KR20190142670A - Electrolyte for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same. The electrolyte comprises: a lithium salt having a molar concentration of 1.8-5 M; and an organic solvent including at least one fluorine-substituted cyclic compound and a linear carbonate-based compound selected from the group consisting of a fluorine-substituted cyclic carbonate-based compound and a fluorine-substituted cyclic ether-based compound. The organic solvent contains the linear carbonate-based compound and the fluorine-substituted cyclic compound in a weight ratio of 90-99 : 1-10, and a dipole moment index of the linear carbonate-based compound is 1.5 to 30.

Description

ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME

The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same. More particularly, a lithium secondary battery electrolyte and a lithium secondary battery including the same, which have improved lifetime characteristics and discharge output by applying a high concentration lithium salt. It is about.

As the development and demand of mobile devices, electric vehicles, and the like increase, the demand for secondary batteries as energy sources is increasing rapidly, among them, high energy density and operating potential, long cycle life, and low self discharge rate Lithium secondary batteries have been commercialized and widely used.

In the lithium secondary battery, charging and discharging proceed while repeating a process in which lithium ions are intercalated and deintercalated from a lithium metal oxide of a positive electrode to a graphite electrode of a negative electrode.

In general, the most commonly used positive electrode of a lithium secondary battery is a porous electrode made of a lithium transition metal oxide as an active material, and thus the electrolyte penetrates into the micropores of the electrode to supply lithium ions and at the same time, the lithium ions are intercalated with the active material. It is in charge of sending and receiving. Basic performances such as operating voltage and energy density of lithium secondary batteries are determined by the materials constituting the positive and negative electrodes, but the ion transfer between the two electrodes is important to obtain excellent battery performance. It's been going on.

Recently, in order to improve the lifespan performance and discharge output of a battery by increasing the amount of ions present in the electrolyte than the electrolyte currently used, studies on a high concentration lithium salt electrolyte having a high yield (transference number) have been conducted. However, when the concentration of the lithium salt is increased, the interaction between the ions is increased, thereby increasing the viscosity of the electrolyte to decrease the ionic conductivity to decrease the performance of the battery.

Accordingly, there is a need for development of a lithium secondary battery electrolyte and a lithium secondary battery including the same, which can improve battery life characteristics and discharge output, and which can maintain an ion conductivity of the electrolyte at a predetermined level or higher.

Republic of Korea Patent Publication No. 10-2017-0008479

The present invention has been made to solve the above problems, and provides a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, while having excellent life characteristics and discharge output by using a cyclic lithium salt, which can maintain a certain level of ion conductivity. do.

According to one embodiment, the present invention is a lithium salt having a molar concentration of 1.8M to 5M; And an organic solvent including at least one fluorine-substituted cyclic compound and a linear carbonate compound selected from the group consisting of a fluorine-substituted cyclic carbonate compound and a fluorine-substituted cyclic ether compound. Wherein the linear carbonate compound and the fluorine-substituted cyclic compound are included in a weight ratio of (90 to 99) :( 1 to 10), wherein the linear carbonate compound is a compound having a dipole moment index of 1.5 to 30. Provided is an electrolyte for a lithium secondary battery.

In this case, the linear carbonate compound may be dimethyl carbonate.

The fluorine-substituted cyclic carbonate compound is selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, fluoropropylene carbonate, difluoropropylene carbonate and trifluoropropylene carbonate It may be at least one.

The fluorine-substituted cyclic ether compound is 2,2-bistrifluoromethyl-1,3-dioxalan, 2- (difluoromethyl) -2- (trifluoromethyl) -1,3- Dioxalane, 2- (fluoromethyl) -2- (trifluoromethyl) -1,3-dioxalan, 2,2-bistrifluoromethyl-1,3-dioxane, 2- (difluoro Methyl) -2- (trifluoromethyl) -1,3-dioxane, 2- (fluoromethyl) -2- (trifluoromethyl) -1,3-dioxane, fluorotetrahydrofuran and di It may be at least one selected from the group consisting of fluorotetrahydrofuran.

On the other hand, the lithium secondary battery electrolyte, as an additive, vinylene carbonate, lithium difluoro oxalate phosphate, lithium bis (oxalato) borate, difluorobenzene, propanesultone, polyphenylene sulfide, succinitrile, It may further comprise at least one compound selected from the group consisting of lithium difluoro (oxalato) borate, propenesultone, vinyl ethylene carbonate, ethylene sulfate and adiponitrile.

