KR20210083061A - Binder for lithium secondary battery, and electrode and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a binder for a lithium secondary battery, an electrode including the same, and a lithium secondary battery, and more specifically, to a binder composition for a lithium secondary battery including N-vinyl imidazolium ionic liquid represented by chemical formula 1 and N-vinylacetamide, a positive electrode active material composition including the binder composition, and a lithium secondary battery including the binder composition and the positive electrode active material. The binder for a lithium secondary battery of the present invention can provide a lithium secondary battery with improved high-temperature thermal stability and charge/discharge characteristics.

Description

A binder for a lithium secondary battery, and an electrode and a lithium secondary battery including the same {BINDER FOR LITHIUM SECONDARY BATTERY, AND ELECTRODE AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a binder for a lithium secondary battery, an electrode comprising the same, and a lithium secondary battery, and more specifically, N-vinyl imidazolium ionic liquid and N represented by the following [Formula 1] A binder composition for a lithium secondary battery comprising -vinylacetamide (N-Vinylacetamide), a cathode active material composition comprising the binder composition, and a lithium secondary battery comprising the binder composition and a cathode active material.

Lithium-ion batteries (LIBs) are attracting attention as promising energy conversion/storage devices in various fields including electric vehicles (EVs) and energy-storage systems (ESSs). However, until now, LIBs have low theoretical capacity of active materials, so their energy density has not met the level required for large-scale applications. Therefore, in order to obtain a high energy density in LIBs, it is necessary to use an advanced material with a higher specific capacity. In this regard, nickel-rich layered oxides (LiNi _x Co _y Mn _z O ₂ , Ni-rich NCMs) have higher specific capacities (> _{180 mA hg −1} ) than those of lithium cobalt oxides (150 mA h g −1 ). Therefore, it has received much attention as an alternative anode material.

Lithium ion batteries are usually manufactured by uniformly mixing and polishing an electroactive material, a conductive agent, and a binder solution to make a slurry, then coating it on a copper or aluminum foil, which is a current collector, followed by drying, consolidation, and other processes. Here, the lithium ion battery binder is one of the indispensable raw materials in the lithium ion battery manufacturing process, and serves to attach the positive and negative electroactive materials and the conductive agent on the current collector.

Lithium-ion battery positive electrode binders are mainly divided into two types. One is an oil-based binder employing an organic solvent as a dispersant, and a fluoropolymer binder that is currently relatively widely applied. For example, there is polyvinylidene fluoride (PVDF) using N-methylpyrrolidone (NMP) as a solvent, and the binder has a large organic solvent capacity and is easy to volatilize during the manufacturing process. At the same time, it pollutes and poses a relatively large risk to the health of workers. Fluoropolymers and their solvents are expensive and thus costly to produce. In addition, the PVDF binder has a problem in that the charging and discharging cycle characteristics are inferior.

Republic of Korea Patent Publication No. 10-2008-0105045

The present invention provides a binder for a lithium secondary battery capable of improving high-temperature thermal stability and charge/discharge characteristics of a lithium secondary battery, a cathode active material composition for a lithium secondary battery comprising the binder, and a lithium secondary battery including the binder.

In order to achieve the object of the present invention, there is provided a binder composition for a lithium secondary battery comprising an ionic liquid represented by the following [Formula 1] and N-vinylacetamide (N-Vinylacetamide),

[Formula 1]

,

,

(wherein R is _{any one of C 2} -C ₁₃ -alkyl, allyl or butenyl, and an anion is BF ₄ ^- , PF ₆ ^- , SbF ₆ ^- , NO ₃ ^- , CF ₃ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- (TFSI), (C ₂ F ₅ SO ₂ ) ₂ N ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ CO ₂ ^- , C ₃ F ₇ CO ₂ ^- , CH ₃ CO ₂ ^- or (CN) ₂ N ^- ).

