Electrolyte for rechargeable lithium battery and rechargeable lithium battery including same

Abstract

An electrolyte for a rechargeable lithium battery, according to embodiments of the present disclosure, comprises: a lithium salt; an organic solvent; and a fluorine-based additive of a specific chemical formula. The rechargeable lithium battery comprises: an electrode assembly which comprises a positive electrode comprising a lithium-transition metal oxide having a lithium molar ratio of 1.1 or more, and a negative electrode facing the positive electrode; and an electrolyte which is impregnated in the electrode assembly and comprises a fluorine-based additive of a specific chemical formula. Through the fluorine-based additive, electrolyte side reactions can be prevented, and the high-voltage charging stability and high-temperature lifespan characteristics of a positive electrode active material can be improved.

Description

Electrolyte for lithium secondary batteries and lithium secondary batteries containing the same

[0001] The present disclosure relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same. More specifically, the present disclosure relates to an electrolyte for a lithium secondary battery comprising an additive and a lithium secondary battery comprising the same.

[0002]

[0003] Secondary batteries, which can be repeatedly charged and discharged, are widely used as power sources for portable electronic communication devices such as camcorders, mobile phones, and laptops, thanks to the advancements in the information and communication and display industries. Furthermore, battery packs containing secondary batteries are being developed and applied as power sources for eco-friendly vehicles such as hybrid vehicles.

[0004] Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, and nickel-hydrogen batteries. Among these, lithium secondary batteries are being actively researched and developed due to their high operating voltage and energy density per unit weight, and their advantages in charging speed and weight reduction.

[0005] A lithium secondary battery may include an electrode assembly including a positive electrode, a negative electrode, and a separator, and an electrolyte impregnating the electrode assembly. The lithium secondary battery may further include an outer packaging material, for example, in the form of a pouch, that accommodates the electrode assembly and the electrolyte.

[0006] The cathode active material of a lithium secondary battery may include a lithium-rich cathode active material in which lithium is present in a transition metal layer of a layered structure. The lithium-rich cathode active material has high capacity characteristics, but requires high voltage operation, which may reduce the lifespan characteristics of the lithium secondary battery.

[0007]

[0008] An object of the present disclosure is to provide an electrolyte for a lithium secondary battery with improved electrochemical stability.

[0009] An object of the present disclosure is to provide a lithium secondary battery having improved capacity characteristics and operating stability.

[0010]

[0011] An electrolyte for a lithium secondary battery according to exemplary embodiments may include a lithium salt, an organic solvent, and a fluorine-based additive. The fluorine-based additive may be represented by the following chemical formula 1.

[0012] [Chemical Formula 1]

[0013]

[0014] In the above chemical formula 1, R1 and R2 can each independently be hydrogen, a hydroxy group, an aliphatic hydrocarbon group having 1 to 10 carbon atoms, or an aromatic hydrocarbon group having 6 to 10 carbon atoms.

[0015] In some embodiments, R1 and R2 in the above formula 1 may each independently be an alkyl group having 1 to 3 carbon atoms.

[0016] In some embodiments, the content of the fluorine-based additive may be 0.1 wt% to 5.0 wt% of the total weight of the electrolyte for the lithium secondary battery.

[0017] In some embodiments, the electrolyte for a lithium secondary battery may further include at least one auxiliary additive selected from the group consisting of a cyclic carbonate compound, a fluorine-substituted cyclic carbonate compound, a sultone compound, and a cyclic sulfonate compound.

[0018] In one embodiment, the content of the auxiliary additive may be 0.01 wt% to 5.0 wt% of the total weight of the electrolyte for the lithium secondary battery.

[0019] In some embodiments, the organic solvent may include a cyclic carbonate compound and a linear carbonate compound.

[0020] In some embodiments, the organic solvent may include at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0021] A lithium secondary battery according to exemplary embodiments may include an electrode assembly including a positive electrode including a lithium metal oxide having a lithium molar ratio of 1.1 or more, and a negative electrode facing the positive electrode; and an electrolyte solution impregnating the electrode assembly, the electrolyte solution including a fluorine-based additive represented by the chemical formula 1.

[0022] In some embodiments, the electrolyte may further include a lithium salt and an organic solvent.

[0023] In some embodiments, the content of the fluorine-based additive may be 0.1 wt% to 5.0 wt% of the total weight of the electrolyte.

[0024] According to exemplary embodiments, the molar ratio of lithium in the lithium metal oxide may exceed 1.1.

[0025] In some embodiments, the lithium metal oxide may further include nickel and manganese.

