阅读说明：本技术 锂二次电池用非水电解质溶液和包含其的锂二次电池 (Non-aqueous electrolyte solution for lithium secondary battery and lithium secondary battery comprising same ) 是由 李贤荣 李哲行 林永敏 李政旻 廉澈殷 韩正求 于 2020-11-13 设计创作，主要内容包括：本发明涉及一种锂二次电池用非水电解质溶液和包含其的锂二次电池,具体而言,本发明的目的在于提供一种包含锂盐、有机溶剂和作为添加剂的式1表示的化合物的锂二次电池用非水电解质溶液和锂二次电池,其中通过包含所述非水电解质溶液,高温下的高倍率充放电特性得到改善。(The present invention relates to a non-aqueous electrolyte solution for a lithium secondary battery and a lithium secondary battery comprising the same, and more particularly, to a non-aqueous electrolyte solution for a lithium secondary battery and a lithium secondary battery comprising a lithium salt, an organic solvent, and a compound represented by formula 1 as an additive, wherein high-rate charge and discharge characteristics at high temperatures are improved by including the non-aqueous electrolyte solution.)

1. A nonaqueous electrolyte solution for a lithium secondary battery, comprising a lithium salt, an organic solvent, and a compound represented by formula 1 as an additive:

[ formula 1]

Wherein, in the formula 1,

R₁to R₆Each independently hydrogen, an alkyl group having 1 to 5 carbon atoms or a-CN group, wherein R₁To R₆At least one of which is a-CN group.

2. The non-aqueous electrolyte solution for lithium secondary batteries according to claim 1, wherein R in formula 1₁To R₆Each independently hydrogen, an alkyl group having 1 to 4 carbon atoms or a-CN group, wherein R₁To R₆At least one of which is a-CN group.

3. The non-aqueous electrolyte solution for lithium secondary batteries according to claim 2, wherein R in formula 1₁Is an alkyl group having 1 to 3 carbon atoms or a-CN group,

R₂is hydrogen or an alkyl group having 1 to 3 carbon atoms, and

R₃to R₆Each independently hydrogen, an alkyl group having 1 to 4 carbon atoms or a-CN group, wherein R₁、R₃To R₆At least one of which is a-CN group.

4. The non-aqueous electrolyte solution for lithium secondary batteries according to claim 1, wherein R in formula 1₁Is a-CN group, R₂Is hydrogen or an alkyl group having 1 to 3 carbon atoms, and R₃To R₆Each independently hydrogen, an alkyl group having 1 to 3 carbon atoms, or a-CN group.

5. The non-aqueous electrolyte solution for lithium secondary batteries according to claim 1, wherein R in formula 1₁Is a-CN group, R₂Is hydrogen, R₃And R₆Each independently is hydrogen or a-CN group, and R₄And R₅Each independently is hydrogen or fluorineAn alkyl group having 1 to 3 carbon atoms or a-CN group.

6. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the compound represented by formula 1 comprises a compound represented by formula 1 a:

[ formula 1a ]

7. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the content of the compound represented by formula 1 is 0.05 wt% or more and less than 1.2 wt% based on the total weight of the nonaqueous electrolyte solution.

8. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 7, wherein the compound represented by formula 1 is contained in an amount of 0.1 to 1% by weight based on the total weight of the nonaqueous electrolyte solution.

9. A lithium secondary battery comprising a negative electrode, a positive electrode, a separator provided between the negative electrode and the positive electrode, and a nonaqueous electrolyte solution,

wherein the nonaqueous electrolyte solution contains the nonaqueous electrolyte solution for a lithium secondary battery according to claim 1.

10. The lithium secondary battery according to claim 9, wherein the positive electrode comprises a positive electrode active material comprising lithium and at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), and aluminum (Al).

Technical Field

Cross Reference to Related Applications

The present application claims priority from korean patent application No. 10-2019-0147431, filed on 18/11/2019, and korean patent application No. 10-2020-0151165, filed on 12/11/2020, the disclosures of which are incorporated herein by reference. The present invention relates to a nonaqueous electrolyte solution for a lithium secondary battery and a lithium secondary battery including the same.

Background

The dependence of modern society on electric energy is gradually increasing, and correspondingly, the production of electric energy is further increasing. In order to solve the environmental problems occurring in this process, renewable energy power generation has been attracting attention as a next-generation power generation system. For renewable energy, a large-capacity energy storage device is essential for stable power supply due to its intermittent power generation feature. Lithium ion batteries have received much attention as devices showing the highest energy density commercialized in energy storage devices at present.

The lithium ion battery is composed of: a positive electrode formed of a transition metal oxide containing lithium, a negative electrode capable of storing lithium, an electrolyte solution containing an organic solvent containing a lithium salt, and a separator.

With regard to the positive electrodes in these elements, energy is stored by redox reactions of the transition metals, wherein this results in the transition metals having to be substantially contained in the positive electrode material.

When the positive electrode active material undergoes structural collapse during repeated charge and discharge, the positive electrode performance is reduced. That is, metal ions that have been dissolved from the surface of the positive electrode during collapse of the positive electrode structure are electrodeposited on the negative electrode, thereby reducing the performance of the battery. This phenomenon is further increased when the potential of the positive electrode is raised or the battery is exposed to high temperature.