The binder composition is an initiator AIBN (azobisisobutyronitrile), ADMVN (azobisdimethylvaleronitrile), DLP (dilauroyl peroxide), BPO (Benzoyl peroxide), acetyl peroxide (acetyl peroxide), t-butyl peracetate (t-butyl peracetate), cumyl per Oxide (cumyl peroxide), t-butyl peroxide (t-butyl peroxide), t- butyl hydroperoxide (t-butyl hydroperoxide) or a combination thereof is characterized.

The binder may further include a fluorine-containing polymer, wherein the fluorine-containing polymer is polyvinylidene fluoride, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polyvinyl It is characterized as a copolymer of leadenefluoride-tetrafluoroethylene (PVdF-TFE) or a combination thereof.

Another embodiment of the present invention, the binder composition; It provides a cathode active material composition for a lithium secondary battery comprising a cathode active material; and a solvent.

The cathode active material composition may include an NCM-based cathode active material represented by the following [Formula 2];

[Formula 2]

Li _a Ni _x Co _y Mn _z O _α

(In the above formula, 0.6≤a≤1.2, 0<x≤1.0, 0<y≤1.0, 0<z≤1.0, α=2, x+y+z=1.0).

In another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode including the binder and a positive electrode active material; a negative electrode including a negative electrode active material; and a non-aqueous electrolyte.

The binder for a lithium secondary battery of the present invention may provide a lithium secondary battery with improved high-temperature thermal stability and charge/discharge characteristics.

1 is a photograph of an ionic liquid synthesized according to the present invention.
2 is a graph of TGA results of the ionic liquid synthesized according to the present invention.
3 is a photograph of the xylene solubility test result of the ionic liquid synthesized according to the present invention.
4 is a photograph showing the results of thermal crosslinking experiments on Al and glass of N-vinylacetamide.
5 is a photograph showing the thermal crosslinking test result of the ionic liquid synthesized according to the present invention.
6 is a graph showing the results of FR-IR analysis of electrodes prepared using N-vinylacetamide and PVdF as binders.
7 is a graph showing the electrochemical performance of an electrode prepared using an ionic liquid, N-vinylacetamide and PVdF as a binder, and an electrode prepared using only PVdF as a binder (monomer = ionic liquid and N-vinyl) acetamide mixture).

Hereinafter, the present invention will be described in more detail through Examples and Experimental Examples. However, these examples are only for helping the understanding of the present invention, and the scope of the present invention is not limited to these examples in any sense.

The binder for a lithium secondary battery according to an embodiment of the present invention may include an ionic liquid represented by the following [Formula 1] and N-vinylacetamide (N-Vinylacetamide);

[Formula 1]

(wherein R is _{any one of C 2} -C ₁₃ -alkyl, allyl or butenyl, and an anion is BF ₄ ^- , PF ₆ ^- , SbF ₆ ^- , NO ₃ ^- , CF ₃ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- (TFSI), (C ₂ F ₅ SO ₂ ) ₂ N ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ CO ₂ ^- , C ₃ F ₇ CO ₂ ^- , CH ₃ CO ₂ ^- or (CN) ₂ N ^- either.)

The ionic liquid of the present invention is generally composed of a cation and an anion, and the cation of the ionic liquid is N-vinyl imidazolium, but is not limited thereto, but if it has high ionic conductivity and low viscosity Pyrrolidinium-based, triazolium-based, tetrazolium-based, morpholinium-based or piperidinium-based cations may be used instead of cations.

(1) pyrrolidinium-based ionic liquid

(2) Triazolium-based ionic liquid

(3) tetrazolium-based ionic liquids

(4) Morpholinium-based ionic liquids

(5) Piperidinium-based ionic liquids

However, the cation of the ionic liquid preferably includes a vinyl functional group to contribute to crosslinking, and more preferably, the vinyl functional group may be connected to a nitrogen atom of the cation of the ionic liquid.

The alkyl chain (R, alkyl chain) _{is preferably any one of C 2} -C ₁₃ -alkyl, allyl or butenyl, but is not limited thereto.

