^{Peel strength measurement was measured by peeling the adhesive tape at a rate of 180°} at a speed of 20 mm/min with an electrode manufactured using a universal physical property tester (Lloyd TA1) having a size of 2.5 cm*5 cm. The peel strength value was calculated by dividing the measured force by the peeled length.

<Preparation Example 1> Synthesis of pentaethylene glycol monomethyl ether

After dissolving tetraethylene glycol (10 g, 0.05 mol) in anhydrous tetrahydrofuran (120 mL) in a 250 mL 2-neck round bottom flask under a nitrogen atmosphere, sodium hydride (2 g, 0.05 mol) at 0°C. mol) was added and stirred for 30 minutes. 2-methylethyl-p-toluenesulfonate (2-methylethyl p-toluenesulfonate, 11.51 g, 0.05 mol) dissolved in anhydrous tetrahydrofuran at 0°C was added dropwise, stirred for 1 hour, and then heated to reflux condition. It reacted for 20 hours. At the end of the reaction, the solvent was removed, extracted with methylene chloride, washed with brine, and the organic layer was dried over magnesium sulfate and column to obtain pentaethylene glycol monomethyl ether as a product.

[Example 1]

Triethylene glycol monomethyl ether methacrylate (4.64 g, 0.02 mol), methyl methacrylate (4.0048 g, 0.04 mol), azobisisobutyronitrile (AIBN, 0.246) in a 250 mL two neck round bottom flask under a nitrogen atmosphere. g, 0.0015 mol) and methyl alcohol (15 mL) were added, followed by reaction for 36 hours under reflux conditions. At the end of the reaction, after cooling to room temperature, methyl alcohol was added to dissolve the polymer, followed by precipitation in ether at 15°C. After washing several times with methyl alcohol and ether, the obtained polymer was vacuum-dried at 60° C. to obtain a graft copolymer. The ethylene glycol content, molecular weight, and PDI values per mole of the synthesized graft copolymer were measured and shown in Table 1.

<Manufacture of coin cell for ion conductivity measurement>

Lithium bis (trifluoromethanesulfonyl) imide (Lithium bis (trifluoromethanesulfonyl) imide, LiTFSi) and the graft copolymer are mixed so that the molar ratio of ethylene glycol group and lithium salt (EO)/Li] is 20, and acetone is Solvent casting was performed on the 16π spacer as a solvent. After drying at room temperature for 24 hours and in a vacuum oven at 40° C. for 2 hours, the thickness is about 0.035 mm, and a coin cell (N coin cell) for ion conductivity measurement using a 2032 type coin cell is prepared as shown in FIG. 1 and the ion conductivity is measured. It was measured and described in Table 2.

<Manufacture of coin cell containing PEGDME for ion conductivity measurement>

A mixture of the graft copolymer and polyethylene glycol dimethyl ether (PEGDME) having a weight average molecular weight of 500 g/mol at a weight ratio of 2:1 was dissolved in acetone, a solvent, and then added to the mixture. The included ethylene glycol group and LiTFSi were mixed in a molar ratio of [EO]/[Li]=20, and subjected to solvent casting on a 16π spacer. After drying at room temperature for 24 hours and in a vacuum oven at 40° C. for 2 hours, the thickness is about 0.035 mm, and a coin cell (P coin cell) for ion conductivity measurement using a 2032 type coin cell is prepared as shown in FIG. 1 and the ion conductivity is measured. It was measured and described in Table 2.

<Manufacture of Half Cell>

NCM811, carbon black (super P) and the graft copolymer were mixed at a weight ratio of 70:8:22, and 2.1 g of methylpyrrolidone was added. After mixing for 30 minutes with an automatic mixer, it was coated on an aluminum foil of a certain standard. After drying in a constant temperature chamber at 120℃ for 2 hours, the dried electrode was compressed and punched with 14π to adjust the loading amount of the active material to 2.3 to 2.7 mg/cm ² , and dried in a vacuum oven at 120℃ for 24 hours to obtain a positive electrode for a lithium ion secondary battery. Was prepared.