The additive may be included in an amount of 0.01 parts by weight to 10 parts by weight based on 100 parts by weight of the electrolyte for a lithium secondary battery.

In another embodiment, the present invention, Lix (Ni _a Co _b Mn _c M _d ) O ₂ (M is at least one selected from the group consisting of Al, Zr, Mg, Zn, Y, Fe, W and Ti) An anode comprising a positive electrode active material comprising a lithium composite metal oxide represented by 0.5? A <1.0, 0? B? 0.5, 0? C? 0.5, 0? D? 0.4, and 0.8? X? 1.2; cathode; A separator interposed between the anode and the cathode; And it provides a lithium secondary battery comprising the electrolyte for the lithium secondary battery.

Preferably, the lithium composite metal oxide may be 0.6 ≦ a <1.0, 0 <b ≦ 0.4, 0 ≦ c ≦ 0.4, 0 ≦ d ≦ 0.4, and 0.8 ≦ x ≦ 1.2.

More preferably, the lithium composite metal oxide may be 0.8≤a <1.0, 0 <b≤0.2, 0≤c≤0.2, 0≤d≤0.2, 0.8≤x≤1.2.

On the other hand, the range of x is preferably 0.9≤x≤1.1, more preferably 0.95≤x≤1.05.

The cathode may be at least one metal electrode selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals, and transition metals.

More specifically, the metal used for the negative electrode may be at least one metal selected from the group consisting of lithium, copper, nickel and indium.

The electrolyte for a lithium secondary battery according to the present invention can increase the amount of ions present in the electrolyte by using a high concentration of lithium salts, thereby improving the lifespan characteristics and the discharge output of the battery, and by using an organic solvent containing a specific compound, An electrolyte for a lithium secondary battery having a viscosity of can be provided.

In addition, by using a positive electrode including a positive electrode active material containing a lithium composite metal oxide containing a high content of Ni together with the electrolyte for a lithium secondary battery according to the present invention, the capacity characteristics of the battery can also be improved.

1 is a graph measuring the number of cycles until the discharge capacity of the lithium secondary battery measured according to Experimental Example 1 becomes 80% of the initial discharge capacity.

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

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

As used herein, the terms "comprise", "comprise" or "have" are intended to indicate that there is a feature, number, step, component, or combination thereof, that is, one or more other features, It should be understood that it does not exclude in advance the possibility of the presence or addition of numbers, steps, components, or combinations thereof.

Electrolyte for Lithium Secondary Battery

The electrolyte for a lithium secondary battery according to the present invention is at least selected from the group consisting of a lithium salt and a fluorine-substituted cyclic carbonate compound and a fluorine-substituted cyclic ether compound having a molar concentration of 1.8M to 5M. At least one fluorine-substituted cyclic compound and an organic solvent comprising a linear carbonate-based compound, wherein the organic solvent, the linear carbonate-based compound and the fluorine-substituted cyclic compound is (90 ~ 99): (1 ~ 10) included in weight ratio. In this case, the linear carbonate-based compound may be a compound having a dipole moment index of 1.5 to 30.

In general, in the case of the lithium salt used in the electrolyte for lithium secondary batteries, a lithium salt having a molar concentration of 1 M or less is used. This is because the higher the concentration of the lithium salt, the higher the viscosity of the electrolyte, the lower the ionic conductivity, the less wettability of the separator, the lower the performance of the battery.

However, in order to improve battery performance, there is an increasing demand for an electrolyte containing a cyclic lithium salt, thereby overcoming the above problems and increasing the need for an electrolyte containing a cyclic lithium salt.

Therefore, in the present invention, a lithium secondary battery electrolyte includes a high concentration of lithium salt, but includes an organic solvent containing a linear carbonate-based compound and a fluorine-substituted cyclic compound to control the viscosity, thereby increasing the amount of ions in the electrolyte The ion conductivity can be maintained above a certain level while improving the lifetime performance and discharge output.

More specifically, the molar concentration of the lithium salt may be 1.8M to 5M, preferably 1.8M to 4M, more preferably 1.8M to 3M. When the molar concentration of the lithium salt is within the above range, the amount of ions in the electrolyte may be increased to have a high yield (transference number) while having a level at which the battery can be driven. In the present invention, when the current flows through the electrolyte, the anion moves to the positive electrode and the positive ion moves toward the negative electrode, where both ions mean a sharing ratio when carrying electricity.