Therefore, in order to control the deterioration behavior of the battery, studies have been made to apply an additive for forming a film on the positive electrode, and in addition, studies are being made to suppress the occurrence of electrodeposition or ion substitution of a dissolved transition metal on the negative electrode.

Disclosure of Invention

Technical problem

An aspect of the present invention provides a non-aqueous electrolyte solution for a lithium secondary battery, which includes an additive that forms a complex with a transition metal ion dissolved from a positive electrode.

Another aspect of the present invention provides a lithium secondary battery in which high-rate charge and discharge characteristics are improved by including a non-aqueous electrolyte solution for a lithium secondary battery.

Technical scheme

In accordance with one aspect of the present invention,

provided is a non-aqueous electrolyte solution for a lithium secondary battery, which includes a lithium salt, an organic solvent, and a compound represented by formula 1 as an additive.

[ formula 1]

Wherein, in the formula 1,

R₁to R₆Each independently hydrogen, an alkyl group having 1 to 5 carbon atoms or a-CN group, wherein R₁To R₆At least one of which is a-CN group.

In accordance with another aspect of the present invention,

there is provided a lithium secondary battery comprising a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and a nonaqueous electrolyte solution, wherein the nonaqueous electrolyte solution comprises the nonaqueous electrolyte solution for lithium secondary batteries of the present invention.

Advantageous effects

The compound represented by formula 1 included in the non-aqueous electrolyte solution of the present invention is a compound having a cyano group in its structure, wherein the cyano group can inhibit electrodeposition of metal ions on a negative electrode by forming a complex with transition metal ions dissolved from the positive electrode of a lithium secondary battery. Since the nonaqueous electrolyte solution containing such an additive is oxidatively decomposed before the organic solvent to form a film on the surface of the positive electrode, it can suppress a continuous decomposition reaction between the positive electrode and the organic solvent. Therefore, if a nonaqueous electrolyte solution is contained, a lithium secondary battery having improved high-rate charge-discharge characteristics can be realized.

Drawings

The following drawings attached to the specification illustrate preferred embodiments of the present invention by way of examples and serve to enable the technical concept of the present invention to be further understood together with the detailed description of the invention given below, and therefore the present invention should not be construed as limited to the matters in the drawings.

Fig. 1 is a graph showing the results of electrochemical stability evaluation of a nonaqueous electrolyte solution in experimental example 1;

fig. 2 is a graph showing the results of measurement of the decomposition start voltages of the nonaqueous electrolyte solutions of example 3 and comparative example 2 in experimental example 2;

fig. 3 is a graph showing differential capacitance curves of the lithium secondary batteries of example 5 and comparative example 3 in experimental example 3;

fig. 4 is a graph showing the impedance evaluation results of the lithium secondary batteries of example 5 and comparative example 3 in experimental example 5; and

fig. 5 is a graph showing the results of evaluation of high-temperature cycle characteristics of the secondary batteries of example 5 and comparative example 3 in experimental example 6.

Detailed Description

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

It should be understood that the words or terms used in the specification and claims should not be construed as meanings defined in a general dictionary, and it will be further understood that the words or terms should be construed as having meanings consistent with their meanings in the context of the related art and the technical idea of the present invention based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the invention.

Conventionally, the transition metal constituting the positive electrode is easily dissolved into the electrolyte solution due to hydrolysis/thermal decomposition of an acid or lithium salt generated by a side reaction between the positive electrode and the electrolyte solution (e.g., hydrofluoric acid (HF)), or the positive electrode structure is changed due to repeated charge and discharge, and the dissolved transition metal ions are again deposited on the positive electrode, resulting in an increase in the resistance of the positive electrode. In addition, since the transition metal moved to the negative electrode through the electrolyte solution is electrodeposited on the negative electrode, thereby self-discharging the negative electrode and breaking a Solid Electrolyte Interface (SEI) that makes the negative electrode have passivation ability, the interfacial resistance of the negative electrode is increased by promoting an additional electrolyte solution decomposition reaction.

Since this series of reactions reduces the amount of available lithium ions in the battery, not only does it lead to a decrease in battery capacity, but it also accompanies decomposition reactions of the electrolyte solution, thus increasing resistance. In addition, in the case where metal impurities are contained in the electrode when the positive electrode is disposed, since the metal foreign matter is dissolved from the positive electrode during initial charging, the dissolved metal ions are electrodeposited on the surface of the negative electrode. The electrodeposited metal ions grow into dendrites to cause internal short circuits of the battery, and thus become a main cause of low voltage failure.

The present invention is directed to a non-aqueous electrolyte solution for a lithium secondary battery, which can form a strong film on the surface of a positive electrode through oxidative decomposition before an organic solvent by containing an additive capable of preventing metal ions from being electrodeposited on the negative electrode (the cause of the above-mentioned deterioration and failure behavior) through the formation of a complex with dissolved metal ions, and a lithium secondary battery in which high-rate charge and discharge at high temperatures are improved by containing the non-aqueous electrolyte solution.

