US20180191021A1 - Lithium secondary battery including non-aqueous electrolyte solution - Google Patents

Lithium secondary battery including non-aqueous electrolyte solution Download PDF

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US20180191021A1
US20180191021A1 US15/740,065 US201615740065A US2018191021A1 US 20180191021 A1 US20180191021 A1 US 20180191021A1 US 201615740065 A US201615740065 A US 201615740065A US 2018191021 A1 US2018191021 A1 US 2018191021A1
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secondary battery
lithium
lithium secondary
electrolyte solution
carbonate
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Ha Eun Kim
Young Min Lim
Chul Haeng Lee
Min Jung Kim
Gwang Yeon Kim
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LG Energy Solution Ltd
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LG Chem Ltd
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Publication of US20180191021A1 publication Critical patent/US20180191021A1/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0567—Liquid materials characterised by the additives
    • H01M2/14—
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
    • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M50/00—Constructional details or processes of manufacture of the non-active parts of electrochemical cells other than fuel cells, e.g. hybrid cells
    • H01M50/40—Separators; Membranes; Diaphragms; Spacing elements inside cells
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0568—Liquid materials characterised by the solutes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/056—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes
    • H01M10/0564—Accumulators with non-aqueous electrolyte characterised by the materials used as electrolytes, e.g. mixed inorganic/organic electrolytes the electrolyte being constituted of organic materials only
    • H01M10/0566—Liquid materials
    • H01M10/0569—Liquid materials characterised by the solvents
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/027—Negative electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0025—Organic electrolyte
    • H01M2300/0028—Organic electrolyte characterised by the solvent
    • H01M2300/0034—Fluorinated solvents
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M2300/00—Electrolytes
    • H01M2300/0017—Non-aqueous electrolytes
    • H01M2300/0065—Solid electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
    • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a lithium secondary battery which includes a non-aqueous electrolyte solution including lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material, a negative electrode, and a separator.
  • a non-aqueous electrolyte solution including lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound
  • LiFSI lithium bis(fluorosulfonyl)imide
  • a fluorobiphenyl compound fluorobiphenyl compound
  • a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material
  • a negative electrode and a separator.
  • lithium secondary batteries having high energy density and high voltage have been commercialized and widely used.
  • a lithium metal oxide is used as a positive electrode active material of a lithium secondary battery, and a lithium metal, a lithium alloy, crystalline or amorphous carbon, or a carbon composite is used as a negative electrode active material.
  • a current collector may be coated with the active material of appropriate thickness and length or the active material itself may be coated in the form of a film, and the resultant product is then wound or stacked with an insulating separator to prepare an electrode assembly. Thereafter, the electrode assembly is put into a can or a container similar thereto, and a secondary battery is then prepared by injecting an electrolyte solution.
  • Solid electrolyte interface SEI
  • the SEI formed at an initial stage of charging may prevent a reaction of the lithium ions with the carbon negative electrode or other materials during charge and discharge.
  • the SEI may only pass the lithium ions by acting as an ion tunnel. The ion tunnel may prevent the collapse of a structure of the carbon negative electrode due to the co-intercalation of the carbon negative electrode and organic solvents of an electrolyte solution having a high molecular weight which solvates lithium ions and moves therewith.
  • the SEI may prevent the reaction of the lithium ions with the negative electrode or other materials during repeated charge and discharge cycles caused by the subsequent use of the battery, and the SEI may act as an ion tunnel that only passes the lithium ions between the electrolyte solution and the negative electrode.
  • an electrolyte solution which does not include an electrolyte solution additive or includes an electrolyte solution additive having poor characteristics, it may he difficult to expect the improvement of low-temperature output characteristics due to the formation of a non-uniform SEI. Furthermore, even in a case in which the electrolyte solution additive is included, if the amount of the added electrolyte solution additive is not adjusted to a required amount, the surface of a positive electrode may be decomposed or an oxidation reaction of the electrolyte solution may occur during a high-temperature reaction due to the electrolyte solution additive, and eventually, irreversible capacity of the secondary battery may be increased and output characteristics may be reduced. Also, since a decomposition reaction of the electrolyte solution occurs when the lithium secondary battery is stored at high temperature, high-temperature storage performance and life performance of the battery may be degraded.
  • the present invention provides a non-aqueous electrolyte solution for a lithium secondary battery which may improve room-temperature and high-temperature cycle characteristics and capacity characteristics after high-temperature storage as well as low-temperature and room-temperature output characteristics, and a lithium secondary battery including the same.