On the other hand, the lithium salt may be used without particular limitation as long as it is a lithium salt commonly used in electrolytes for lithium secondary batteries. Specifically, LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , CF ₃ SO ₃ Li, LiC (CF ₃ SO ₂ ) ₃ , LiC ₄ BO _8, may include at least one or more compounds selected from the group consisting of LiTFSI (lithium bis (trifluoro methane sulfonyl ) imide) and LiFSI (lithium bis (fluoro sulfonyl) imide) and LiClO _4. More specifically, the lithium salt may be used alone or in combination of two or more kinds as necessary.

The organic solvent includes at least one fluorine-substituted cyclic compound and a linear carbonate compound selected from the group consisting of a fluorine-substituted cyclic carbonate compound and a fluorine-substituted cyclic ether compound.

As an organic solvent used in an electrolyte for a lithium secondary battery, it is common to use a linear carbonate compound and a cyclic carbonate compound together, but in the case of the electrolyte of the present invention to which a cyclic lithium salt is applied, the viscosity of the electrolyte may be excessively increased. . In this case, the mobility of ions in the liquid electrolyte is inversely proportional to the viscosity of the organic solvent according to Stokes' law. When the cyclic lithium salt is applied to the electrolyte, the viscosity is relatively higher than that of the conventional electrolyte. In the present invention, in order to adjust the viscosity of the electrolyte to a predetermined level or less, it is preferable to use a linear carbonate-based compound as one configuration of the organic solvent.

For example, the linear carbonate compound may be dimethyl carbonate (DMC).

When using an organic solvent containing dimethyl carbonate, when the lithium salt dissociates in the organic solvent, the dielectric constant may increase rapidly.

This is related to the formation of isomers by dissociation of lithium salts. More specifically, when the lithium salt is dissociated, the compound molecules constituting the organic solvent may undergo isomerization into isomers having higher energy, and include dipole moments depending on the form of the isomers. Polarity and dielectric constant of the entire electrolyte may change.

Dimethyl carbonate has a dipole moment of about 0.32D when present as an energy-stable cis-cis form of dimethyl carbonate, but is relatively stable among isomers through isomerization due to dissociation of lithium salts. When present as a dimethyl carbonate, it has a dipole moment of about 3.98D, and the dielectric constant also increases significantly.

However, among the same linear carbonates, diethyl carbonate (DEC) isomerized due to dipole moment (about 0.64D) and lithium salt dissociation when present in the form of energy-stable cis-cis. Dipoles of 0.48D, 0.23D, and 0.46D, respectively, when they exist as isomers of cc (tg ^± ), cc (g ^± g), and cc (g ^± g ^± ) (c: cis, t: trans, g: gauche) Since it has a moment, it has a lower dipole moment than the cis-cis form. Therefore, it is difficult to expect a synergistic effect of the dipole moment through dissociation of lithium salts.

As a result, in order to increase the dielectric constant of the electrolyte, when the lithium salt is dissociated in the organic solvent, the dipole moment is increased, and the linear carbonate compound is preferably dimethyl carbonate as the linear carbonate compound.

Therefore, when the high lithium salt is to be included in the electrolyte, it is preferable to use a compound that can increase the dielectric constant of the electrolyte as the organic solvent as the dipole moment is improved as the lithium salt dissociates as mentioned above.

Therefore, the present invention can use a compound having a dipole moment index of 1.5 to 30 as a linear carbonate compound.

The dipole moment index is defined by the following formula.

In this case, A means the dipole moment value of the compound itself before the lithium salt is dissociated, and A 'means the dipole moment of the isomer formed when the lithium salt is dissociated.

Accordingly, the linear carbonate compound may be a compound having a dipole moment index of 1.5 to 30, preferably 1.5 to 25, more preferably 2 to 25.

On the other hand, the organic solvent contains a cyclic carbonate compound substituted with fluorine. Specifically, the fluorine-substituted cyclic carbonate compound, a group consisting of fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, fluoropropylene carbonate, difluoropropylene carbonate and trifluoropropylene carbonate It may include at least one selected from. However, as mentioned, the cyclic carbonate compound in which fluorine is substituted is also substituted with fluorine in the cyclic carbonate compound, and has a relatively high viscosity as compared with the linear carbonate compound, so that the ion conductivity is lowered when included in the electrolyte. Can be.