Nonaqueous electrolyte solution for lithium secondary battery

Specifically, in an embodiment of the present invention, there is provided a nonaqueous electrolyte solution for a lithium secondary battery, including:

a lithium salt, an organic solvent, and a compound represented by formula 1 as an additive,

[ formula 1]

In the formula 1, the first and second groups,

R₁to R₆Each independently hydrogen, an alkyl group having 1 to 5 carbon atoms or a-CN group, wherein R₁To R₆At least one of which is a-CN group.

Lithium salt

First, in the non-aqueous electrolyte solution for a lithium secondary battery of the present invention, any lithium salt that is generally used in a lithium secondary battery as an electrolyte solution may be used without limitation, and for example, the lithium salt may include Li⁺As a cation, and comprises a compound selected from the group consisting of F^-、Cl^-、Br^-、I^-、NO₃ ^-、N(CN)₂ ^-、BF₄ ^-、ClO₄ ^-、B₁₀Cl₁₀ ^-、AlCl₄ ^-、AlO₄ ^-、PF₆ ^-、CF₃SO₃ ^-、CH₃CO₂ ^-、CF₃CO₂ ^-、AsF₆ ^-、SbF₆ ^-、CH₃SO₃ ^-、(CF₃CF₂SO₂)₂N^-、(CF₃SO₂)₂N^-、(FSO₂)₂N^-、BF₂C₂O₄ ^-、BC₄O₈ ^-、PF₄C₂O₄ ^-、PF₂C₄O₈ ^-、(CF₃)₂PF₄ ^-、(CF₃)₃PF₃ ^-、(CF₃)₄PF₂ ^-、(CF₃)₅PF^-、(CF₃)₆P^-、C₄F₉SO₃ ^-、CF₃CF₂SO₃ ^-、CF₃CF₂(CF₃)₂CO^-、(CF₃SO₂)₂CH^-、CF₃(CF₂)₇SO₃ ^-And SCN^-At least one of the group consisting of as an anion.

Specifically, the lithium salt may include one or more compounds selected from the group consisting of LiCl, LiBr, LiI, and LiBF₄、LiClO₄、LiB₁₀Cl₁₀、LiAlCl₄、LiAlO₄、LiPF₆、LiCF₃SO₃、LiCH₃CO₂、LiCF₃CO₂、LiAsF₆、LiSbF₆、LiCH₃SO₃LiFSI (lithium bis (fluorosulfonyl) imide), LiN (SO)₂F)₂) LiBETI (lithium bis (perfluoroethanesulfonyl) imide), LiN (SO)₂CF₂CF₃)₂) And lithium bis (trifluoromethanesulfonyl) imide (litz), LiN (SO)₂CF₃)₂) A single material of the group, or a mixture of two or more thereof. In addition to this, any lithium salt commonly used in an electrolyte solution of a lithium secondary battery may be used without limitation.

The lithium salt may be appropriately changed within a normally usable range, but may be contained in the electrolyte solution at a concentration of 0.8M to 4.0M (e.g., 1.0M to 3.0M) to obtain the best effect of forming a film for preventing corrosion of the electrode surface. If the concentration of the lithium salt is less than 0.8M, the effects of improving the low-temperature output of the lithium secondary battery and improving the cycle characteristics during high-temperature storage are insignificant, and if the concentration of the lithium salt is greater than 4.0M, the permeability of the electrolyte solution may be reduced due to the increase in viscosity of the nonaqueous electrolyte solution.

(2) Organic solvent

In the nonaqueous electrolyte solution for a lithium secondary battery of the present specification, the organic solvent may include a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, or a mixed organic solvent thereof.

The cyclic carbonate-based organic solvent is an organic solvent that can well dissociate lithium salts in the electrolyte due to a high dielectric constant as a high-viscosity organic solvent, wherein specific examples of the cyclic carbonate-based organic solvent may be at least one organic solvent selected from the group consisting of Ethylene Carbonate (EC), Propylene Carbonate (PC), 1, 2-butylene carbonate, 2, 3-butylene carbonate, 1, 2-pentylene carbonate, 2, 3-pentylene carbonate, and vinylene carbonate, wherein the cyclic carbonate-based organic solvent may include ethylene carbonate.

Further, the linear carbonate-based organic solvent is an organic solvent having a low viscosity and a low dielectric constant, wherein typical examples of the linear carbonate-based organic solvent may be at least one organic solvent selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethylmethyl carbonate (EMC), methylpropyl carbonate, and ethylpropyl carbonate, and the linear carbonate-based organic solvent may specifically include ethylmethyl carbonate (EMC).

In order to prepare an electrolyte solution having high ionic conductivity, a mixed organic solvent of a cyclic carbonate-based organic solvent and a linear carbonate-based organic solvent may be used as the organic solvent.

In addition, the organic solvent may include a linear ester-based organic solvent and/or a cyclic ester-based organic solvent in addition to the cyclic carbonate-based organic solvent and/or the linear carbonate-based organic solvent.

Specific examples of the linear ester-based organic solvent may be at least one organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

In addition, the cyclic carbonate-based organic solvent may include at least one organic solvent selected from the group consisting of gamma-butyrolactone, gamma-valerolactone, gamma-caprolactone, sigma-valerolactone and epsilon-caprolactone.