Next, a half cell was fabricated using the positive electrode for a lithium ion secondary battery as a working electrode and a lithium foil as a reference electrode and a counter electrode. A 2032 type coin cell was manufactured and performed, and was assembled in an argon-filled glove box. This test cell was assembled as shown in FIG. 2 by sandwiching a separator impregnated with a solid polymer electrolyte between two electrodes.

As the separator, a microporous nonwoven fabric separator having a porosity of 75% and a thickness of 0.0304 mm was used. In the solid polymer electrolyte, LiTFSi was added in a molar ratio of [EO]/[Li]=15 to a mixture of PEGDME and Bisphenol A in a weight ratio of 8: 2, and additionally, a fluoroethylene carbonate (FEC) additive was used as a solid. It was prepared by adding so as to be 5 wt% of the total weight of the polymer electrolyte.

The prepared half-cell was described in Table 3 by measuring the charge/discharge efficiency at 25° C. for the charge/discharge measurement evaluation, and the charge/discharge efficiency was measured at 45° C. and described in Table 4.

<Manufacture of electrodes for measuring peel strength>

NCM811, carbon black (super P) and the graft copolymer were mixed at a weight ratio of 70:8:22, and 2.1 g of methylpyrrolidone was added. After mixing for 30 minutes with an automatic mixer, it was coated on an aluminum foil of a certain standard. After drying in a constant temperature chamber at 120° C. for 2 hours, the dried electrode was compressed, cut to a size of 2.5 cm*5 cm, and dried in a vacuum oven at 120° C. for 24 hours to prepare a positive electrode for measuring peel strength. Thereafter, after the tape was adhered, the peel strength was measured and shown in FIG. 5.

[Example 2]

Except for the synthesis of the composition of triethylene glycol methyl ether methacrylate (9.29 g, 0.04 mol)) and methyl methacrylate (2.0024 g, 0.02 mol) in the preparation of the binder precursor in Example 1, the same was carried out. I did. In addition, the physical properties were measured and shown in Tables 1 to 4 below.

[Example 3]

In Example 1, when preparing the binder precursor, polyethylene glycol methyl ether methacrylate (Mn 300), 9 g, 0.03 mol), methyl methacrylate (1.5 g, 0.015 mol) and azobisiso Except for the synthesis of the composition of butyronitrile (AIBN, 0.185 g, 0.001125 mol) was carried out in the same manner. In addition, the physical properties were measured and shown in Tables 1 to 4 below.

[Example 4]

In the preparation of the binder precursor in Example 1, polyethylene glycol methyl ether methacrylate (Mn 500), 15 g, 0.03 mol), methyl methacrylate (1.5 g, 0.015 mol) and azobisiso Except for the synthesis of the composition of butyronitrile (AIBN, 0.185 g, 0.001125 mol) was carried out in the same manner. In addition, the physical properties were measured and shown in Tables 1 to 4 below.

[Example 5]

In Example 1, when preparing the binder precursor, pentaethylene glycol monomethyl ether methacrylate synthesized in Preparation Example 1 (3.12 g, 0.00974 mol), methyl methacrylate (0.486 g, 0.00487 mol) and azobisisobutyro Except for the synthesis of the composition of nitrile (AIBN, 0.060g, 0.000365 mol) was carried out in the same manner. In addition, the physical properties were measured and shown in Tables 1 to 4 below.