[0026] In one embodiment, the molar ratio of manganese in the lithium metal oxide may be greater than the molar ratio of nickel.

[0027] In some embodiments, the lithium metal oxide may be represented by the following chemical formula 2.

[0028] [Chemical Formula 2]

[0029] Li a [M x Ni y Mn z ]O b

[0030] In chemical formula 2, M is at least one selected from the group consisting of Co, Mg, V, Ti, Al, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0≤x≤0.9, 0≤y≤0.9, 0.1≤z≤0.9, x+y>0, 1.8≤a+x+y+z≤2.2, 1.05≤a / (x+y+z)≤1.95, and 1.8≤b≤2.2.

[0031] In some embodiments, y≤z in the above chemical formula 2.

[0032]

[0033] The cathode according to embodiments of the present disclosure comprises a cathode active material comprising lithium metal oxide particles containing an excess of lithium. In this case, lithium may be present in a transition metal layer within the layered structure of the lithium metal oxide particles. Accordingly, the capacity characteristics of the cathode active material may be improved.

[0034] According to exemplary embodiments, a lithium secondary battery includes a non-aqueous electrolyte. The non-aqueous organic solvent of the non-aqueous electrolyte includes a fluorinated organic solvent in a predetermined content range. Accordingly, the oxidation resistance of the non-aqueous electrolyte is enhanced, thereby improving the lifespan characteristics of the lithium secondary battery and reducing the rate of increase in resistance due to repeated charge and discharge cycles.

[0035]

[0036] FIG. 1 and FIG. 2 are schematic plan views and cross-sectional views, respectively, showing lithium secondary batteries according to exemplary embodiments.

[0037]

[0038] Embodiments of the present disclosure provide a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte.

[0039] Lithium secondary batteries according to embodiments of the present disclosure can be widely applied in green technology fields such as electric vehicles, battery charging stations, and other battery-powered solar and wind power generation. Furthermore, lithium secondary batteries according to embodiments of the present disclosure can be used in eco-friendly electric vehicles, hybrid vehicles, and the like, which aim to prevent climate change by suppressing air pollution and greenhouse gas emissions.

[0040] Hereinafter, embodiments of the present disclosure will be described in detail. However, these are merely exemplary and the present disclosure is not limited to the specific embodiments described as examples.

[0041] A lithium secondary battery according to exemplary embodiments may include an electrode assembly including a positive electrode and a negative electrode. The electrode assembly may further include a separator interposed between the positive electrode and the negative electrode. The electrode assembly may be contained within a case together with an electrolyte solution including an electrolyte and may be impregnated with the electrolyte solution.

[0042] The positive electrode may include a positive electrode active material layer formed by applying a positive electrode active material to a positive electrode current collector. The positive electrode active material may include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0043] The above positive electrode active material may include a lithium metal oxide. For example, the lithium metal oxide may include nickel (Ni) and may further include at least one of manganese (Mn) and cobalt (Co).

[0044] According to exemplary embodiments, the lithium metal oxide may include a lithium-rich oxide having a molar ratio of lithium exceeding 1. For example, when the total molar number of metals included in the lithium metal oxide is 2, the molar number of lithium included in the lithium metal oxide particle may exceed 1.

[0045] For example, lithium may be present at some of the transition metal sites of the lithium metal oxide. Accordingly, the capacity characteristics of the positive electrode active material may be improved to be similar to the theoretical capacity (250 mAh / g) of the layered structure.

[0046] In some embodiments, the molar ratio of lithium in the lithium metal oxide may be 1.05 or more, 1.1 or more, or greater than 1.1. In one embodiment, the molar ratio of lithium in the lithium metal oxide may be 1.15 or more, 1.2 or more, and 1.95 or less, 1.8 or less, or 1.6 or less. Within the above range, lithium may be sufficiently present in the transition metal layer in addition to the lithium layer in the layered structure of the lithium metal oxide particle.

[0047] In some embodiments, the lithium metal oxide may comprise a lithium-nickel-manganese oxide comprising nickel and manganese.

[0048] In one embodiment, the molar ratio of manganese in the lithium metal oxide may be greater than the molar ratio of nickel. For example, the molar fraction of manganese in the lithium metal oxide may be 0.5 or greater, for example, 0.5 to 0.75. By including a high content of manganese, high conductivity and capacity can be secured.

[0049] In some embodiments, the lithium metal oxide may include a chemical structure or crystal structure represented by the following chemical formula 2. For example, the lithium metal oxide may include a layered structure represented by the following chemical formula 2.