Accordingly, the organic solvent is a cyclic compound substituted with the linear carbonate compound and the fluorine (90 to 99): (1 to 10) weight ratio, preferably, (95 to 99): (1 to 5) weight ratio It is preferable to be included as. When the linear carbonate compound and the cyclic carbonate compound in which the fluorine is substituted are included in the above range in the organic solvent, ion conductivity of the electrolyte may be maintained at a predetermined level or more.

On the other hand, the electrolyte for a lithium secondary battery according to the present invention, as an additive, vinylene carbonate, lithium difluoro oxalate phosphate, lithium bis (oxalato) borate, difluorobenzene, propanesultone, polyphenylene sulfide, succinate It may further include at least one compound selected from the group consisting of nitrile, lithium difluoro (oxalato) borate, propenesultone, vinyl ethylene carbonate, ethylene sulfate and adiponitryl. According to the type of the additive used, the SEI (Solid Electrolyte Interphase) layer is rapidly formed on the surface of the cathode and then kept stable, thereby suppressing side reactions with the electrolyte and suppressing the oxidation reaction of the electrolyte generated on the surface of the anode. By controlling the anodic decomposition reaction, the output characteristics of the battery can be effectively improved. In addition, as the additive is used, battery life characteristics may be improved by suppressing corrosion of the metal electrode and / or the metal electrode current collector.

The additive may be included in an amount of 0.01 parts by weight to 10 parts by weight, preferably 0.05 parts by weight to 7 parts by weight, and more preferably 0.1 parts by weight to 5 parts by weight, based on 100 parts by weight of the electrolyte for a lithium secondary battery. . When the additive is included in the above range, it is possible to prevent side reactions due to unreacted additives and to improve output characteristics and lifespan characteristics of the battery by using the additives.

Lithium secondary battery

Next, a lithium secondary battery according to the present invention will be described. Lithium secondary battery according to an embodiment of the present invention, a positive electrode, a negative electrode, a separator and the electrolyte for the lithium secondary battery, the positive electrode, Lix (Ni _a Co _b Mn _c M _d ) O ₂ (M is Al, Zr, Mg, Zn, Y, Fe, W, and at least one element selected from the group consisting of Ti, 0.5≤a <1.0, 0 <b≤0.5, 0≤c≤0.5, 0≤d≤0.5, 0.8 It includes a positive electrode active material containing a lithium composite metal oxide represented by ≤ x ≤ 1.2).

The positive electrode may be prepared by coating a positive electrode active material slurry including a positive electrode active material, a binder, a conductive material, a solvent, and the like on a positive electrode current collector.

The positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, the positive electrode current collector may be formed of stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon on the surface of aluminum or stainless steel. Surface treated with nickel, titanium, silver, or the like may be used.

On the other hand, the lithium composite metal oxide included in the positive electrode active material may be 0.6≤a <1.0, 0 <b≤0.4, 0≤c≤0.4, 0≤d≤0.4, 0.8≤x≤1.2, or , 0.8 ≦ a ≦ 1.0, 0 ≦ b ≦ 0.2, 0 ≦ c ≦ 0.2, 0 ≦ d ≦ 0.2, 0.8 ≦ x ≦ 1.2.

As the lithium composite metal oxide, when the lithium composite metal oxide having an atomic fraction of Ni in the metal oxide containing nickel is used as the positive electrode active material, the battery capacity of the positive electrode can be improved, and the energy density of the battery can be improved.

More specifically, the lithium composite metal oxide is lithium nickel manganese cobalt oxide (for example, Li (Ni) in that the capacity characteristics and stability of the battery can be improved_0.5Mn_0.3Co_0.2) O₂, Li (Ni_0.6Mn_0.2Co_0.2) O₂, Li (Ni_0.7Mn_0.15Co_0.15) O₂, Or Li (Ni_0.8Mn_0.1Co_0.1) O₂ Etc.) or lithium nickel cobalt aluminum oxide (e.g., LiNi_0.8Co_0.15Al_0.05O₂ And the like, and the lithium composite metal oxide is Li (Ni) in view of the remarkable effect of improvement according to the type and content ratio control of the element forming the lithium composite metal oxide._0.6Mn_0.2Co_0.2) O₂, Li (Ni_0.7Mn_0.15Co_0.15) O₂, Or Li (Ni_0.8Mn_0.1Co_0.1) O₂ And any one or a mixture of two or more thereof may be used.