  • a lithium secondary battery including: a non-aqueous electrolyte solution including lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material, a negative electrode, and a separator.
  • LiFSI lithium bis(fluorosulfonyl)imide
  • fluorobiphenyl compound a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material
  • a negative electrode and a separator.
  • a lithium secondary battery including a non-aqueous electrolyte solution for a lithium secondary battery of the present invention since the non-aqueous electrolyte solution may form a robust solid electrolyte interface (SEI) on a negative electrode during initial charge and may prevent decomposition of the surface of a positive electrode and an oxidation reaction of the electrolyte solution during a high-temperature cycle, excellent low-temperature output characteristics as well as improved high-temperature storage characteristics and life characteristics may be achieved.
  • SEI solid electrolyte interface
  • FIG. 1 illustrates the results of overcharge tests on batteries of Example 1 and Comparative Example 5.
  • a lithium secondary battery of the present invention includes a non-aqueous electrolyte solution including lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material, a negative electrode, and a separator.
  • LiFSI lithium bis(fluorosulfonyl)imide
  • fluorobiphenyl compound a positive electrode including a lithium-nickel-manganese-cobalt-based oxide as a positive electrode active material
  • a negative electrode and a separator.
  • the non-aqueous electrolyte solution includes lithium bis(fluorosulfonyl)imide (LiFSI) and a fluorobiphenyl compound, and, since the lithium bis(fluorosulfonyl)imide is added as a lithium salt to the non-aqueous electrolyte solution to form a robust thin solid electrolyte interface (SEI) on the negative electrode, the lithium bis(fluorosulfonyl)imide may not only improve low-temperature output characteristics, but also may inhibit decomposition of the surface of the positive electrode, which may occur during a high-temperature cycle, and may prevent an oxidation reaction of the electrolyte solution.
  • LiFSI lithium bis(fluorosulfonyl)imide
  • SEI thin solid electrolyte interface
  • the fluorobiphenyl compound is added to the electrolyte solution and decomposed in the positive electrode and the negative electrode of the lithium secondary battery including the fluorobiphenyl compound to form a thin film and the thin film plays a role in protecting the positive electrode to reduce metal dissolution of the positive electrode active material and increase porosity of a negative electrode film, lithium ions may more smoothly move, and thus, long life and storage characteristics of the secondary battery including the fluorobiphenyl compound may be improved.
  • the fluorobiphenyl compound may improve room-temperature capacity characteristics and output characteristics, and, since the fluorobiphenyl compound may form a film near 4.62 V during overcharging to short the battery at a low state of charge (Sac), the fluorobiphenyl compound may prevent heat generation and subsequent ignition of the battery. Since the SEI formed on the negative electrode is thin, the lithium ions in the negative electrode may more smoothly move, and, accordingly, output of the secondary battery may be improved.
  • a concentration of the lithium bis(fluorosulfonyl)imide in the non-aqueous electrolyte solution may be in a range of 0.01 mol/L to 2 mol/L, particularly, 0.01 mol/L to 1 mol/L.
  • concentration of the lithium bis(fluorosulfonyl)imide is less than 0.01 mol/L, effects of improving the low-temperature output and high-temperature cycle characteristics of the lithium secondary battery may be insignificant.
  • the concentration of the lithium bis(fluorosulfonyl)imide is greater than 2 mol/L, since side reactions in the electrolyte solution may excessively occur during charge and discharge of the battery, a swelling phenomenon may occur and corrosion of a positive electrode or negative electrode collector formed of a metal may occur in the electrolyte solution.
  • the non-aqueous electrolyte solution of the present invention may further include a lithium salt excluding the lithium bis(fluorosulfonyl)imide.
  • a lithium salt excluding the lithium bis(fluorosulfonyl)imide.
  • Any lithium salt commonly used in the art may be used as the lithium salt, and, for example, the lithium salt may include any one selected from the group consisting of LiPF 6 , LiAsF 6 , LiCF 3 SO 3 , LiBF 6 , LiSbF 6 , LiN(C 2 F 5 SO 2 ) 2 , LiAlO 4 , LiAlCl 4 , LiSO 3 CF 3 and LiClO 4 , or a mixture of two or more thereof.
  • a mixing ratio of the lithium salt and the lithium bis(fluorosulfonyl)imide may be in a range of 1:0.01 to 1:1 as a molar ratio.