[0050] [Chemical Formula 2]

[0051] Li a [M x Ni y Mn z ]O b

[0052] Chemical formula 2 may represent the average composition of the lithium metal oxide. In chemical formula 2, M may be at least one of Co, Mg, V, Ti, Al, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi.

[0053] In chemical formula 2, 0≤x≤0.9, 0≤y≤0.9, 0.1≤z≤0.9, x+y>0, 1.8≤a+x+y+z≤2.2, 1.05≤a / (x+y+z)≤1.95, and 1.8≤b≤2.2 may be satisfied.

[0054] In some embodiments, y≤z in formula 2, or y <z일 수 있다. 일 실시예에 있어서, 0.5≤z≤0.9, 또는 0.5≤z≤0.75일 수 있다.

[0055] In one embodiment, 1.05≤a / (x+y+z)≤1.8 1.1≤a / (x+y+z)≤1.95, 1.1

[0056] The chemical structure represented by Chemical Formula 2 represents the bonding relationship contained within the layered structure or crystal structure of lithium metal oxide particles and does not exclude other additional elements. For example, Ni, Mn, and M of Chemical Formula 2 can serve as the main active elements of the positive electrode active material. Chemical Formula 2 is provided to express the bonding relationship of the main active elements and should be understood as a formula encompassing the introduction and substitution of additional elements.

[0057] ​In one embodiment, in addition to the main active element, auxiliary elements may be further included to enhance the chemical stability of the lithium metal oxide particles or the layered structure / crystal structure. The auxiliary elements may be incorporated into the layered structure / crystal structure to form bonds, and in this case, it should be understood that they are also included within the chemical structure range represented by Chemical Formula 2.

[0058] The auxiliary element may include at least one selected from the group consisting of, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra and P.

[0059] In one embodiment, the lithium-excess oxide may be a solid solution phase in which a Li2MnO3 structure and LiMO2 are mixed. Accordingly, lithium ions within the lithium metal oxide particles may also exist in the transition metal layer of the Li2MnO3 phase, thereby improving the capacity characteristics of the positive electrode active material.

[0060] As described above, by employing a lithium-rich oxide with a relatively high lithium content as the cathode active material, some of the transition metal sites can be replaced by lithium. The lithium-rich oxide can induce rearrangement of its crystal structure when charged in a relatively high voltage range (e.g., 4.4 V or higher or 4.5 V or higher). Therefore, charging is possible in the high voltage range and can provide high reversible capacity.

[0061] However, lithium-excess oxide can relatively reduce electrode density due to high lithium content, thereby reducing capacity efficiency per electrode volume. Furthermore, lithium impurities (e.g., LiOH, Li2CO3) can more easily form on the surface of the positive electrode active material. These lithium impurities can cause side reactions with the electrolyte, resulting in gas generation and battery expansion during repeated charge / discharge cycles.

[0062] In addition, when exposed to harsh environments of high or low temperatures, the crystal structure of the transition metal may become unstable, resulting in a decrease in discharge capacity and a voltage drop.

[0063] According to exemplary embodiments of the present disclosure, the above-described lithium-excess oxide and electrolyte additive can be combined to reduce or suppress the above-described electrical and chemical instability.

[0064] An electrolyte for a lithium secondary battery according to exemplary embodiments may include a lithium salt, an organic solvent, and a fluorine-based additive.

[0065] According to exemplary embodiments, the fluorine-based additive may include a compound represented by the following chemical formula 1.

[0066] [Chemical Formula 1]

[0067]

[0068] In the above chemical formula 1, R1 and R2 can each independently be hydrogen, a hydroxy group, an aliphatic hydrocarbon group having 1 to 10 carbon atoms, or an aromatic hydrocarbon group having 6 to 10 carbon atoms.

[0069] The term "aliphatic hydrocarbon group" as used in this application is used to encompass an organic group that does not contain an aromatic ring throughout the compound. For example, the aliphatic hydrocarbon group may include a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, or a substituted or unsubstituted alkynyl group.

[0070] The term "aromatic hydrocarbon group" as used in this application is used to encompass compounds having aromaticity throughout the compound or compounds in which a portion of the molecular structure of the compound has aromaticity, such as a benzene ring. For example, the aromatic hydrocarbon group may include a substituted or unsubstituted aryl group, a substituted or unsubstituted arylalkyl group, or a substituted or unsubstituted alkylaryl group.

[0071] The term "substituted" as used in this application may mean that at least one of the hydrogen atoms of the compound is substituted with a substituent such as a halogen group, a hydroxy group, a heteroalkyl group, a heterocycloalkyl group, a heteroaryl group, an amine group, a nitrile group, a nitro group, or a silyl group.

