KR101485944B1 - Composition for preparing anode for lithium secondary battery and lithium secondary battery using the same - Google Patents

Abstract

The present invention relates to a composition for forming a negative electrode for a lithium secondary battery and a lithium secondary battery produced using the same. The present invention provides a negative electrode composition for a lithium secondary battery comprising a negative electrode active material, a binder, a solvent, and an SEI film former. The lithium secondary battery manufactured using the composition for forming a negative electrode of the present invention can significantly reduce the content of the SEI film former added to the electrolytic solution more than necessary.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composition for forming a negative electrode for a lithium secondary battery, and a lithium secondary battery produced using the same. BACKGROUND ART [0002]

The present invention relates to a composition for forming a negative electrode for a lithium secondary battery containing an additive and maximizing the effect of use of the additive, and a lithium secondary battery produced using the same.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs, and electric vehicles further expand, there is a growing demand for higher energy density of batteries used as power sources for such electronic devices. Lithium secondary batteries are the ones that can best meet these demands, and researches on them are actively under way.

Among the currently applied secondary batteries, the lithium secondary batteries developed in the early 1990s include a cathode made of a carbon material capable of absorbing and desorbing lithium ions, a cathode made of a lithium-containing oxide, and a mixed organic solvent, And a non-aqueous electrolytic solution.

A typical lithium secondary battery has an average discharge voltage of about 3.6 to 3.7 V, which is one of advantages of other alkaline batteries and nickel-cadmium batteries. In order to achieve such a high driving voltage, an electrochemically stable electrolyte composition is required at a charge / discharge voltage range of about 0 to 4.2 V. For this purpose, a mixed solvent in which a cyclic carbonate compound such as ethylene carbonate or propylene carbonate and a linear carbonate compound such as dimethyl carbonate, ethylmethyl carbonate or diethyl carbonate are appropriately mixed is used as a solvent for the electrolyte solution. As the solute of the electrolytic solution, a lithium salt such as LiPF ₆ , LiBF ₄ , LiClO ₄ or the like is usually used, and these act as a source of lithium ions in the battery, thereby enabling the operation of the lithium battery.

During the initial charging of the lithium secondary battery, lithium ions from the cathode active material such as lithium metal oxide migrate to the anode active material such as graphite and are inserted between the layers of the anode active material. At this time, since lithium is highly reactive, the electrolyte composing the anode active material reacts with carbon on the surface of the anode active material such as graphite to produce compounds such as Li ₂ CO ₃ , Li ₂ O, and LiOH. These compounds form a SEI (Solid Electrolyte Interface) film on the surface of a negative electrode active material such as graphite.

The SEI membrane acts as an ion tunnel, allowing only lithium ions to pass through. The SEI membrane is an effect of this ion tunnel, which prevents organic solvent molecules moving together with lithium ions in the electrolyte from being intercalated between the layers of the negative electrode active material and destroying the negative electrode structure. Therefore, the electrolytic solution is not decomposed by preventing the contact between the electrolytic solution and the negative electrode active material, and the amount of lithium ions in the electrolytic solution is reversibly maintained, so that stable charge / discharge is maintained.

However, in the thin prismatic type battery, there arises a problem that the cell thickness is expanded during charging due to gases such as CO, CO ₂ , CH ₄ and C ₂ H ₆ generated from the decomposition of the carbonate-based solvent during the SEI film formation reaction described above do. Also, as the time elapses from the high-temperature standing state in the full charge state, the SEI film gradually collapses due to the increased electrochemical energy and heat energy, and the side reaction in which the exposed cathode surface and the surrounding electrolyte reacts is continuously generated. In this case, the internal pressure of the battery is increased due to the continuous gas generation. As a result, in the case of the prismatic battery and the pouch battery, the thickness of the battery increases, causing problems in electronic devices such as cellular phones and notebook computers. That is, the safety at high temperature is poor.

Various attempts have been made to change the shape of the SEI film formation reaction by adding an additive to the electrolyte to suppress the rise of the internal pressure of the cell, and various additives for SEI film formation have been introduced.

However, when the additive is added to the electrolytic solution, the additive is distributed not only in the cathode in which the additive acts but also in the anode and the separator.