The cathode active material may be included in an amount of 60 wt% to 98 wt%, preferably 70 wt% to 98 wt%, and more preferably 80 wt% to 98 wt%, based on the total weight of the solids excluding the solvent in the cathode active material slurry. have.

The binder is a component that assists the bonding of the active material and the conductive material to the current collector, and is typically 1% to 20% by weight, preferably 1% by weight based on the total weight of the solids excluding the solvent in the cathode active material slurry. % To 15% by weight, more preferably 1% to 10% by weight may be included.

Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, various copolymers, and the like.

The conductive material is a component for further improving the conductivity of the positive electrode active material, 1 wt% to 20 wt%, preferably 1 wt% to 15 wt%, more preferably based on the total weight of the solids excluding the solvent in the positive electrode active material slurry. May be included in an amount of 1 wt% to 10 wt%.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, graphite; Carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. Specific examples of commercially available conductive materials include Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, Ketjenblack and EC, which are acetylene black series. Family (Armak Company), Vulcan XC-72 (manufactured by Cabot Company) and Super P (manufactured by Timcal).

The solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that becomes a desirable viscosity when including the positive electrode active material and optionally a binder and a conductive material.

For example, the negative electrode may be prepared by coating a negative electrode active material slurry including a negative electrode active material, a binder, a conductive material, a solvent, and the like on a negative electrode current collector, and an alkali metal, an alkaline earth metal, a rare earth metal, and a transition metal or these At least one metal electrode selected from the group consisting of an alloy of may be used as the cathode.

In this case, the metal constituting the metal electrode may be at least one metal electrode selected from the group consisting of lithium (Li), copper (Cu), nickel (Ni) and indium (In) or alloys thereof as a cathode. have.

On the other hand, when the lithium electrode is used as the negative electrode, the cathode reduction potential is low while having a high theoretical capacity, which is higher than when using a metal electrode having a different energy density.

On the other hand, when the negative electrode is prepared by coating a negative electrode active material slurry on the negative electrode current collector, the negative electrode current collector generally has a thickness of 3㎛ to 500㎛. Such a negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver, and the like on the surface, aluminum-cadmium alloy and the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

As the negative electrode active material, natural graphite, artificial graphite, carbonaceous material; Metals (Me) that are lithium-containing titanium composite oxide (LTO), Si, Sn, Li, Zn, Mg, Cd, Ce, Ni, or Fe; Alloys composed of the metals (Me); Oxides of the metals (Me) (MeOx); And one or two or more negative electrode active materials selected from the group consisting of a complex of the metals (Me) and carbon.

The negative electrode active material may be included in an amount of 60 wt% to 98 wt%, preferably 70 wt% to 98 wt%, and more preferably 80 wt% to 98 wt%, based on the total weight of the solids excluding the solvent in the negative active material slurry. have.

The binder, the conductive material, and the solvent are the same as those described above, and detailed description thereof is omitted.

On the other hand, among the negative electrodes, as a negative electrode for a lithium secondary battery using the lithium secondary battery electrolyte to which the cyclic lithium salt according to the present invention is applied, unlike a negative electrode of another kind, lithium ions may be deposited more uniformly. As a result, the performance of the battery can be further improved.

The separator is a conventional porous polymer film conventionally used as a separator, for example, polyolefin-based, such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer The porous polymer film made of a polymer may be used alone or by laminating them, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. no.

In addition, in order to improve heat resistance and mechanical strength of the separator, an inorganic material may be further included. In this case, the organic material and the inorganic material may be mixed to form a separator in the form of a single layer, or the separator may be formed in a multilayer form such as coating an inorganic material on an organic material or the like.

The inorganic material may be used without limitation as long as it is a material capable of uniformly controlling the pores of the separator and improving heat resistance. For example, non-limiting examples of the inorganic material include SiO ₂ , Al ₂ O ₃ , TiO ₂ , BaTiO ₃ , Li ₂ O, LiF, LiOH, Li ₃ N, BaO, Na ₂ O, Li ₂ CO ₃ , CaCO ₃ , At least one selected from the group consisting of LiAlO ₂ , SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , SiC, and derivatives thereof and mixtures thereof.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the following examples are only examples to help understanding of the present invention, but do not limit the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the present disclosure, and such variations and modifications are within the scope of the appended claims.