If necessary, the organic solvent may be used by adding it to an electrolyte solution generally used for a lithium secondary battery, without limitation. For example, the organic solvent may further include at least one selected from an ether-based organic solvent, an amide-based organic solvent, and a nitrile-based organic solvent.

(3) Additive agent

The nonaqueous electrolyte solution for a lithium secondary battery of the present invention may contain a compound represented by the following formula 1 as an additive.

[ formula 1]

In the formula 1, the first and second groups,

R₁to R₆Each independently hydrogen, an alkyl group having 1 to 5 carbon atoms or a-CN group, wherein R₁To R₆At least one of which is a-CN group.

Specifically, in formula 1, R₁To R₆Can each independently be hydrogen, an alkyl group having 1 to 4 carbon atoms or a-CN group, wherein R₁To R₆At least one of which may be a-CN group.

Further, in formula 1, R₁May be an alkyl group having 1 to 3 carbon atoms or a-CN group, R₂May be hydrogen or an alkyl group having 1 to 3 carbon atoms, R₃To R₆May each independently be hydrogen, an alkyl group having 1 to 4 carbon atoms or a-CN group, wherein R is₁、R₃To R₆At least one of which may be a-CN group.

Further, in formula 1, R₁May be a-CN group, R₂May be hydrogen or an alkyl group having 1 to 3 carbon atoms, and R₃To R₆May each independently be hydrogen, an alkyl group having 1 to 3 carbon atoms, or a-CN group.

In addition, in the formulaIn 1, R₁May be a-CN group, R₂May be hydrogen, R₃And R₆May each independently be hydrogen or a-CN group, and R₄And R₅May each independently be hydrogen, an alkyl group having 1 to 3 carbon atoms, or a-CN group.

Further, in formula 1, R₁May be a-CN group, R₂May be hydrogen, R₃And R₆May each independently be hydrogen, and R₄And R₅May each independently be hydrogen or a-CN group.

Preferably, the compound represented by formula 1 may be a compound represented by the following formula 1a, for example, coumarin-3-carbonitrile.

[ formula 1a ]

In the present invention, the compound represented by formula 1 included as an additive for an electrolyte solution is a compound having a cyano group in its structure, wherein the cyano group can inhibit electrodeposition of metal ions on a negative electrode by forming a complex with the metal ions dissolved from the positive electrode of a lithium secondary battery. In addition, the additive may form a strong film on the surface of the positive electrode by oxidative decomposition before the organic solvent, and the film may inhibit a continuous decomposition reaction between the positive electrode and the organic solvent. Therefore, a lithium secondary battery having improved high-rate charge and discharge can be realized by including a nonaqueous electrolyte solution (including an additive).

The content of the compound of formula 1 may be 0.05 wt% or more to less than 1.2 wt%, for example, 0.1 wt% to 1 wt%, based on the total weight of the nonaqueous electrolyte solution.

In the case where the content of the compound represented by formula 1 is within the above range, a secondary battery having more improved overall performance can be prepared. For example, in the case where the content of the compound represented by formula 1 is 0.05 wt% or more and less than 1.2 wt%, it may remove metal ions and complexes and may simultaneously form a strong film on the surface of the positive electrode while suppressing disadvantages such as side reactions caused by additives, a decrease in initial capacity, and an increase in resistance as much as possible. If the content of the compound represented by formula 1 is 1.2% by weight or more, side reactions due to the additive may occur or a decrease in initial capacity due to an increase in resistance may occur because the solubility of the additive in the non-aqueous organic solvent decreases.

Lithium secondary battery

In addition, in another embodiment of the present invention, there is provided a lithium secondary battery comprising the non-aqueous electrolyte solution for a lithium secondary battery of the present invention.

The lithium secondary battery of the present invention may be prepared as follows: an electrode assembly in which a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode are sequentially stacked is formed, the electrode assembly is accommodated in a battery case, and then the nonaqueous electrolyte solution of the present invention is injected.

As a method for preparing the lithium secondary battery of the present invention, typical methods known in the art may be used, and specifically, a method for preparing the lithium secondary battery of the present invention is as follows.

(1) Positive electrode

The positive electrode may be prepared by coating a positive electrode current collector with a positive electrode slurry including a positive electrode active material, a binder, a conductive agent, and a solvent, and then drying and roll-pressing the coated positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, or the like may be used.

The positive active material is a compound capable of reversibly intercalating and deintercalating lithium, wherein the positive active material may specifically include a lithium composite metal oxide including lithium and at least one metal selected from the group consisting of nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), and aluminum (Al).