Accordingly, an object of the present invention is to provide a composition for forming a negative electrode for a lithium secondary battery and a lithium secondary battery manufactured using the composition, which can maximize the effect of an additive while minimizing the amount of additives used.

In order to solve the above problems, the present invention provides a negative electrode active material composition comprising a negative electrode active material, a binder, a solvent, and an SEI film former, wherein the SEI film former is selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, vinylethylene carbonate, Which is a mixture of at least one selected from the group consisting of sulphone, sulphone, sulphone, unsaturated sulphone, and acyclic sulphone, or a mixture of two or more thereof.

The present invention can maintain the excellent effect of forming the SEI film by increasing the degree of dispersion of the additive while minimizing the amount of the additive used by adding the additive for SEI film formation to the composition for forming the negative electrode, which has been added to the conventional electrolyte solution.

The cyclic sulfite may be represented, for example, by the following formula 1:

Wherein R ³ to R ⁶ are each independently a hydrogen atom, a halogen atom, a C ₁ to C ₆ alkyl group, or a C ₁ to C ₆ haloalkyl group, and n is an integer of 1 to 3.

The saturated sulphone may be represented, for example, by the following formula (2): < EMI ID =

In the general formula (2), R ⁷ to R ¹² are each independently a hydrogen atom, a halogen atom, a C ₁ to C ₆ alkyl group, or a C ₁ to C ₆ haloalkyl group, and n is an integer of 0 to 3.

The unsaturated sulphone may be represented, for example, by the following formula (3)

In Formula 3, R ¹³ to R ¹⁶ are each independently a hydrogen atom, a halogen atom, a C ₁ to C ₆ alkyl group, or a C ₁ to C ₆ haloalkyl group, and n is an integer of 0 to 3.

The non-cyclic sulfone can be represented, for example, by the following formula (4)

Wherein R ¹⁷ and R ¹⁸ are each independently selected from the group consisting of a C ₁ to C ₆ alkyl group, a C ₁ to C ₆ haloalkyl group, a C ₂ to C ₆ alkenyl group, a C ₂ to C ₆ haloalkenyl group, A C ₆ to C ₁₈ aryl group, or a C ₆ to C ₁₈ haloaryl group.

The SEI film forming agent according to the present invention may be contained in the composition for forming a negative electrode in an amount of 0.1 to 5 parts by weight, preferably 0.5 to 2 parts by weight, based on 100 parts by weight of the negative electrode active material.

Alternatively, the composition for forming a negative electrode for a lithium secondary battery of the present invention may further comprise a carbonate, an alcohol, or a mixture thereof as an auxiliary solvent in order to increase the solubility of the SEI film former.

The composition for forming a negative electrode for a lithium secondary battery of the present invention may be applied to at least one surface of an anode current collector and then dried to be a cathode. The prepared anode can be formed into an SEI film without adding an SEI film forming agent to the electrolyte Therefore, it can be usefully used for a lithium secondary battery.

The composition for forming a negative electrode for a lithium secondary battery according to the present invention contains an SEI film former, which can significantly reduce the content of an SEI film former added more than necessary in a conventional electrolytic solution. In addition, since the SEI film forming agent can be present not only in the negative electrode active material particles on the electrode surface but also in the vicinity of the negative electrode active material particles therein, the use effect of the additive can be maximized.

Therefore, a lithium secondary battery can be manufactured more economically than in the prior art, and it is possible to prevent an excessive amount of the lithium secondary battery from remaining in the battery as an impurity or side reaction.

1 is a graph showing lifetime characteristics of lithium secondary batteries manufactured in Examples and Comparative Examples.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

The negative electrode active material composition for a lithium secondary battery of the present invention comprises a negative electrode active material, a binder, a solvent, and an SEI film former.

As described above, the SEI film is formed on the surface of the negative electrode active material. However, in the prior art, since the SEI film forming agent is added to the electrolyte and injected into the battery, it has been impossible to distribute the SEI film forming agent only around the target negative active material. Therefore, the SEI film forming agent inevitably has to be used in an excessive amount, and the SEI film forming agent remaining in the electrolyte solution without forming the SEI film is distributed in the separator, the anode, etc., and has a problem of acting as an impurity or causing side reactions.