Anions included in the ionic liquid are BF ₄ ^- , PF ₆ ^- , SbF ₆ ^- , NO ₃ ^- , CF ₃ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- (TFSI), (C ₂ F ₅ SO _{2 )} ) ₂ N ⁻ , (CF ₃ SO ₂ ) ₃ C ⁻ , CF ₃ CO ₂ ⁻ , C ₃ F ₇ CO ₂ ⁻ , CH ₃ CO ₂ ⁻ or (CN) ₂ N ⁻ may include any one of, Preferably _{either BF 4} ^- or (CF ₃ SO ₂ ) ₂ N ^- (TFSI).

The present inventors confirmed that the thermal decomposition temperature of the binder composition of the present invention is different depending on the anion of the ionic liquid. Specifically, in the case of the same alkyl chain (R), the binder composition including ^{TFSI −} _{as an anion had better thermal stability than the binder composition including BF 4} ⁻ , and in the case of including the same anion, the binder by the alkyl chain (R) It was observed that the thermal stability of the composition did not change significantly.

In addition, as a result of evaluating the solubility in xylene solvent, most ionic liquids showed low solubility in xylene solvent regardless of the alkyl chain (R) and anion, but TFSI ^- was included as an anion. It was confirmed that when it contains an alkyl chain (R) having 12 carbon atoms, it is dissolved in a xylene solvent. In addition, the ionic liquid is TFSI in the storage test with the solid electrolyte of LPS to evaluate the chemical stability ^- those containing the containing the anion and the number of carbon atoms is 12 alkyl chain (R) or butenyl decomposition behavior of the binder composition not observed.

In addition, the binder composition further includes an initiator, and the initiator is azobisisobutyronitrile (AIBN), azobisdimethylvaleronitrile (ADMVN), dilauroyl peroxide (DLP), Benzoyl peroxide (BPO), acetyl peroxide, t-butyl peracetate ( t-butyl peracetate), cumyl peroxide, t-butyl peroxide, t-butyl hydroperoxide, or a combination thereof. The initiator may generate radicals by heat or ultraviolet rays and initiate cross-linking.

The initiator is preferably mixed in an amount of 0.1 parts by weight based on 1 part by weight of the mixture of the ionic liquid and N-vinylacetamide.

The binder may further include a fluorine-containing polymer, wherein the fluorine-containing polymer is polyvinylidene fluoride (PVdF), polyvinylidene fluoride polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP) , a copolymer of polyvinylidene fluoride-tetrafluoroethylene (PVdF-TFE) or a combination thereof. The fluorine-containing polymer serves to facilitate adhesion between the particles of the positive electrode active material including the binder. In such a fluorine-containing polymer, the polymer can be melted in the rolling process during the manufacturing process of the electrode, and in particular, when the rolling process is performed above the polymer melting point, the polymer is almost melted, and the adhesion between the active material particles can be further improved. have.

The present inventors measured the binding force only with N-vinylacetamide without adding PVdF, and confirmed that the powder was separated from the current collector due to insufficient binding force. In order to increase the binding force of the binder composition of the present invention PVdF was added.

The binder composition is preferably mixed with an ionic liquid and N-vinylacetamide in a weight ratio of 1:1, and contains a mixture of the ionic liquid and N-vinylacetamide and fluorine The polymer is preferably mixed in a weight ratio of 1:1.

In another embodiment of the present invention, the binder composition; It provides a cathode active material composition for a lithium secondary battery comprising a cathode active material; and a solvent, wherein the cathode active material composition may include an NCM-based cathode active material represented by the following [Formula 2];

[Formula 2]

Li _a Ni _x Co _y Mn _z O _α

(In the above formula, 0.6≤a≤1.2, 0<x≤1.0, 0<y≤1.0, 0<z≤1.0, α=2, x+y+z=1.0).