  • the mixing ratio of the lithium salt and the lithium bis(fluorosulfonyl)imide is above the molar ratio range, since the side reactions in the electrolyte solution may excessively occur during the charge and discharge of the battery, the swelling phenomenon may occur, and, in a case in which the mixing ratio is below the molar ratio range, the output of the secondary battery generated may be reduced.
  • the mixing ratio of the lithium salt and the lithium bis(fluorosulfonyl)imide is less than 1:0.01 as a molar ratio
  • a large amount of irreversible reaction may occur during a process of forming the SEI in the lithium-ion battery and a process of intercalating lithium ions, which are solvated by a carbonate-based solvent, into the negative electrode, and the effects of improving the low-temperature output as well as the cycle characteristics and capacity characteristics after high-temperature storage of the secondary battery may be insignificant due to the exfoliation of a negative electrode surface layer (e.g., carbon surface layer) and the decomposition of the electrolyte solution.
  • a negative electrode surface layer e.g., carbon surface layer
  • the mixing ratio of the lithium salt and the lithium bis(fluorosulfonyl)imide is greater than 1:1 as a molar ratio, since an excessive amount of the lithium bis(fluorosulfonyl)imide is included in the electrolyte solution to cause the corrosion of the electrode collector during the charge and discharge, stability of the secondary battery may be affected.
  • the positive electrode active material as the lithium-nickel-manganese-cobalt-based oxide, may include an oxide represented by Formula 1 below.
  • the positive electrode active material as the lithium-nickel-manganese-cobalt-based oxide, is used in the positive electrode, the positive electrode active material may be combined with the lithium bis(fluorosulfonyl)imide to have a synergistic effect.
  • the lithium-nickel-manganese-cobalt-based oxide positive electrode active material since a phenomenon (cation mixing), in which a position of Li +1 ion and a position of Ni +2 ion in a layered structure of the positive electrode active material are changed during the charge and discharge as an amount of nickel (Ni) among transition metals is increased, occurs, the structure is collapsed, and, thus, the positive electrode active material may cause a side reaction with the electrolyte solution or a dissolution phenomenon of the transition metal may occur.
  • Ni nickel
  • the reason for this is that sizes of the Li +1 ion and the Ni +2 ion are similar. Eventually, performance of the battery is easily degraded due to the depletion of the electrolyte solution in the secondary battery and the structural collapse of the positive electrode active material caused by the side reaction.
  • a layer is formed of a component from the LiFSI on the surface of the positive electrode, and thus, a range, in which a sufficient amount of the Ni transition metal for securing capacity of the positive electrode active material may be secured while suppressing the cation mixing phenomenon of the Li +1 ion and Ni + ion, has been found.
  • the positive electrode active material including the oxide according to Formula 1 of the present invention the side reaction with the electrolyte solution and the metal dissolution phenomenon may be effectively suppressed when the LiFSI-containing electrolyte solution is used.
  • the nickel transition metal having a d orbital in an environment, such as high temperature depending on the variation of oxidation number of the Ni must have an octahedral structure when coordination bonded, but the order of energy levels is reversed or the oxidation number is changed (heterogenization reaction) by external energy supply to form a distorted octahedron.
  • the probability of dissolution of the nickel metal in the positive electrode active material is increased.
  • the present inventors found that excellent efficiency in high-temperature stability and capacity characteristics is achieved while generating high output when the positive electrode active material including the oxide in the range according to Formula 1 and the LiFSI salt are combined.
  • the electrolyte solution may be decomposed in a high output environment and collapse of the negative electrode may be induced.
  • high-temperature stability of the secondary battery generated may be secured.
  • the electrolyte solution having insufficient thermal stability may be easily decomposed in the battery to form LIF and PF 5 .
  • the LiF salt may reduce conductivity and the number of free Li + ions to increase the resistance of the battery, and, as a result, the capacity of the battery is reduced.
  • the non-aqueous electrolyte solution includes a non-aqueous organic solvent
  • the non-aqueous organic solvent is not limited as long as it may minimize the decomposition due to the oxidation reaction during the charge and discharge of the battery and may exhibit desired characteristics with the additive.
  • the non-aqueous organic solvent may include a nitrile-based solvent, cyclic carbonate, linear carbonate, ester, ether, or ketone. These materials may be used alone or in combination of two or more thereof.
  • carbonate-based organic solvents may be easily used.
  • the cyclic carbonate may be any one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), or a mixture of two or more thereof
  • examples of the linear carbonate may be any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), and ethylpropyl carbonate (EPC), or a mixture of two or more thereof.