However, the present invention solves the above problems by including the SEI film former in the composition for forming an anode. By adding the SEI film forming agent to the composition for forming the negative electrode, the SEI film forming agent can be positioned only on the surface and in the vicinity of the negative electrode active material, so that the usage amount of the SEI film forming agent can be remarkably reduced. Therefore, not only can the manufacturing cost of the battery be reduced, but also the above problems due to the remaining SEI film forming agent can be solved.

In the present invention, the SEI film former is any one or a mixture of two or more selected from the group consisting of vinylene carbonate, fluoroethylene carbonate, vinylethylene carbonate, cyclic sulfite, saturated sulphone, unsaturated sulphone, and acyclic sulphone .

Among the SEI film former according to the present invention, the cyclic sulfite may be represented by, for example,

[Chemical Formula 1]

Wherein R ³ to R ⁶ are each independently a hydrogen atom, a halogen atom, a C ₁ to C ₆ alkyl group, or a C ₁ to C ₆ haloalkyl group, and n is an integer of 1 to 3.

Non-limiting examples of cyclic sulfites of Formula 1 above include ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, , 4,5-dimethylpropylene sulfite, 4,5-diethylpropylene sulfite, 4,6-dimethylpropylene sulfite, 4,6-diethylpropylene sulfite and 1,3-butylene glycol sulfite can do. These compounds may be used alone or in combination of two or more.

Among the SEI film formers according to the present invention, the saturated sulphone can be represented, for example, by the following formula (2)

(2)

In the general formula (2), R ⁷ to R ¹² are each independently a hydrogen atom, a halogen atom, a C ₁ to C ₆ alkyl group, or a C ₁ to C ₆ haloalkyl group, and n is an integer of 0 to 3.

Non-limiting examples of saturated sulphones of formula (2) above include, but are not limited to, 1,3-propane sulphone and 1,4-butane sulphone. These compounds may be used alone or in combination of two or more.

Among the SEI film formers according to the present invention, the unsaturated sulphones may be represented by, for example,

(3)

In Formula 3, R ¹³ to R ¹⁶ are each independently a hydrogen atom, a halogen atom, a C ₁ to C ₆ alkyl group, or a C ₁ to C ₆ haloalkyl group, and n is an integer of 0 to 3.

Non-limiting examples of the unsaturated sulphon of the formula (3) may include ethene sulphone, 1,3-propene sulphone, 1,4-butene sulphone, and 1-methyl-1,3-propene sulphone. These compounds may be used alone or in combination of two or more.

Among the SEI film forming agents according to the present invention, the acyclic sulfone can be represented, for example, by the following formula (4)

[Chemical Formula 4]

Wherein R ¹⁷ and R ¹⁸ are each independently selected from the group consisting of a C ₁ to C ₆ alkyl group, a C ₁ to C ₆ haloalkyl group, a C ₂ to C ₆ alkenyl group, a C ₂ to C ₆ haloalkenyl group, A C ₆ to C ₁₈ aryl group, or a C ₆ to C ₁₈ haloaryl group.

Non-limiting examples of sulfone of the above formula (4) may include divinyl sulfone, dimethyl sulfone, diethyl sulfone, methyl ethyl sulfone, and methyl vinyl sulfone. These compounds may be used alone or in combination of two or more.

The content of the SEI film forming agent according to the present invention can be variously determined depending on the kind of the specific SEI film forming agent used, the kind of the negative electrode active material, the use conditions of the battery, and the like. For example, 0.1 to 5 parts by weight, preferably 0.5 to 2 parts by weight based on 100 parts by weight of the negative electrode active material may be included in the composition for forming a negative electrode so as to effectively form an SEI film while minimizing the residual amount in the battery. It is not.

As the negative electrode active material used in the composition for forming a negative electrode for a lithium secondary battery of the present invention, a carbon material capable of occluding and releasing lithium ions, lithium, lithium alloy, silicon, silicon alloy, tin, tin alloy, , And metal oxides such as TiO ₂ and SnO ₂ having a potential for lithium of less than 2V are also possible. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes.

As the binder used in the composition for forming a negative electrode for a lithium secondary battery of the present invention, a commonly used binder may be used without limitation. For example, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, SBR (styrene butadiene rubber), carboxymethyl cellulose (CMC), and the like can be used.