Specifically, the positive active material is preferably _{LiNi 0.8} Co _0.1 Mn _0.1 O _{2 .}

In another embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode including the binder and a positive electrode active material; a negative electrode including a negative electrode active material; and an electrolyte. As a result of evaluating the electrochemical performance of a lithium secondary battery including a positive electrode made of the binder, the cell to which the binder is added has a higher coulombic efficiency than the cell to which the binder is not added (only PVdF is added as a binder) in the initial formation. and showed higher coulombic efficiency in subsequent cycles as compared to cells prepared only with PVdF.

Hereinafter, the present invention will be described in more detail through Examples, and the present invention is not limited by the Examples.

<Example>

(1) Synthesis of ionic liquid (crosslinking agent)

A two-step synthesis method was devised for the simple synthesis of an ionic liquid, and the ionic liquid was synthesized through alkylation (step 1) and anion substitution reaction (step 2) of the starting material.

[STEP 1]

During the initial alkylation, imidazolium bromide (Br-) is formed as an intermediate, and it was confirmed that the pattern of product obtained varies depending on the type of alkyl halide. In the case of C-12, the rate of chemical reaction is very high. The experiment was carried out under the slow reflux condition.

[STEP 2]

In the case of TFSI anion, the product has hydrophobic properties, and the by-products formed together have hydrophilic properties. Therefore, hydrophilic water was used as a solvent to induce separation between the product and the by-product. In _{the case of BF 4} anion, both the product and the formed NaBr by-product have hydrophobic properties, but acetone, which can relatively lower the solubility of NaBr, was used as a solvent to induce separation of the product and the by-product. After the reaction was completed, the separated product was dissolved in DCM (dichloromethane), and then impurities were removed by column chromatography purification using _{acidic, basic and neutral Al 2} O _{3 as a stationary phase.}

10 kinds of ionic liquids of [Fig. 1] were prepared by the above method (VPI, 1-vinyl-3-propylimidazolium; VBI, 1-vinyl-3-butylimidazolium; VAI, 1-vinyl-3-allylimidazolium; VHAI) , 1-vinyl-3-homoallylimidazolium; VDI, 1-vinyl-3-dodecylimidazolium).

<Experimental example>

(1) Thermal stability (TGA) evaluation of ionic liquids

To evaluate the thermal stability of the synthesized ionic liquid, TGA was measured, and it was confirmed that the thermal decomposition temperature was different depending on the anion and the alkyl group (FIG. 2 and Table 1).

ionic liquid T _d (℃) VPI TFSI ~300 VPI BF ₄ ~230 VAI TFSI ~320 VAI BF ₄ ~230 VDI TFSI ~300

Specifically, in the case of the same alkyl chain (R), the binder composition including ^{TFSI −} _{as an anion had better thermal stability than the binder composition including BF 4} ⁻ , and in the case of including the same anion, the binder by the alkyl chain (R) It was observed that the thermal stability of the composition did not change significantly.

(2) Determination of solubility of ionic liquids

In order to determine whether the synthesized ionic liquid can be used in the electrode manufacturing process, solubility evaluation of the ionic liquid in a xylene solvent was performed (FIG. 3 and Table 2).

state Ionic liquid capacity Xylene capacity Solubility VPI TFSI Liquid 0.5g 5ml layer separation VPI BF ₄ Liquid 0.1g 5ml layer separation VBI TFSI Liquid 0.5g 5ml layer separation VBI BF ₄ Liquid 0.5g 5ml layer separation VAI TFSI Liquid 0.5g 5ml layer separation VAI BF ₄ solid 0.5g 5ml does not melt VHAI TFSI Liquid 0.5g 5ml layer separation VHAI BF ₄ Liquid 0.5g 5ml layer separation VDI TFSI Liquid 5g 5ml record VDI BF ₄ solid 0.5g 5ml layer separation

As a result of the experiment, in the case of an ionic liquid containing propyl, allyl, butyl, and homoallyl, the solubility in xylene was low regardless of the anion, and specifically, the alkyl group It was confirmed that when the number of carbons is 12 and TFSI ^- has an anion, it is dissolved in xylene.