[0072] The above fluorine-based additive can form a coordination bond by contacting with the positive electrode active material. For example, since the difluorophosphine group (-PF2) is exposed at the terminal of the fluorine-based additive, the difluorophosphine group can freely contact the positive electrode active material and form a coordination bond.

[0073] Therefore, a CEI (Cathode Electrolyte Interphase) layer is formed between the interface of the positive active material and the electrolyte, thereby protecting the surface of the positive active material or the positive electrode surface. In addition, the crystal structure / chemical structure instability on the surface of the lithium-excess oxide described above can be reduced or suppressed through the interaction between the difluorophosphine group of the fluorine-based additive and the transition metal.

[0074] In some embodiments, the oxidation resistance of the electrolyte may be improved due to the fluorine ions derived from the fluorine-based additive. Accordingly, the lifespan characteristics of a lithium secondary battery may be improved, and the rate of increase in resistance due to repeated charge and discharge cycles may be reduced.

[0075] In addition, the fluorine-based additive contains an amine group (-NR1R2) bonded to a difluorophosphine group, and thus can have a relatively high boiling point and chemical stability. Accordingly, the fluorine-based additive can stably form a protective film at high temperatures, and the high-temperature and high-voltage operation stability of the secondary battery can be improved.

[0076] In some embodiments, R1 and R2 in the above chemical formula 1 may be an aliphatic hydrocarbon group having 1 to 10 carbon atoms, or an alkyl group having 1 to 3 carbon atoms. In this case, a film having high thermal stability may be formed on the electrode surface due to the alkyl group structure. Accordingly, the ionic conductivity and electrode protection properties of the non-aqueous electrolyte may be improved.

[0077] Because fluorine-based additives contain short-chain alkyl groups, they can ensure heat resistance and flame retardancy while suppressing increases in resistance. Consequently, the passivation, surface stabilization, and ionic conductivity of the cathode active material are amplified, improving the high-temperature stability and lifespan stability of lithium secondary batteries and further enhancing capacity characteristics.

[0078] As described above, during repeated charge / discharge cycles, the protective film formed by the fluorine-based additive can suppress gas generation due to side reactions caused by the elution of transition metal ions from the positive electrode active material. Therefore, even when using lithium-excess oxide, the crystal structure instability of the transition metal can be reduced or suppressed, thereby preventing a decrease in discharge capacity and voltage drop. Consequently, stable capacity retention of the secondary battery can be ensured even under high-temperature and high-voltage conditions.

[0079] In some embodiments, the content of the fluorine-based additive in the total weight of the electrolyte may be less than about 10 wt% and may be 0.1 wt% or more.

[0080] For example, in consideration of implementing sufficient passivation and stabilization effects, the content of the fluorine-based additive may be adjusted to 0.1 wt% or more, 0.2 wt% or more, 0.3 wt% or more, 0.4 wt% or more, 0.5 wt% or more, or 1 wt% or more. In addition, in consideration of lithium ion movement and active material activity in the electrolyte, the content of the fluorine-based additive may be adjusted to 9 wt% or less, 7 wt% or less, 6 wt% or less, 5 wt% or less, 4.5 wt% or less, 4 wt% or less, 3.5 wt% or less, 3 wt% or less, or 2 wt% or less.

[0081] In one embodiment, the content of the fluorine-based additive may be 0.1 wt% to 5 wt%, more preferably 0.5 wt% to 2 wt%. Within the above range, the passivation and surface stabilization effects of the positive electrode active material described above may be sufficiently achieved, while not excessively inhibiting the movement of lithium ions and the activity of the positive electrode active material.

[0082] In some embodiments, the electrolyte may further include an auxiliary additive. For example, the auxiliary additive may include a cyclic carbonate compound containing a double bond, a fluorine-substituted cyclic carbonate compound, a sultone compound, a cyclic sulfonate compound, and the like.

[0083] The cyclic carbonate compound containing the above double bond may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc.

[0084] The above fluorine-substituted cyclic carbonate compound may include fluoroethylene carbonate (FEC).

[0085] The above sultone compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0086] The above cyclic sulfonate compound may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0087] In one embodiment, a fluorine-substituted cyclic carbonate compound and a cyclic carbonate compound may be used together as the auxiliary additive. In this case, the stability and durability of the protective film may be further enhanced. In one embodiment, a sultone compound may be used together with a fluorine-substituted cyclic carbonate compound and a cyclic carbonate compound.