EXAMPLE

Example 1

(1) Preparation of electrolyte for lithium secondary battery

Dimethyl carbonate (DMC): fluoroethylene carbonate (FEC) = 95: 5 LiFSI was dissolved in an organic solvent mixed in a weight ratio of 2.8 M concentration to prepare a lithium secondary battery electrolyte.

(2) lithium secondary battery manufacturing

10 parts by weight of a solvent, N-methyl-2-pyrrolidone (NMP), a lithium composite metal oxide ((Li (Ni _0.8 Co _0.1 Mn _0.1 ) O ₂ ), NCM811), carbon black as a conductive material and polyvinyl as a binder A positive electrode active material slurry was prepared by adding 90 parts by weight of a mixture of lidene fluoride (PVDF) at a ratio of 96: 2: 2 (wt%). The positive electrode active material slurry was applied to a positive electrode current collector (Al thin film) having a thickness of 10 μm, dried, and roll pressed to prepare a positive electrode.

A roll of 20 μm thick lithium metal on a 10 μm thick copper current collector was prepared as a negative electrode.

After preparing the electrode assembly by sequentially stacking the positive electrode and the negative electrode prepared by the above method with a polyethylene porous film as a separator, in the order of anode / separator / cathode, and then placing the electrode assembly in a battery case, the lithium secondary The battery electrolyte was injected and then sealed to manufacture a lithium secondary battery.

2. Example 2

When preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the dimethyl carbonate (DMC): fluoroethylene carbonate (FEC) was mixed in a weight ratio of 90:10. It was.

3. Example 3

In preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFSI was dissolved to a concentration of 3M.

4. Example 4

In preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFSI was dissolved to a concentration of 3.5 M.

5. Example 5

In preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFSI was dissolved to a concentration of 4M.

6. Example 6

In preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1 except that LiPF ₆ was dissolved in a concentration of 2.5 M instead of LiFSI.

7. Example 7

An electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that ((Li (Ni _0.6 Co _0.2 Mn _0.2 ) O ₂ ), NCM622) was used as a lithium composite metal oxide when preparing a lithium secondary battery. Was prepared.

8. Example 8

An electrolyte for a lithium secondary battery and a lithium secondary battery in the same manner as in Example 1, except that ((Li (Ni _0.5 Co _0.3 Mn _0.2 ) O ₂ ), NCM532) was used as a lithium composite metal oxide in manufacturing a lithium secondary battery. Was prepared.

[Comparative Example]

1. Comparative Example 1

In preparing an electrolyte for a lithium secondary battery, LiPF ₆ was dissolved in an organic solvent mixed in an ethyl methyl carbonate (EMC): fluoroethylene carbonate (FEC) = 70:30 weight ratio to prepare a lithium secondary battery electrolyte. Thereafter, a lithium secondary battery was manufactured in the same manner as in Example 1.

2. Comparative Example 2

In preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiFSI was dissolved to a concentration of 1 M.

3. Comparative Example 3

In preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1, except that LiPF ₆ was dissolved to a concentration of 6M.

4. Comparative Example 4

When preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery was prepared in the same manner as in Example 1, except that an organic solvent mixed in a diethyl carbonate (DEC): fluoroethylene carbonate (FEC) = 95: 5 weight ratio was used. A lithium secondary battery was produced.

5. Comparative Example 5

When preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were manufactured in the same manner as in Example 1 except that dimethyl carbonate (DMC) was used alone as an organic solvent.

6. Comparative Example 6

When preparing an electrolyte for a lithium secondary battery, an electrolyte for a lithium secondary battery and a lithium secondary battery were used in the same manner as in Example 1, except that an organic solvent mixed in a dimethyl carbonate (DMC): ethylene carbonate (EC) = 95: 5 weight ratio was used. Was prepared.