[Comparative Example 1]

In Example 1, the preparation of the binder precursor was carried out in the same manner, except that PVdF was used instead of the graft copolymer. In addition, the physical properties were measured and shown in Tables 2 to 4 below.

nEGMA:
MMA molar ratio AIBN Reaction time
(time) Mn Mw PDI Example 1 1:2 2.5 mol% 36 26,000 81,000 3.1 Example 2 2:1 2.5 mol% 36 49,000 173,000 3.5 Example 3 2:1 2.5 mol% 36 37,000 163,000 4.3 Example 4 2:1 2.5 mol% 36 26,000 87,000 3.3 Example 5 2:1 2.5 mol% 36 34,000 99,000 2.9

In Table 1, nEGMA means ethylene glycol oligomer monomethyl ether methacrylate used in Examples 1 to 5. The number of repeating units of ethylene glycol contained in one molecule of ethylene glycol oligomer monomethyl ether methacrylate used in Examples 1 to 5 is as follows.

Examples 1 and 2: [EO]/mol = 3

Example 3: [EO]/mol = 4.5

Example 4: [EO]/mol = 9.0

Example 5: [EO]/mol = 5

N coin cell / ion conductivity (S/cm) P coin cell / ion conductivity (S/cm) Binder precursor 25 °C (3.35)* 60 ℃ (3.00)* 25 ℃ (3.35)* 60 ℃ (3.00)* Example 1 6.47X10 ^-8 3.05X10 ^-6 1.68X10 ^-6 3.96X10 ^-5 Example 2 1.45X10 ^-7 7.05X10 ^-6 1.19X10 ^-5 1.96X10 ^-4 Example 3 5.02X10 ^-7 1.29X10 ^-5 1.26X10 ^-4 4.87X10 ^-4 Example 4 9.44X10 ^-6 1.27X10 ^-4 2.62X10 ^-4 2.10X10 ^-3 Example 5 5.15X10 ^-7 1.75X10 ^-5 2.49X10 ^-4 4.24X10 ^-4 Comparative Example 1 3.60X10 ^-10 1.00X10 ^-8 1.12X10 ^-9 3.64X10 ^-8

In Table 2, the * value in parentheses means a value obtained by converting the temperature (℃) into 1000/K units.

25 ^o C
Cycle charge
Volume
(mAh/g) Discharge
Volume
(mAh/g) Early
efficiency
(%) 100th cycle
When
Charging capacity
(mAh/g) 100th cycle
When
Discharge capacity
(mAh/g) 100th cycle
When
efficiency
(%) 100th cycle
Capacity retention rate when
(%) Example 1 132 115 87 114 113 99 98 Example 2 145 120 83 101 100 99 83 Example 3 132 108 82 103 101 98 94 Example 4 148 117 79 140 102 73 87 Example 5 158 128 81 113 112 99 88 Comparative Example 1 148 120 81 95 94 99 78

45 ^o C
Cycle charge
Volume
(mAh/g) Discharge
Volume
(mAh/g) Early
efficiency
(%) 100th cycle
When
Charging capacity
(mAh/g) 100th cycle
When
Discharge capacity
(mAh/g) 100th cycle
When
efficiency
(%) 100th cycle
Capacity retention rate when
(%) Example 1 158 145 92 120 119 99 82 Example 2 158 144 91 117 117 100 81 Example 3 153 138 90 110 109 99 79 Example 4 162 144 89 112 111 99 77 Example 5 145 120 83 107 106 99 88 Comparative Example 1 156 132 85 88 87 99 66

As shown in Table 2, it can be seen that the example has higher ionic conductivity than the comparative example. In particular, it was confirmed that the ionic conductivity of the example measured in the N coin cell differs from 170 times to as much as 26,000 times less than the ionic conductivity of the comparative example measured in the N coin cell. In addition, it was confirmed that the ionic conductivity of the Example measured in the P coin cell differs from 1,500 times to as much as 230,000 times less than the ionic conductivity of the Comparative Example measured in the P coin cell. As can be seen in Fig. 4 and Table 2, it was confirmed that the ionic conductivity measurements of the Example in the N coin cell and the Comparative Example in the P coin cell differ by 50 to 8,000 times. Although PEGDME contains the same polyethylene glycol as the graft copolymer according to the present invention, it has low ionic conductivity when mixed with PVDF, whereas the examples including the graft copolymer according to the present invention have very high ionic conductivity. You can see what you have. This suggests that the graft copolymer according to the present invention contains polyethylene glycol as a side chain, so that it has excellent binding properties compared to a binder containing polyethylene glycol as a main chain, and has a remarkable effect of high ionic conductivity.