[0088] The above-described auxiliary additive can further enhance the durability and stability of the protective film formed by the fluorine-based additive. The above-described auxiliary additive may be included in an appropriate amount within a range that does not impede the passivation effect of the above-described fluorine-based additive and the movement of lithium ions within the electrolyte.

[0089] In some embodiments, the content of the auxiliary additive may be 0.01 wt% to 5 wt%, preferably 0.05 wt% to 5 wt%, and more preferably 0.1 wt% to 4 wt% of the total weight of the non-aqueous electrolyte. Within the above range, the durability of the protective film can be improved without inhibiting the film formation of the fluorine-based additive.

[0090] The above lithium salt is, for example, Li + X - is expressed as and the anion of the lithium salt (X - ) as a non-limiting example of F - , Cl - , Br -, I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - These can be used alone or in combination of two or more lithium salts.

[0091] In one embodiment, the lithium salt may comprise LiBF4 and / or LiPF6.

[0092] In one embodiment, the lithium salt may be included in the organic solvent at a concentration of 0.01 M to 5 M, preferably 0.01 M to 2 M, and more preferably 0.1 M to 1.5 M. Within this range, the transfer of lithium ions and / or electrons during charging and discharging of a lithium secondary battery may be promoted, thereby securing improved capacity.

[0093] The organic solvent may include an organic compound that provides sufficient solubility for the lithium salt and the fluorine-based additive and is not reactive with the lithium secondary battery. In some embodiments, a non-aqueous organic solvent is used, and the electrolyte may be provided as a non-aqueous electrolyte.

[0094] According to exemplary embodiments, the organic solvent may include a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, an aprotic solvent, etc. These may be used alone or in combination of two or more.

[0095] Examples of the above carbonate solvent include linear carbonate compounds such as dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), and dipropyl carbonate, or cyclic carbonate compounds such as propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), and butylene carbonate.

[0096] Examples of the above ester solvents include methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), gamma-butyrolactone (GBL), decanolide, valerolactone, mevalonolactone, caprolactone, etc.

[0097] Examples of the above ether organic solvents include dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxy ethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc.

[0098] An example of the above ketone solvent is cyclohexanone. Examples of the above alcohol solvent are ethyl alcohol, isopropyl alcohol, etc.

[0099] The above aprotic solvent may include a nitrile solvent, an amide solvent such as dimethyl formamide (DMF), a dioxolane solvent such as 1,3-dioxolane, a sulfolane solvent, etc.

[0100] In some embodiments, a carbonate-based solvent may be used as the organic solvent. For example, the organic solvent may include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or a combination thereof.

[0101] In one embodiment, a combination of at least two of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) may be used as the organic solvent. For example, the organic solvent may include a combination of a cyclic carbonate compound and a linear carbonate compound.

[0102] Among the above electrolytes, the organic solvent may be used as a residual amount or as a surplus excluding the above-described components.

[0103] Hereinafter, a lithium secondary battery including the positive electrode and electrolyte described above will be described with reference to FIGS. 1 and 2.

[0104] Referring to FIGS. 1 and 2, a lithium secondary battery may include a positive electrode (100) including a positive electrode active material including the lithium metal oxide particles described above, and a negative electrode (130) facing the positive electrode (100).

[0105] The positive electrode (100) may include a positive electrode active material layer (110) formed by applying a positive electrode active material including the lithium metal oxide particles described above to a positive electrode current collector (105).

[0106] For example, a positive electrode slurry can be prepared by mixing and stirring a positive electrode active material with a binder, a conductive agent, and / or a dispersing agent in a solvent. The positive electrode slurry can be coated on at least one surface of a positive electrode current collector (105), and then dried and rolled to prepare a positive electrode (100).

[0107] The positive electrode current collector (105) may include stainless steel, nickel, aluminum, titanium, or an alloy thereof. The positive electrode current collector (105) may also include aluminum or stainless steel surface-treated with one or more of carbon, nickel, titanium, and silver.

[0108] The binder may include, for example, an organic binder such as vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, or an aqueous binder such as styrene-butadiene rubber (SBR), and may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0109] For example, a PVDF-based binder may be used as a binder for forming a positive electrode. In this case, the amount of binder for forming a positive electrode active material layer (110) may be reduced, and the amount of positive electrode active material may be relatively increased. Accordingly, the output and capacity of the secondary battery may be improved.

[0110] The conductive material may be included to promote electron transfer between active material particles. For example, the conductive material may include a carbon-based conductive material such as graphite, carbon black, graphene, carbon nanotubes, and / or a metal-based conductive material including a perovskite material such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3.