The solvent used in the composition for forming a negative electrode for a lithium secondary battery of the present invention can be used without limitation as long as it is used as a solvent of a composition for forming a negative electrode in the art. For example, n-methylpyrrolidone, dimethylsulfoxide, cresol, water and the like may be used alone or in admixture of two or more thereof, but not limited thereto.

Alternatively, the composition for forming a negative electrode for a lithium secondary battery of the present invention may further comprise a carbonate, an alcohol or a mixture thereof as an auxiliary solvent. The co-solvents are used to control the polarity to increase the solubility of the SEI film former.

Examples of the carbonate that can be used as the co-solvent include linear carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; Cyclic carbonates such as ethylene carbonate and propylene carbonate; And mixtures thereof. Examples of the alcohol include isopropyl alcohol, isobutyl alcohol, and the like, but the present invention is not limited thereto.

The composition for forming a negative electrode of the present invention may be applied to at least one surface of a current collector for a negative electrode and dried to be used as a negative electrode of a lithium secondary battery. At this time, the SEI film former is located on the surface of the negative electrode active material or in the vicinity of the active material. Therefore, when the electrolyte is injected at the time of assembling the battery, the SEI film can be formed on the surface of the negative electrode active material by reacting with the electrolyte solution reaching the negative electrode.

The lithium secondary battery according to the present invention is manufactured as a lithium secondary battery by injecting a non-aqueous electrolyte into the electrode structure composed of the negative electrode, the positive electrode, and the separator interposed between the positive electrode and the negative electrode. In addition to the negative electrode according to the present invention, the positive electrode, the separator, and the nonaqueous electrolyte may be those conventionally used in the production of lithium secondary batteries.

The nonaqueous electrolyte solution according to the present invention comprises an ionizable lithium salt and an organic solvent.

The lithium salt contained as an electrolyte in a non-aqueous liquid electrolyte of the present invention can be used without limitation, those which are commonly used in a lithium secondary battery electrolyte, such as the lithium salt, the anion 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 ^- .

Examples of the organic solvent included in the non-aqueous electrolyte of the present invention include those commonly used in an electrolyte for a lithium secondary battery, such as an ether, an ester, an amide, a linear carbonate, a cyclic carbonate, etc., Two or more of them may be used in combination.

Among them, a carbonate compound which is typically a cyclic carbonate, a linear carbonate, or a mixture thereof may be included. Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, and halides thereof, or a mixture of two or more thereof. Specific examples of the linear carbonate compound include a group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate and ethyl propyl carbonate Any one selected, or a mixture of two or more thereof may be used as typical examples, but the present invention is not limited thereto.

In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates in the carbonate-based organic solvent, can be preferably used because they have high permittivity as a high viscosity organic solvent and dissociate the lithium salt in the electrolyte well, and dimethyl carbonate and diethyl carbonate When a low viscosity and a low dielectric constant linear carbonate are mixed in an appropriate ratio, an electrolyte having a high electric conductivity can be prepared, and thus it can be more preferably used.

As the ether in the organic solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether and ethyl propyl ether or a mixture of two or more thereof may be used , But is not limited thereto.

Examples of the ester in the organic solvent include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate,? -Butyrolactone,? -Valerolactone,? -Caprolactone,? -Valerolactone, ε-caprolactone, or a mixture of two or more thereof, but the present invention is not limited thereto.

Also, in the positive electrode, the negative electrode and the separator forming the electrode structure, in addition to the negative electrode prepared according to the present invention, the positive electrode and the separator may be those conventionally used in the manufacture of lithium secondary batteries.

As a specific example, the cathode active material include lithium-containing transition and the metal oxide is preferably used, for example, _{Li x CoO 2 (0.5 <x} <1.3), Li x NiO 2 (0.5 <x <1.3), Li x MnO _{2 (0.5 <x <1.3)} , Li x Mn 2 O 4 (0.5 <x <1.3), Li x (Ni a Co b Mn c) O 2 (0.5 <x <1.3, 0 <a <1, 0 < 1, 0 <c <1, a + b + c = 1), Li _x Ni _1-y Co _y O ₂ (0.5 <x <1.3, 0 <y <1), Li _x Co _1-y Mn _{y O 2 (0.5 <x <} 1.3, 0≤y <1), Li x Ni 1-y Mn y O 2 (0.5 <x <1.3, O≤y <1), Li x (Ni a Co b Mn c ) O ₄ (0.5 <x <1.3, 0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), Li _x Mn _{2 -z} Ni _z O ₄ x <1.3, 0 <z < 2), Li x Mn 2-z Co z O 4 (0.5 <x <1.3, 0 <z <2), Li x CoPO 4 (0.5 <x <1.3) and Li _x FePO ₄ (0.5 < x < 1.3), or a mixture of two or more thereof. The lithium-containing transition metal oxide may be coated with a metal such as aluminum (Al) or a metal oxide . In addition to the lithium-containing transition metal oxide, sulfide, selenide and halide may also be used.