(3) Chemical stability of ionic liquids (LPS dissolution behavior and NMR analysis)

In order to induce even dispersion of the ionic liquid and LPS in the electrode manufacturing process to form a binder network inside the electrode, it is important that LPS has high solubility and that chemical decomposition does not occur.

To evaluate the chemical stability of the synthesized ionic liquid, a storage test was performed with LPS, a solid electrolyte, and the chemical shift change of the crosslinking agent was compared through NMR analysis of the solution.

Ionic liquid capacity LPS capacity dissolution behavior Decomposition behavior ( ¹ H) VPI TFSI 1.0g 0.01g record 6% VPI BF ₄ 1.0g 0.01g record 5% VBI TFSI 1.0g 0.01g record 7% VBI BF ₄ 1.0g 0.01g record 8% VAI TFSI 1.0g 0.01g record 8% VHAI TFSI 1.0g 0.01g Dispersion not observed VHAI BF ₄ 1.0g 0.01g record 18% VDI TFSI 1.0g 0.01g Dispersion not observed

Result was confirmed that the behavior with LPS vary depending on the anionic type of the ionic liquid is TFSI ^- decomposition behavior of the binder composition if it contains as anion a carbon number pohan 12 alkyl chain (R) or butenyl are observed It didn't happen.

(4) Evaluation of crosslinking power of N-vinylacetamide

After adding N-vinylacetamide together with AIBN to the NMP solvent, it was applied to Cu foil and dried at a temperature of 120°C. In this case, the viscosity of the mixed solution was low, so it was difficult to control the thickness, but after oven drying, a transparent and hard film was observed, indicating that crosslinking has progressed.

Therefore, the present inventors conducted an experiment on Al and glass due to the weak interaction of N-Vinylacetamide and Cu, and tried to form a stable film by changing the amount of NMP solvent. After 1 hour, regardless of the amount of solvent, all were in a completely solid state, and it was confirmed that as the amount of solvent was increased, a thicker film was formed (FIG. 4).

(5) Evaluation of crosslinking power of ionic liquids

After mixing the synthesized ionic liquid and the initiator (AIBN) in a ratio of 9:1, the result was observed by storing in an oven at 70° C. for a long time (FIG. 5)

As a result of the experiment, the VPI ionic liquid started the reaction, and after 2 hours, it completely hardened with color change and changed to a hard form. After overnight storage, the VPI ionic liquid changed to a darker color.

The VAI ionic liquid started the reaction with bubbles generated after 30 minutes, and although the color of the interface changed after 1 hour, it did not show any solidified form and appeared black after overnight.

(6) Evaluation of binding force of N-vinylacetamide

An electrode was prepared by replacing PVdF, which is a commonly used binder, with N-vinylacetamide, and binding strength was compared. At this time, since the initiator added for crosslinking has instability to heat, it was added in the last step and coated on Al.

After dissolving 0.4 g of N-vinylacetamide in NMP solution, 0.2 g of the conductive agent Super-P, 3.4 g of NCM811 positive electrode active material, and 0.04 g of AIBN were sequentially added to coat on Al, dried in an oven at 120° C. As a result, when only N-vinylacetamide was used as a binder without PVdF, it was observed that the powder came off from the current collector due to insufficient binding force.

Therefore, in order to increase the binding force, the present inventors prepared an electrode by mixing N-vinylacetamide with PVdF, which is a conventional binder, in a weight ratio of 1:1, and as a result, it was confirmed that the binding force was increased and binding to the current collector.

Since the N-vinylacetamide is cross-linked through an intramolecular double bond, FT-IR analysis was performed in anticipation of a change in the C=C bond after polymerization. Referring to FIG. 6 , the C=C functional group of N-vinylacetamide was observed in the vicinity of 1644 cm-1, but it was determined that the film was formed by the disappearance of the C=C bond peak in the electrode state.

(6) Electrochemical performance evaluation of electrodes using ionic liquids, N-vinylacetamide and PVdF as binders

Then, the present inventors mixed the ionic liquid with N-vinylacetamide, and prepared a positive electrode by using the mixture together with PVdF as a binder composition, and then manufactured a battery including the positive electrode.