[0111] The negative electrode (130) may include a negative electrode current collector (125) and a negative electrode active material layer (120) formed by coating a negative electrode active material on at least one surface of the negative electrode current collector (125).

[0112] The above-mentioned negative active material can be any material known in the art capable of absorbing and desorbing lithium ions without particular limitation. For example, carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, and carbon fibers, lithium alloys, silicon, or tin can be used.

[0113] The amorphous carbon may include, for example, hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500°C or lower, mesophase pitch-based carbon fibers (MPCF), etc. The crystalline carbon may include, for example, graphitized carbon such as natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc. The elements included in the lithium alloy may include, for example, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0114] The negative electrode current collector (125) may include, for example, gold, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, and preferably may include copper or a copper alloy.

[0115] In some embodiments, the negative electrode active material may be mixed and stirred with a binder, a conductive agent, and / or a dispersing agent in a solvent to produce a slurry. The slurry may be coated on at least one surface of the negative electrode current collector, followed by drying and rolling to produce the negative electrode (130).

[0116] Materials substantially identical to or similar to the above-described binder and conductive agent may be used. In some embodiments, the binder for forming the negative electrode may include, for example, an aqueous binder such as styrene-butadiene rubber (SBR) for compatibility with the carbon-based active material, and may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0117] In some embodiments, a separator (140) may be interposed between the positive electrode (100) and the negative electrode (130). The separator (140) may include a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, or the like. The separator (140) may also include a nonwoven fabric formed of high-melting-point glass fibers, polyethylene terephthalate fibers, or the like.

[0118] According to exemplary embodiments, an electrode cell is defined by an anode (100), a cathode (130), and a separator (140), and a plurality of the electrode cells may be repeatedly arranged to form an electrode assembly (150). In some embodiments, the electrode assembly (150) may be of a winding type, a stacking type, a z-folding type, or a stack-folding type.

[0119] A lithium secondary battery may be defined by housing an electrode assembly (150) together with the electrolyte described above within a case (160). For example, the electrolyte described above may impregnate the electrode assembly (150).

[0120] As illustrated in FIG. 1, electrode tabs (positive electrode tabs and negative electrode tabs) may protrude from the positive electrode collector (105) and negative electrode collector (125) belonging to each electrode cell and extend to one side of the case (160). The electrode tabs may be fused together with the one side of the case (160) to form electrode leads (positive electrode leads (107) and negative electrode leads (127)) that extend or are exposed to the outside of the case (160).

[0121] The above lithium secondary battery can be manufactured in a cylindrical, square, pouch or coin shape using, for example, a can.

[0122] Hereinafter, preferred embodiments are presented to help understand the present disclosure, but these embodiments are only illustrative of the present disclosure and do not limit the scope of the appended claims. It will be apparent to those skilled in the art that various changes and modifications to the embodiments are possible within the scope and technical idea of ​​the present disclosure, and it is natural that such changes and modifications fall within the scope of the appended claims.

[0123]

[0124] Examples and Comparative Examples

[0125] Example 1

[0126] (1) Preparation of electrolyte

[0127] A 1.05 M LiPF6 solution (EC / EMC mixed solvent with a volume ratio of 25:75) was prepared. 2 wt% of fluoroethylene carbonate (FEC), 1 wt% of LiPO2F2(W3), 1.0 wt% of 1,3-propane sultone (PS), and 0.3 wt% of 1,3-propene sultone (PRS) were added to the LiPF6 solution based on the total weight of the electrolyte, and 1 wt% of a compound of the following chemical formula 1-1 was mixed as a fluorine-based additive.

[0128] [Chemical Formula 1-1]

[0129]

[0130] (2) Manufacturing of lithium secondary battery samples

[0131] Li as a cathode active material 1.15 Ni 0.30 Mn 0.55 O2, carbon black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 92:5:3 to prepare a slurry. The slurry was uniformly applied to a 15 μm thick aluminum foil, vacuum-dried at 130°C, and rolled to prepare a positive electrode for a lithium secondary battery.

[0132] An anode slurry was prepared, comprising 95 wt% of a negative active material mixed with artificial graphite and natural graphite in a weight ratio of 7:3, 1 wt% of Super-P as a conductive agent, 2 wt% of styrene-butadiene rubber (SBR) as a binder, and 2 wt% of carboxymethyl cellulose (CMC) as a thickener. The anode slurry was uniformly coated on a 15 μm thick copper foil, dried, and pressed to prepare an anode.