More specifically, the lithium composite metal oxide may include: lithium-manganese-based oxides (e.g. LiMnO)₂、LiMn₂O₄Etc.), lithium-cobalt based oxides (e.g., LiCoO)₂Etc.), lithium-nickel based oxides (e.g., LiNiO)₂Etc.), lithium-nickel-manganese-based oxides (e.g., LiNi)_1-YMn_YO₂(wherein 0)<Y<1)、LiMn_2-ZNi_ZO₄(wherein, 0<Z<2) Lithium-nickel-cobalt based oxides (e.g., LiNi)_1-Y1Co_Y1O₂(wherein 0)<Y1<1) Lithium-manganese-cobalt based oxides (e.g., LiCo)_1-Y2Mn_Y2O₂(wherein 0)<Y2<1)、LiMn_2-Z1Co_Z1O₄(wherein 0)<Z1<2) Lithium-nickel-manganese-cobalt based oxides (e.g., Li (Ni))_pCo_qMn_r1)O₂(wherein 0)<p<1、0<q<1、0<r1<1 and p + q + r1 ═ 1) or Li (Ni)_p1Co_q1Mn_r2)O₄(wherein 0)<p1<2、0<q1<2、0<r2<2 and p1+ q1+ r2 ═ 2), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li (Ni)_p2Co_q2Mn_r3M_S2)O₂(wherein M is selected from the group consisting of aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and p2, q2, r3, and S2 are atomic fractions of each independent element, where 0 is<p2<1、0<q2<1、0<r3<1、0<S2<1 and p2+ q2+ r3+ S2 ═ 1), and any one or two or more compounds thereof may be included. Among these materials, the lithium composite metal oxide may include LiCoO in terms of improving capacity characteristics and stability of a battery₂、LiMnO₂、LiNiO₂Lithium nickel manganese cobalt oxide (e.g., Li (Ni))_0.6Mn_0.2Co_0.2)O₂、Li(Ni_0.5Mn_0.3Co_0.2)O₂Or Li (Ni)_0.8Mn_0.1Co_0.1)O₂) Or lithium nickel cobalt aluminum oxide (e.g., LiNi)_0.8Co_0.15Al_0.05O₂Etc.), the lithium composite metal oxide may include Li (Ni) in consideration of significant improvement by controlling the type and content ratio of elements constituting the lithium composite metal oxide_0.6Mn_0.2Co_0.2)O₂、Li(Ni_0.5Mn_0.3Co_0.2)O₂、Li(Ni_0.7Mn_0.15Co_0.15)O₂Or Li (Ni)_0.8Mn_0.1Co_0.1)O₂Any one of them or a mixture of two or more thereof may be used.

The positive electrode active material may be contained in an amount of 80 to 99 wt%, for example, 90 to 99 wt%, based on the total weight of solids in the positive electrode slurry. When the amount of the positive electrode active material is 80 wt% or less, the energy density decreases, and thus the capacity decreases.

The binder is a component that contributes to adhesion between the active material and the conductive agent and adhesion to the current collector, wherein the binder is generally added in an amount of 1 to 30 wt% based on the total weight of solids in the cathode slurry. Examples of the binder may be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, fluororubber, various copolymers, and the like.

Further, the conductive agent is a material that provides conductivity without causing adverse chemical changes in the battery, wherein the conductive agent may be added in an amount of 1 to 20 wt% based on the total weight of solids in the positive electrode slurry.

As typical examples of the conductive agent, the following conductive materials can be used, for example: carbon powders, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black or thermal black; graphite powder such as natural graphite, artificial graphite or graphite having a developed crystal structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or a polyphenylene derivative.

In addition, the solvent may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that a desired viscosity is obtained when the cathode active material and optional binder and conductive agent are included. For example, the solvent may be contained in an amount such that the concentration of solids in the slurry including the positive electrode active material and optionally the binder and the conductive agent is in the range of 10 to 60 wt%, for example, 20 to 50 wt%.

(2) Negative electrode

The anode may be prepared by coating an anode current collector with an anode slurry including an anode active material, a binder, a conductive agent, and a solvent, and then drying and roll-pressing the coated anode current collector.

The anode current collector generally has a thickness of 3 to 500 μm. The anode current collector is not particularly limited as long as it has high conductivity without causing adverse chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, fired carbon; copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, and the like; aluminum cadmium alloys, and the like. In addition, the anode current collector may have fine surface roughness to improve adhesive strength with the anode active material, similar to the cathode current collector, and the anode current collector may be used in various shapes, such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric body, and the like.

In addition, the anode active material may include at least one selected from the group consisting of metallic lithium, a carbon material capable of reversibly intercalating/deintercalating lithium ions, a metal or an alloy of lithium and a metal, a metal composite oxide, a material that may be doped and undoped with lithium, and a transition metal oxide.

As the carbon material capable of reversibly intercalating/deintercalating lithium ions, a carbon-based negative electrode active material generally used in a lithium ion secondary battery may be used without particular limitation, and as typical examples, crystalline carbon, amorphous carbon, or both thereof may be used. Examples of crystalline carbon may be graphite, such as irregular, planar, flaky, spherical or fibrous natural or artificial graphite, and examples of amorphous carbon may be soft carbon (low-temperature sintered carbon) or hard carbon, mesophase pitch carbide and fired coke.

As the metal or the alloy of lithium and metal, a metal selected from the group consisting of copper (Cu), nickel (Ni), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), silicon (Si), antimony (Sb), lead (Pb), indium (In), zinc (Zn), barium (Ba), radium (Ra), germanium (Ge), aluminum (Al), and tin (Sn), or an alloy of lithium and metal may Be used.