The composition of the lithium composite metal oxide, the organic solvent, and the type and concentration of the lithium salt of the positive electrode according to the Examples and Comparative Examples are shown in Table 1 below.

anode Organic solvent Lithium salt Example 1 NCM811 FEC / DMC = 5/95 2.8M LiFSI Example 2 NCM811 FEC / DMC = 10/90 2.8M LiFSI Example 3 NCM811 FEC / DMC = 5/95 3M LiFSI Example 4 NCM811 FEC / DMC = 5/95 3.5M LiFSI Example 5 NCM811 FEC / DMC = 5/95 4M LiFSI Example 6 NCM811 FEC / DMC = 5/95 2.5M LiPF ₆ Example 7 NCM622 FEC / DMC = 5/95 2.8M LiFSI Example 8 NCM523 FEC / DMC = 5/95 2.8M LiFSI Comparative Example 1 NCM811 FEC / EMC = 30/70 1M LiPF ₆ Comparative Example 2 NCM811 FEC / DMC = 5/95 1M LiFSI Comparative Example 3 NCM811 FEC / DMC = 5/95 6M LiFSI Comparative Example 4 NCM811 FEC / DEC = 5/95 2.8M LiFSI Comparative Example 5 NCM811 DMC = 100 2.8M LiFSI Comparative Example 6 NCM811 EC / DMC = 5/95 2.8M LiFSI

Experimental Example

Experimental Example 1 Evaluation of Life Characteristics of Lithium Secondary Battery

Each lithium secondary battery prepared in Examples 1 to 8 and Comparative Examples 1 to 6 was stored for 1 day to perform the wetting process of the electrolyte, and then repeated charging / discharging at 0.1 C rate twice. (formation) was performed. The discharge capacity at this time was set to the initial discharge capacity. Thereafter, charging / discharging was performed one time by charging at 0.2C rate and discharging at 0.5C rate as 1 cylcle. It was. The measurement results are shown in Table 2 below, and the measurement results of Example 1 and Comparative Examples 5 and 6 are shown in FIG. 1.

Cycle number when discharge capacity reaches 80% of initial discharge capacity Example 1 162 Example 2 104 Example 3 160 Example 4 143 Example 5 127 Example 6 117 Example 7 145 Example 8 73 Comparative Example 1 40 Comparative Example 2 7 Comparative Example 3 15 Comparative Example 4 4 Comparative Example 5 2 Comparative Example 6 3

Referring to Table 2, it can be seen that the number of cycles of the embodiments is greater than the number of cycles of the comparative examples. Through this, when using the electrolyte of the embodiments it can be seen that the life characteristics of the lithium secondary battery is further improved. Furthermore, referring to FIG. 1, when only linear carbonate is used as the organic solvent (Comparative Example 5), when compared with the case where no cyclic carbonate substituted with fluorine is used as the organic solvent (Comparative Example 6), as the organic solvent, the linear It can be seen that the life characteristics are significantly inferior to Example 1 using the carbonate and the cyclic carbonate substituted with fluorine.

2. Experimental Example 2: Evaluation of Capacity Characteristics of Lithium Secondary Battery

Each lithium secondary battery prepared in Examples 1 to 8 and Comparative Examples 1 to 6 was stored for 1 day to perform the wetting process of the electrolyte, and then repeated charging / discharging at 0.1 C rate twice. The initial discharge capacity after the (formation) was measured and shown in Table 3 below.

Initial discharge capacity (mAh) Example 1 5.21 Example 2 5.17 Example 3 5.2 Example 4 5.14 Example 5 5.15 Example 6 5.2 Example 7 4.89 Example 8 4.32 Comparative Example 1 5.19 Comparative Example 2 4.98 Comparative Example 3 4.87 Comparative Example 4 4.66 Comparative Example 5 3.82 Comparative Example 6 5.17

Referring to Table 3 above, when Examples 1 to 6 and Comparative Examples 1 to 6 using NC811 as the cathode active material are compared, it can be confirmed that the initial capacity is high or equivalent. In Comparative Example 6, although the initial capacity is somewhat higher than other comparative examples, referring to Experimental Example 1, the number of cycles when the discharge capacity becomes 80% of the discharge capacity is remarkably high. You can see that it is low.

3. Experimental Example 3: Discharge Output Experiment

In Example 1, 6 and Comparative Example 1, each lithium secondary battery was stored for 1 day to perform the wetting process of the electrolyte, and then repeated charging / discharging twice at 0.1 C rate to form (formation). Proceeded. The discharge capacity at this time was set to the initial discharge capacity. After that, the battery is charged at 0.1C rate and discharged at 1C rate, and then charged / discharged at 0.1C rate again to discharge capacity at 1C discharge capacity, charged at 0.1C rate, and discharged at 2C rate, and then again at 0.1C rate. The discharge capacity after charging / discharging with 2C discharge capacity, charged at 0.1C rate, discharged at 3C rate, and then discharged after charging / discharging at 0.1C rate was measured as 3C discharge capacity. At this time, the discharge capacity and the retention ratio of the discharge capacity relative to the initial discharge capacity were measured and shown in Table 4 below.