[0133] The positive and negative electrodes manufactured as described above were each cut to a predetermined size and laminated, and a separator (polyethylene, 20 μm thick) was interposed between the positive and negative electrodes to form an electrode assembly, and then the tab portions of the positive and negative electrodes were each welded.

[0134] The above electrode assembly was placed in a pouch and sealed on three sides except for the electrolyte injection side. At this time, the part with the electrode tab was included in the sealing part. The electrolyte prepared in (1) was injected through the remaining sides except for the sealing part, and the remaining sides were sealed, and then impregnated for more than 12 hours to prepare a lithium secondary battery sample.

[0135] Example 2

[0136] A secondary battery sample was manufactured in the same manner as Example 1, except that the compound of the following chemical formula 1-2 was used as a fluorine-based additive.

[0137] [Chemical Formula 1-2]

[0138]

[0139] Example 3

[0140] Li[Ni as a cathode active material 0.6 Co 0.2 Mn 0.2 ]O2 and Li[Ni 0.8 Co 0.1 Mn 0.1 ]A secondary battery sample was prepared in the same manner as in Example 1, except that a blend containing O2 was used in a weight ratio of 6:4.

[0141] Comparative Example 1

[0142] A secondary battery sample was prepared in the same manner as in Example 1, except that the fluorine-based additive was omitted when preparing the electrolyte.

[0143] Comparative Example 2

[0144] A secondary battery sample was manufactured in the same manner as Example 1, except that the compound of the following chemical formula 4 was used instead of the compound of the above chemical formula 1-1 as an additive.

[0145] [Chemical Formula 4]

[0146]

[0147] Comparative Example 3

[0148] A secondary battery sample was manufactured in the same manner as Example 1, except that the compound of the following chemical formula 5 was used instead of the compound of the above chemical formula 1-1 as an additive.

[0149] [Chemical Formula 5]

[0150]

[0151] Comparative Example 4

[0152] Li[Ni as a cathode active material 0.6 Co 0.2 Mn 0.2 ]O2 and Li[Ni 0.8 Co 0.1 Mn 0.1 ]A secondary battery sample was manufactured in the same manner as in Comparative Example 2, except that a blend containing O2 was used in a weight ratio of 6:4.

[0153]

[0154] Experimental example

[0155] (1) Capacity maintenance rate evaluation

[0156] The lithium secondary batteries manufactured according to the above-described examples and comparative examples were subjected to 50 cycles of charging (CC / CV 1C 4.6V 0.05C CUT-OFF) and discharging (CC 1C 2.0V CUT-OFF) at 45°C.

[0157] The capacity retention rate was calculated as a percentage by dividing the discharge capacity measured at the 100th discharge by the discharge capacity measured at the 1st discharge.

[0158] [Formula 1]

[0159] Capacity retention rate (%) = (100th discharge capacity / 1st discharge capacity) × 100

[0160]

[0161] (2) Evaluation of resistance increase rate

[0162] For lithium secondary batteries manufactured according to the above-described examples and comparative examples, the surface resistance of the positive electrode was measured as follows to evaluate the resistance increase rate.

[0163] Specifically, two lithium secondary batteries manufactured according to the above-described Examples and Comparative Examples were prepared for each of the Examples and Comparative Examples (hereinafter, the two lithium secondary batteries are referred to as the first battery and the second battery, respectively). After placing the first battery in a 45°C chamber, it was charged at a C-rate of 1 C in the voltage range of 2.0 V to 4.3 V and discharged at a C-rate of 1 C. The charging and discharging were repeated 100 times.

[0164] After the above charging and discharging were completed, the electrochemical impedance of the first battery was measured using a potentiostat (Biologic, VMP-3), and the surface resistance (Rct1) of the positive electrode was measured. The second battery was placed in a 45°C chamber, and then charged at a C-rate of 1 C in the voltage range of 2.0 V to 4.6 V, and discharged at a C-rate of 1 C. The charging and discharging were repeated 100 times.

[0165] After the above charging and discharging were completed, the electrochemical impedance of the second battery was measured with the potential changer to confirm the surface resistance (Rct2) of the positive electrode. According to Equation 1 below, the ratio of the positive electrode surface resistance (Rct1) of the voltage section cycle from 2.0 V to 4.3 V to the positive electrode surface resistance (Rct2) of the voltage section cycle from 2.0 V to 4.6 V was calculated as a percentage and evaluated as the resistance increase rate (%).

[0166] [Formula 2]

[0167] Resistance increase rate (%) = (Rct2 / Rct1) × 100

[0168] The evaluation results are shown in Table 1 below.