Can be selected from PbO, PbO₂、Pb₂O₃、Pb₃O₄、Sb₂O₃、Sb₂O₄、Sb₂O₅、GeO、GeO₂、Bi₂O₃、Bi₂O₄、Bi₂O₅、Li_xFe₂O₃(0≤x≤1)、Li_xWO₂(x is more than or equal to 0 and less than or equal to 1) and Sn_xMe_1-xMe'_yO_z(Me: manganese (Mn), Fe, Pb or Ge; Me': Al, boron (B), phosphorus (P), Si, elements of groups I, II and III of the periodic Table of the elements or halogen; 0<x is less than or equal to 1; y is more than or equal to 1 and less than or equal to 3; 1. ltoreq. z. ltoreq.8) may be used as the metal composite oxide.

The material capable of doping and undoped lithium can comprise Si and SiO_x(0<x is less than or equal to 2), Si-Y alloy (wherein Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO₂And Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), SiO may also be used₂And mixtures with at least one thereof. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, scandium (Sc), yttrium (Y), Ti, zirconium (Zr), hafnium (Hf),(Rf), V, niobium (Nb), Ta,(Db), Cr, Mo, tungsten (W),(Sg), technetium (Tc), rhenium (Re),(Bh), Fe, Pb, ruthenium (Ru), osmium (Os),(Hs), rhodium (Rh), iridium (Ir), palladium (Pd), platinum (Pt), Cu, silver (Ag), gold (Au), Zn, cadmium (Cd), B, Al, gallium (Ga), Sn, In, Ge, P, arsenic (As), Sb, bismuth (Bi), sulfur (S), selenium (Se), tellurium (Te), polonium (Po), and combinations thereof.

The transition metal oxide may include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

The content of the negative electrode active material may be 80 to 99% by weight based on the total weight of solids in the negative electrode slurry.

The binder is an ingredient contributing to adhesion between the conductive agent, the active material, and the current collector, wherein the binder is generally added in an amount of 1 to 30 wt% based on the total weight of solids in the anode slurry. Examples of the binder may be polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

The conductive agent is an ingredient for further improving the conductivity of the anode active material, wherein the conductive agent may be added in an amount of 1 to 20 wt% based on the total weight of solids in the anode slurry. Any conductive agent may be used without particular limitation so long as it has conductivity without inducing adverse chemical changes in the battery, and for example, the following conductive materials may be used, for example: carbon powders, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black or thermal black; graphite powder such as natural graphite, artificial graphite or graphite having a developed crystal structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as fluorocarbon powders, aluminum powders, and nickel powders; conductive whiskers such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides such as titanium oxide; or a polyphenolic derivative.

The solvent may include water or an organic solvent, such as NMP and alcohol, and the amount of the solvent used may be such that a desired viscosity is obtained when the anode active material and optional binder and conductive agent are included. For example, the solvent may be contained in an amount such that the concentration of solids in the anode slurry including the anode active material and optionally the binder and the conductive agent is in a range of 50 wt% to 75 wt%, for example, 50 wt% to 65 wt%.

(3) Diaphragm

A typical porous polymer film that is generally used, for example, a porous polymer film made of polyolefin-based polymers (e.g., ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer) may be used alone or laminated, and as a separator included in the lithium secondary battery of the present invention, a typical porous non-woven fabric, for example, a non-woven fabric formed of high-melting glass fibers or polyethylene terephthalate fibers, may be used, but the present invention is not limited thereto.

The shape of the lithium secondary battery of the present invention is not particularly limited, and a cylindrical type, a prismatic type, a pouch type, or a coin type using a can may be used.

Hereinafter, the present invention will be described in more detail according to examples. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these example embodiments are provided so that this description will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

Examples

I. Preparation of non-aqueous electrolyte solution for lithium secondary battery

Comparative example 1

After Ethylene Carbonate (EC) and ethylmethyl carbonate (EMC) were mixed at a volume ratio of 1:2, LiPF was dissolved₆So that the LiPF₆Was 1.0M, thereby preparing an electrolyte solution (a-1).

Comparative example 2

After mixing Ethylene Carbonate (EC), Propylene Carbonate (PC), Ethyl Propionate (EP) and Propyl Propionate (PP) in a volume ratio of 2:1:2.5:4.5Dissolving LiPF₆And LiFSI to make LiPF₆And the concentrations of LiFSI were 0.8M and 0.2M, respectively, to prepare an electrolyte solution (A-2).

Example 1

A nonaqueous electrolyte solution (B-1) for a lithium secondary battery of the present invention was prepared by adding 0.1g of coumarin-3-carbonitrile to 99.9g of the electrolyte solution (A-1) of comparative example 1.

Example 2

A nonaqueous electrolyte solution (B-2) for a lithium secondary battery of the present invention was prepared by adding 1.0g of coumarin-3-carbonitrile to 99.0g of the electrolyte solution (A-1) of comparative example 1.

Example 3

A nonaqueous electrolyte solution (B-3) for a lithium secondary battery of the present invention was prepared by adding 0.2g of coumarin-3-carbonitrile to 99.8g of the electrolyte solution (A-2) of comparative example 2.

Example 4

A nonaqueous electrolyte solution (B-4) for a lithium secondary battery of the present invention was prepared by adding 1.2g of coumarin-3-carbonitrile to 98.8g of the electrolyte solution (A-2) of comparative example 2.