The composition for forming an anode may be the same as the binder used in the negative electrode.

As the separator, a conventional porous polymer film conventionally used as a separator, for example, a polyolefin such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer and an ethylene / methacrylate copolymer A porous polymer film made of a high molecular weight polymer may be used alone or in a laminated manner, or a nonwoven fabric made of a conventional porous nonwoven fabric such as a glass fiber having a high melting point, a polyethylene terephthalate fiber or the like may be used. It is not.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example

&Lt; Preparation of composition for negative electrode formation >

96.0 parts by weight of natural graphite, 1.0 part by weight of carboxymethylcellulose, 2.0 parts by weight of styrene-butadiene rubber, and 1.0 part by weight of conductive carbon were successively mixed with water in a mortar and stirred sufficiently. To this mixture, vinylene carbonate was added in an amount of 2.0% by weight, that is, 1.92 parts by weight of natural graphite, which is an anode active material, and then re-crosslinked to prepare a uniform composition for forming an anode.

&Lt; Preparation of negative electrode for lithium secondary battery &

The thus-prepared composition for anode formation was coated on a copper foil having a thickness of 10 탆 to a thickness of 250 탆 and dried in an oven at about 120 캜 for about 1 hour to prepare a negative electrode.

&Lt; Production of lithium secondary battery >

A coin cell of the CR2016 standard was prepared using lithium as an anode and a polyethylene porous film having a thickness of 20 mu m as a separator. As the electrolytic solution, ethylene carbonate and ethyl methyl carbonate were mixed in a volume ratio of 1: 2, and LiPF6 1M was dissolved and used to prepare a lithium secondary battery.

Comparative Example

A lithium secondary battery was produced in the same manner as in Example except that vinylene carbonate corresponding to 2.0% by weight of natural graphite, which is an anode active material, was added to the electrolyte without adding vinylene carbonate at the time of preparing the negative electrode .

&Lt; Evaluation of charging / discharging cycle performance &

The life characteristics of the coin cells prepared in the above examples and comparative examples were measured in terms of ratio (%) relative to the initial capacity in each cycle under the conditions of a charge-discharge cycle of 1.0 C, and the measurement results are shown in Fig.

Referring to FIG. 1, in the case where vinylene carbonate was added at the same weight ratio to the active material in the electrode preparation and in the comparative example in which the electrolyte was added to the electrolyte, the results obtained when the charge / discharge cycle was directly injected into the active material Can be confirmed.

Claims (10)

Anode active material; bookbinder; menstruum; And an SEI film former,
Wherein the SEI film former is a fluoroethylene carbonate, vinyl ethylene carbonate, or a mixture thereof,
The SEI film forming agent is contained in an amount of 0.5 to 2 parts by weight based on 100 parts by weight of the negative electrode active material,
Wherein the binder is at least one of SBR (styrene butadiene rubber) and CMC (carboxymethyl cellulose), and the solvent is water. delete delete delete delete delete The method according to claim 1,
Carbonate, an alcohol, or a mixture thereof as an auxiliary solvent. The method according to claim 1,
The negative electrode active material may be any one selected from the group consisting of carbon materials capable of lithium ion intercalation and deintercalation, lithium, lithium alloy, silicon, silicon alloy, tin and tin alloy, or a mixture of two or more thereof. By weight based on the weight of the negative electrode. And a negative electrode active material layer formed by applying the negative electrode composition of any one of claims 1, 7, or 8 on at least one surface of the current collector and the current collector and drying the negative electrode active material layer. A lithium secondary battery comprising a negative electrode, a positive electrode and a non-aqueous electrolyte,
Wherein the negative electrode is the negative electrode of claim 9.

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