[0169]

[0170] Capacity retention rate (45 ℃) (%, 100cyc) Resistance increase rate (45 ℃) (%) Example 175106 Example 272109 Example 368112 Comparative example 146135 Comparative example 260129 Comparative example 355132 Comparative example 440142

[0171]

[0172] Referring to Table 1, in the case of secondary batteries of examples including an electrolyte containing a fluorine-based additive of Chemical Formula 1, the capacity retention rate and resistance increase rate were improved compared to the secondary batteries of comparative examples.

[0173] Additionally, in the case of the secondary batteries of the examples, the capacity characteristics and internal resistance were improved even under conditions of lithium-excessive cathode material and high-voltage charging.

[0174] In the comparative examples, the capacity retention rate in a high-voltage environment decreased and the resistance increased rapidly.

[0175] The secondary batteries of Examples 1 and 2 contained lithium-rich cathode active materials and exhibited relatively high high-voltage charge stability. The secondary battery of Example 1, in which the fluorine-based additive contained a relatively short-chain alkyl group, exhibited high capacity retention and low resistance increase.

Claims

1. Lithium salt; organic solvents; and An electrolyte for a lithium secondary battery, comprising a fluorine-based additive represented by the following chemical formula 1: [Chemical Formula 1] (In the above chemical formula 1, R1 and R2 are each independently hydrogen, a hydroxy group, an aliphatic hydrocarbon group having 1 to 10 carbon atoms, or an aromatic hydrocarbon group having 6 to 10 carbon atoms).

2. An electrolyte for a lithium secondary battery, wherein in claim 1, R1 and R2 in the chemical formula 1 are each independently an alkyl group having 1 to 3 carbon atoms.

3. An electrolyte for a lithium secondary battery, wherein the content of the fluorine-based additive in claim 1 is 0.1 wt% to 5.0 wt% of the total weight of the electrolyte for a lithium secondary battery.

4. An electrolyte for a lithium secondary battery, further comprising at least one auxiliary additive selected from the group consisting of a cyclic carbonate compound, a fluorine-substituted cyclic carbonate compound, a sultone compound, and a cyclic sulfonate compound, according to claim 1.

5. In claim 4, the content of the auxiliary additive is 0.01 wt% to 5.0 wt% of the total weight of the electrolyte for a lithium secondary battery.

6. An electrolyte for a lithium secondary battery according to claim 1, wherein the organic solvent comprises a cyclic carbonate compound and a linear carbonate compound.

7. An electrolyte for a lithium secondary battery according to claim 1, wherein the organic solvent comprises at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

8. An electrode assembly comprising a positive electrode comprising a lithium metal oxide having a lithium molar ratio of 1.1 or more, and a negative electrode facing the positive electrode; and A lithium secondary battery comprising an electrolyte solution impregnating the electrode assembly and including a fluorine-based additive represented by the following chemical formula 1: [Chemical Formula 1] (In the above chemical formula 1, R1 and R2 are each independently hydrogen, an aliphatic hydrocarbon group having 1 to 10 carbon atoms, or an aromatic hydrocarbon group having 6 to 10 carbon atoms).

9. A lithium secondary battery according to claim 8, wherein the electrolyte further comprises a lithium salt and an organic solvent.

10. A lithium secondary battery according to claim 9, wherein the content of the fluorine-based additive is 0.1 wt% to 5.0 wt% of the total weight of the electrolyte.

11. A lithium secondary battery according to claim 8, wherein the molar ratio of lithium in the lithium metal oxide exceeds 1.

1.

12. A lithium secondary battery according to claim 8, wherein the lithium metal oxide further contains nickel and manganese.

13. A lithium secondary battery according to claim 12, wherein the molar ratio of manganese among the lithium metal oxides is greater than the molar ratio of nickel.

14. In claim 8, the lithium metal oxide is represented by the chemical formula 2, a lithium secondary battery: [Chemical Formula 2] Li a [M x Ni y Mr z ]O b (In Chemical Formula 2, M is at least one selected from the group consisting of Co, Mg, V, Ti, Al, Ru, Zr, W, Sn, Nb, Mo, Cu, Zn, Cr, Ga, V, and Bi, and 0≤x≤0.9, 0≤y≤0.9, 0.1≤z≤0.9, x+y>0, 1.8≤a+x+y+z≤2.2, 1.05≤a / (x+y+z)≤1.95, and 1.8≤b≤2.2).

15. A lithium secondary battery according to claim 14, wherein y≤z in chemical formula 2.

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