Preparation of Secondary Battery

Example 5

A positive electrode active material (Li (Ni))_0.8Co_0.1Mn_0.1)O₂) Carbon black as a conductive agent and polyvinylidene fluoride as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 98:1:1 to prepare a positive electrode slurry (solid content 40 wt%). A 20 μm thick positive electrode current collector (Al thin film) was coated with the positive electrode slurry, dried, and rolled to prepare a positive electrode.

A negative electrode active material (artificial graphite: natural graphite: SiO 85:10:5, weight ratio), carbon black as a conductive agent, SBR as a binder, and CMC as a thickener were added to NMP at a weight ratio of 95.6:1:2.3:1.1 to prepare a negative electrode slurry (solid matter: 90 wt%). A 10 μm thick copper (Cu) film (as an anode current collector) was coated with the anode slurry, dried, and rolled to prepare an anode.

After preparing an electrode assembly by sequentially stacking the positive electrode, the separator formed of the porous polyethylene film, and the negative electrode prepared above, the electrode assembly was put into a pouch-type battery case, and the nonaqueous electrolyte solution (B-3) for a lithium secondary battery prepared in example 3 was injected thereto, thereby preparing a pouch-type lithium secondary battery.

Example 6

A pouch type lithium secondary battery was prepared in the same manner as in example 5, except that the nonaqueous electrolyte solution for a lithium secondary battery (B-4) of example 4 was used instead of the nonaqueous electrolyte solution for a lithium secondary battery (B-3) of example 3.

Comparative example 3

A pouch type lithium secondary battery was prepared in the same manner as in example 5, except that the electrolyte solution (a-2) of comparative example 2 was used instead of the nonaqueous electrolyte solution (B-3) for a lithium secondary battery of example 3.

Examples of the experiments

Experimental example 1 evaluation of Metal (Co) ion electrodeposition

By mixing 0.1g of cobalt (II) tetrafluoroborate hexahydrate (Co (BF)₄)₂·6H₂O) (metallic foreign matter as an optional ingredient) was added to 99.9g of the nonaqueous electrolyte solutions (B-1 and B-2) for lithium secondary batteries prepared in examples 1 and 2, respectively, to prepare nonaqueous electrolyte solutions for lithium secondary batteries of examples 1-1 and 2-1 for metal ion electrodeposition evaluation (see table 1 below).

In addition, by mixing 0.1g of cobalt (II) tetrafluoroborate hexahydrate (Co (BF)₄)₂·6H₂O) (metallic foreign matter as an optional ingredient) was added to 99.9g of the electrolyte solution (a-1) prepared in comparative example 1, and the nonaqueous electrolyte solution for a lithium secondary battery of comparative example 1-1 was prepared for metal ion electrodeposition evaluation (see table 1 below).

[ Table 1]

Then, the electrochemical stability of the electrolyte solution (a-1) not containing the metallic foreign substance prepared in comparative example 1 and the nonaqueous electrolyte solutions for lithium secondary batteries (containing the metallic foreign substance for metal ion electrodeposition evaluation) of examples 1-1 and 2-1 and comparative example 1-1 were measured using Linear Sweep Voltammetry (LSV) to evaluate the effect of removing the transition metal (Co) ions.

In this case, the working electrode was a platinum (Pt) disk (. PHI.1.6 mm) electrode, the reference electrode was lithium metal, a Pt wire electrode was used as the auxiliary electrode, and measurement was performed at a scan rate of 10mV/s in the Open Circuit Voltage (OCV) range of about 0.2V. The measurement was performed in a glove box (moisture and oxygen concentration at 23 ℃ C. is 10ppm or less) under an argon (Ar) atmosphere, and the results are shown in FIG. 1.

Referring to fig. 1, with respect to the nonaqueous electrolyte solution for a lithium secondary battery of comparative example 1, which does not contain metallic foreign matter, used as a reference ratio, it can be understood that the current between 0.5V and 2.5V does not vary much.

With respect to the non-aqueous electrolyte solution for a lithium secondary battery of comparative example 1-1 (containing only metallic foreign materials, containing no additives), since not only the concentration of free metal (Co) ions in the electrolyte solution increases, but also side reactions occur due to electrodeposition of excessive metal ions on the surface of the Pt disk electrode, it was confirmed that the current rapidly increased between 0.5V and 2.5V.

In contrast, with respect to the nonaqueous electrolyte solutions for lithium secondary batteries of examples 1-1 and 2-1 of the present invention (containing the additive and the metallic foreign substance), even if the metallic foreign substance is contained, the rapid increase in current is suppressed, and in particular, with respect to the nonaqueous electrolyte solution for lithium secondary batteries of example 2-1 (in which the amount of the additive is large), since the side reaction due to the metallic foreign substance is more effectively suppressed, it is understood that the current flowing is lower than that of the nonaqueous electrolyte solution for lithium secondary batteries of example 1-1.

The reason for this is due to the fact that: since the amount of the additive in the nonaqueous electrolyte solution for a lithium secondary battery of example 2 is larger than that in the nonaqueous electrolyte solution for a lithium secondary battery of example 1, a complex with a metal ion is formed better, thereby reducing the concentration of free Co ions in the electrolyte solution.