After preparing a binder composition by mixing an ionic liquid (VPI BF ₄ ), N-vinylacetamide, PVdF and AIBN in a weight ratio of 1:1:2:0.2, NCM811 positive electrode, the binder composition and carbon black (Super P ) was mixed in a ratio of 85:10:5 (wt%) to prepare a positive electrode. Cycling performance was measured using a mixture of ethylene carbonate (EC) and ethylmethyl carbonate (EMC) with each anode, Li metal cathode, poly(ethylene) (PE) separator (Celgard), and electrolyte (1:2 (v/v) PanaxEtec) was investigated using a 2032 coin cell composed of 1 M LiPF _{6 ).} ^{Cells were charged/discharged in the potential range of 3.0-4.3V (vs.Li/Li +} ) at 0.1 C-rate for 2 cycles (formation phase) and they were charged/discharged at 25°C at 1.0C by a charge/discharge unit (WBCS3000, Wonatech). -rate (180 mA g ^-1 ) (cycle steps).

Referring to FIG. 7 , _{the cell to which ionic liquid (VPI BF 4} ), N-vinylacetamide was added increased the coulombic efficiency in the initial formation (PVdF: 89.64%, ionic liquid (VPI BF ₄ ) and N -Vinylacetamide mixed binder: 90.77%) showed higher coulombic efficiency than the existing PVdF cell even after the cycle.

Claims (7)

A binder composition for a lithium secondary battery comprising an ionic liquid represented by the following [Formula 1] and N-vinylacetamide (N-Vinylacetamide);
[Formula 1]

(wherein R is _{any one of C 2} -C ₁₃ -alkyl, allyl or butenyl, and an anion is BF ₄ ^- , PF ₆ ^- , SbF ₆ ^- , NO ₃ ^- , CF ₃ SO ₃ ^- , (CF ₃ SO ₂ ) ₂ N ^- (TFSI), (C ₂ F ₅ SO ₂ ) ₂ N ^- , (CF ₃ SO ₂ ) ₃ C ^- , CF ₃ CO ₂ ^- , C ₃ F ₇ CO ₂ ^- , CH ₃ CO ₂ ^- or (CN) ₂ N ^- either.)
According to claim 1,
The binder composition further includes an initiator, and the initiator is azobisisobutyronitrile (AIBN), azobisdimethylvaleronitrile (ADMVN), dilauroyl peroxide (DLP), Benzoyl peroxide (BPO), acetyl peroxide, t-butyl peracetate (t) -Butyl peracetate), cumyl peroxide (cumyl peroxide), t-butyl peroxide (t-butyl peroxide), t-butyl hydroperoxide (t-butyl hydroperoxide), or a binder for a lithium secondary battery, characterized in that a combination thereof.
According to claim 1,
The binder is a lithium secondary battery binder, characterized in that it further comprises a fluorine-containing polymer.
4. The method of claim 3,
The fluorine-containing polymer is polyvinylidene fluoride (PVdF), polyvinylidene fluoride polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP), polyvinylidene fluoride-tetrafluoroethylene A binder for a lithium secondary battery, characterized in that a polymer (PVdF-TFE) or a combination thereof.
The binder composition of any one of claims 1 to 4; A cathode active material; and a cathode active material composition for a lithium secondary battery comprising a solvent.
6. The method of claim 5,
The cathode active material composition is a cathode active material composition for a lithium secondary battery, characterized in that it comprises an NCM-based cathode active material represented by the following [Formula 2];
[Formula 2]
Li _a Ni _x Co _y Mn _z O _α
(In the above formula, 0.6≤a≤1.2, 0<x≤1.0, 0<y≤1.0, 0<z≤1.0, α=2, x+y+z=1.0).
A lithium secondary battery comprising: a positive electrode including the binder of any one of claims 1 to 4 and a positive electrode active material; a negative electrode including a negative electrode active material; and a non-aqueous electrolyte.

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