Initial discharge capacity 1C discharge capacity 2C discharge capacity 3C discharge capacity Volume
(mAh) Retention rate
(%) Volume
(mAh) Retention rate
(%) Volume
(mAh) Retention rate
(%) Volume
(mAh) Retention rate
(%) Example 1 5.83 100 5.34 91.6 5.21 89.4 5.01 85.9 Example 6 5.81 100 5.19 89.3 5.02 86.4 4.86 83.6 Comparative Example 1 5.67 100 4.91 86.6 4.26 75.1 2.81 49.6

Referring to Table 4, it can be seen that the initial discharge capacity and capacity retention rate of the embodiments using the cyclic lithium salt compared to the comparative example is higher.

Claims (10)

Lithium salt in a molar concentration of 1.8 M to 5 M; And
An organic solvent including at least one fluorine-substituted cyclic compound and a linear carbonate compound selected from the group consisting of a fluorine-substituted cyclic carbonate compound and a fluorine-substituted cyclic ether compound,
The organic solvent includes the linear carbonate-based compound and the fluorine-substituted cyclic compound in a weight ratio of (90 to 99) :( 1 to 10),
The linear carbonate-based compound is a lithium secondary battery electrolyte that is a compound having a dipole moment index of 1.5 to 30.
The method of claim 1,
The linear carbonate compound is dimethyl carbonate electrolyte for a lithium secondary battery.
The method of claim 1,
The fluorine-substituted cyclic carbonate compound is selected from the group consisting of fluoroethylene carbonate, difluoroethylene carbonate, trifluoroethylene carbonate, fluoropropylene carbonate, difluoropropylene carbonate and trifluoropropylene carbonate At least one electrolyte for lithium secondary battery.
The method of claim 1,
The fluorine-substituted cyclic ether compound is 2,2-bistrifluoromethyl-1,3-dioxalan, 2- (difluoromethyl) -2- (trifluoromethyl) -1,3- Dioxalane, 2- (fluoromethyl) -2- (trifluoromethyl) -1,3-dioxalan, 2,2-bistrifluoromethyl-1,3-dioxane, 2- (difluoro Methyl) -2- (trifluoromethyl) -1,3-dioxane, 2- (fluoromethyl) -2- (trifluoromethyl) -1,3-dioxane, fluorotetrahydrofuran and di At least one selected from the group consisting of fluorotetrahydrofuran electrolyte for a lithium secondary battery.
The method of claim 1,
As an additive, vinylene carbonate, lithium difluoro oxalate phosphate, lithium bis (oxalato) borate, difluorobenzene, propanesultone, polyphenylene sulfide, succinitrile, lithium difluoro (oxalato) An electrolyte for a lithium secondary battery further comprising at least one compound selected from the group consisting of borate, propenesultone, vinyl ethylene carbonate, ethylene sulfate and adiponitryl.
The method of claim 5,
The additive is a lithium secondary battery electrolyte, which is contained in 0.01 parts by weight to 10 parts by weight based on 100 parts by weight of the electrolyte for lithium secondary batteries.
Lix (Ni _a Co _b Mn _c M _d ) O ₂ (M is at least one element selected from the group consisting of Al, Zr, Mg, Zn, Y, Fe, W and Ti, 0.5 ≦ a <1.0, 0 A positive electrode including a positive electrode active material containing a lithium composite metal oxide represented by <b ≦ 0.5, 0 ≦ c ≦ 0.5, 0 ≦ d ≦ 0.5, and 0.8 ≦ x ≦ 1.2);
cathode;
A separator interposed between the anode and the cathode; And
A lithium secondary battery comprising the electrolyte for lithium secondary batteries according to claim 1.
The method of claim 7, wherein
The lithium composite metal oxide is 0.6 ≤ a <1.0, 0 <b ≤ 0.4, 0 ≤ c ≤ 0.4, 0 ≤ d ≤ 0.4, 0.8 ≤ x ≤ 1.2.
The method of claim 8,
The lithium composite metal oxide is 0.8 ≤ a <1.0, 0 <b ≤ 0.2, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.2, 0.8 ≤ x ≤ 1.2.
The method of claim 7, wherein
The negative electrode is at least one metal electrode selected from the group consisting of alkali metals, alkaline earth metals, rare earth metals and transition metals.

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