Experimental example 2 decomposition initiation Voltage measurement

Decomposition starting voltages of the nonaqueous electrolyte solution for lithium secondary battery (B-3) prepared in example 3 and the electrolyte solution (a-2) prepared in comparative example 2 were measured using Linear Sweep Voltammetry (LSV).

In this case, the working electrode was a platinum (Pt) disk (. PHI.1.6 mm) electrode, the reference electrode was lithium metal, a Pt wire electrode was used as the auxiliary electrode, and measurements were made at a scan rate of 20mV/s over an Open Circuit Voltage (OCV) of about 6V. The measurement was performed in a glove box (moisture and oxygen concentration at 23 ℃ C. is 10ppm or less) under an argon (Ar) atmosphere, and the results are shown in FIG. 2.

Referring to fig. 2, with respect to the nonaqueous electrolyte solution (B-3) for a lithium secondary battery of example 3, it can be understood that the oxidation current starts at a lower potential than the electrolyte solution (a-2) of comparative example 2.

From these results, since the additive contained in the nonaqueous electrolytic solution for a lithium secondary battery of the present invention is oxidatively decomposed before the organic solvent during overcharge of the lithium secondary battery to form a film on the surface of the positive electrode, and the film can suppress decomposition of the organic solvent to suppress gas generation due to decomposition of the organic solvent, it can be predicted that the stability of the lithium secondary battery during overcharge can be secured.

EXAMPLE 3 evaluation of SEI formation (1)

After each of the secondary batteries prepared in example 5 and comparative example 3 was initially charged (formed) at a constant current of 0.1C rate for 3 hours using a PNE-0506 charging and discharging device (manufacturer: PNE SOLUTION co., ltd.,5V, 6A), a differential capacitance curve obtained by first-order differentiating the capacity-voltage curve thus obtained is shown in fig. 3.

Referring to fig. 3, with respect to the lithium secondary battery of example 5, which includes the nonaqueous electrolyte solution of the present invention containing an additive, a decomposition peak, in which the electrolyte solution is decomposed at about 1.6V, was confirmed, as compared to the secondary battery of comparative example 3, which includes a nonaqueous electrolyte solution containing no additive. According to this behavior, it can be indirectly confirmed that the additive contained in the nonaqueous electrolyte solution of the present invention additionally forms another type of SEI on the surface of the negative electrode while decomposing earlier than other components.

Experimental example 4 initial Capacity evaluation test

Each of the secondary batteries prepared in examples 5 and 6 and comparative example 3 was charged to 4.2V at a rate of 0.3C under a constant current-constant voltage (CC-CV) condition at room temperature (23 ℃) and discharged to 2.5V at a rate of 0.3C under the CC condition, and each of the secondary batteries was charged to 1C/4.2V under a constant current-constant voltage (CC/CV) condition at room temperature (23 ℃) until the current reached 1/20(mA) of 1C, and then discharged to 2.5V at a current of 1C again to measure the initial capacity. The results are shown in table 2 below.

[ Table 2]

	0.33C Capacity (mAh)
		Example 5	102.5
Example 6	98.2
		Comparative example 3	93.0

As shown in table 2, it can be understood that the initial capacity of the secondary battery of comparative example 3 is reduced as compared with the initial capacities of the secondary batteries of examples 5 and 6.

EXAMPLE 5 evaluation of film formation (2)

After the initial capacity evaluation in experimental example 4 was completed, the Electrochemical Impedance Spectrum (EIS) of each of the secondary batteries of example 5 and comparative example 3 was measured using a potentiostat.

Specifically, after the secondary battery of example 5 and each secondary battery of comparative example 3 were charged to a charged State (SOC) of 50% with a current of 54mA, a small voltage (14mv) was applied in a frequency range of 50mHz to 200kHz to measure the resulting current response, the results of which are shown in fig. 4.

Referring to fig. 4, with respect to the secondary battery of example 5, since the film was formed, it was confirmed that the impedance was increased as compared with the impedance of the secondary battery of comparative example 3. That is, from these results, it was confirmed that a strong film was formed by the additive contained in the nonaqueous electrolyte solution of the present invention.

Experimental example 6 evaluation of high temperature (45 ℃ C.) cycle characteristics

The secondary battery prepared in example 5 and each of the secondary batteries prepared in comparative example 3 were charged to 4.2V at a rate of 1C under a constant current/constant voltage (CC-CV) condition at 45C until the current reached 1/20(mA) at 1C, and then discharged to 2.5V at a current of 1C. The above charge and discharge was set to one cycle, and 200 cycles were repeated. Next, the discharge capacity retention rate was calculated using the following equation 1, and the result thereof is shown in fig. 5.

[ equation 1]

Discharge capacity retention (%) (discharge capacity after nth charge and discharge/discharge capacity after 1 st charge and discharge) × 100

Referring to fig. 5, with respect to the secondary battery of example 5 including the additive of the present invention, it can be understood that the 1C discharge capacity retention rate after the 200 th charge-discharge cycle is greater than that of the secondary battery of comparative example 3. From this, it was confirmed that when coumarin-3-carbonitrile was used as an additive in a nonaqueous electrolyte solution, the high-rate discharge capacity retention rate at high temperatures could be improved.