On the other hand, as shown in Table 3, Table 4, and Figure 3, the capacity retention rate of the example was very high compared to the comparative example. This suggests that the graft copolymer according to the present invention has the same or similar binding properties as compared to the fluorine-based polymer binder, and this excellent capacity retention is due to the high flexibility and high ionic conductivity of the ethylene glycol oligomer contained in the graft copolymer. It is feed.

In addition, Figure 5 is for testing the binding force, and as shown in the figure, the peel strength of the Example was higher than that of the Comparative Example. The peel strength of Example 1 (48.92±1.34 N/m) and the peel strength of Example 2 (44.96±0.20 N/m) were about 2.5 times higher than the peel strength of Comparative Example 1 (18.34±1.43 N/m). As a value, this suggests that the graft copolymer according to the present invention has very excellent binding properties compared to the fluorine-based polymer binder.

Claims (10)

A graft copolymer including an ethylene glycol oligomer represented by the following Formula 1 as a side chain on the current collector and including an acrylate as a main chain; And a positive electrode for a lithium ion secondary battery on which a positive electrode active material layer including a lithium metal oxide-based positive electrode active material is formed.
[Formula 1]
R ₁ -(O-CH ₂ -CH ₂ )n-*
(In Formula 1, R ₁ is hydrogen or a C ₁ to C ₆ alkyl group, and n is an integer of 3 or more and 20 or less.) The method of claim 1,
The graft copolymer is a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer including a repeating unit derived from an alkyl acrylate-based monomer is formed. The method of claim 1,
The graft copolymer includes a repeating unit derived from an acrylate-based monomer including an ethylene glycol oligomer and an alkyl acrylate-based monomer, and the molar ratio of the acrylate-based monomer and the alkyl acrylate-based monomer including the ethylene glycol oligomer A positive electrode for a lithium ion secondary battery having a positive electrode active material layer of 1:0.1 to 1:5 is formed. The method of claim 1,
The graft copolymer is a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer including a repeating unit derived from an acrylate monomer including an ethylene glycol oligomer represented by Chemical Formula 2 and a methyl methacrylate is formed.
[Formula 2]
R ₂ -(O-CH ₂ -CH ₂ ) _m -OC(O)-CH(R ₃ )=CH ₂
(In Formula 1, R ₂ is a C ₁ to C ₄ alkyl group, R ₃ is a hydrogen or a methyl group, and m is an integer of 3 or more and 15 or less.) The method of claim 1,
A positive electrode for a lithium ion secondary battery having a positive electrode active material layer having a weight average molecular weight of 20,000 to 500,000 of the graft copolymer. The method of claim 1,
The positive electrode for a lithium ion secondary battery is a positive electrode for a lithium ion secondary battery in which a positive electrode active material layer substantially not containing a fluorine-based polymer is formed. The method of claim 1,
The ionic conductivity of the graft copolymer has a range of 1X10 ^-8 to 1X10 ^-6 S/cm at 25°C, and a range of 1X10 ^-6 to 1X10 ^-4 S/cm at 60°C, lithium ion having a positive electrode active material layer Anode for secondary batteries. In a lithium ion secondary battery comprising a positive electrode, a separator, and a negative electrode,
The positive electrode is the positive electrode for a lithium-ion secondary battery according to any one of claims 1 to 7, a lithium-ion secondary battery. The method of claim 8,
The negative electrode is a lithium ion secondary battery having a structure in which a negative electrode current collector and a metal foil including lithium are laminated on the upper portion thereof. The method of claim 8,
The lithium ion secondary battery having a capacity retention rate of 90% or more at 25°C and 70% or more at 45°C of the lithium ion secondary battery at the 100th cycle.

Images