  • the nitrile-based solvent may include at least one selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-f luorophenylacetonitrile, and 4-fluorophenylacetonitrile, and acetonitrile-based solvent may be used as the non-aqueous solvent according to an embodiment of the present invention.
  • the fluorobiphenyl compound may be a compound represented by the following Formula 2.
  • n may be an integer of 1 to 5, and may specifically be 2.
  • the fluorobiphenyl compound may be 2,3-difluorobiphenyl.
  • the non-aqueous electrolyte solution included in the lithium secondary battery of the present invention includes the fluorobiphenyl compound
  • the non-aqueous electrolyte solution may improve the room-temperature capacity characteristics and output characteristics and may prevent the heat generation and subsequent ignition of the battery by shorting the battery at a low SOC by forming a film near 4.62 V during overcharging.
  • An amount of the fluorobiphenyl compound may be in a range of 0.5 wt % to 10 wt %, particularly 1 wt % to 7 wt %, and more particularly 3 wt % to 5 wt %, based on a total weight of the non-aqueous electrolyte solution.
  • the amount of the fluorobiphenyl compound is 0.5 wt % or more, an effect of shorting the battery during the overcharging of the battery as well as an appropriate effect of improving room-temperature capacity characteristics and output characteristics may be obtained, and, in a case in which the amount of the fluorobiphenyl compound is 10 wt % or less, problems, for example, an increase in irreversible capacity of the battery or an increase in resistance of the negative electrode, may be prevented while having a moderate effect.
  • the amount of the fluorobiphenyl compound may be adjusted according to the amount of the lithium bis(fluorosulfonyl)imide added, and, accordingly, the lithium bis(fluorosulfonyl)imide and the fluorobiphenyl compound may be used in a weight ratio of 1:0.02 to 1:10, particularly 1:0.03 to 1:9, and more particularly 1:0.05 to 1:7.5.
  • the fluorobiphenyl compound may appropriately suppress the side reaction in the electrolyte solution during the charge and discharge of the lithium secondary battery at room temperature which may occur due to the addition of the lithium bis(fluorosulfonyl)imide, and may solve a performance imbalance problem, for example, the reduction of the output in comparison to capacity retention after high-temperature storage or the reduction of the capacity retention in comparison to the output, or a problem such as a decrease in life characteristics improvement effect, when the mixing ratio is outside the above range.
  • the lithium secondary battery according to an embodiment of the present invention may include a negative electrode, a separator disposed between the positive electrode and the negative electrode, and the non-aqueous electrolyte solution.
  • the positive electrode and the negative electrode may respectively include the positive electrode active material according to the embodiment of the present invention and a negative electrode active material.
  • a porous polymer film for example, a porous polymer film prepared from a polyolefin-based polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, may be used alone or in a lamination of two or more thereof as the separator.
  • a typical porous nonwoven fabric for example, a nonwoven fabric formed of high melting point glass fibers or polyethylene terephthalate fibers may be used, but the present invention is not limited thereto.
  • the secondary battery may have various shapes, such as a cylindrical shape, a prismatic shape, or a pouch shape, depending on purposes, and is not limited to a configuration known in the art.
  • the lithium secondary battery according to the embodiment of the present invention may be a pouch type secondary battery.
  • a non-aqueous electrolyte solution was prepared by adding 0.9 mol/L of LiPF 6 , as a lithium salt, based on a total amount of the non-aqueous electrolyte solution and adding 0.1 mol/L of lithium bis(fluorosulfonyi)imide and 3 wt % of 2,3-difluorobiphenyi to a non-aqueous organic solvent having a composition in which a volume ratio of ethylene carbonate (EC):ethylmethyl carbonate (EMC) was 3:7.
  • EC ethylene carbonate
  • EMC ethylmethyl carbonate
  • a positive electrode mixture slurry was prepared by adding 92 wt % of Li(Ni 0.6 Co 0.2 Mn 0.2 )O 0.2 )O 2 as a positive electrode active material, 4 wt % of carbon black as a conductive agent, and 4 wt % of polyvinylidene fluoride (PVdF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent.
  • PVdF polyvinylidene fluoride
  • NMP N-methyl-2-pyrrolidone
  • An about 20 ⁇ m thick aluminum (Al) thin film as a positive electrode collector was coated with the positive electrode mixture slurry and dried, and the coated Al thin film was then roll-pressed to prepare a positive electrode.
  • a negative electrode mixture slurry was prepared by adding 96 wt % of carbon powder as a negative electrode active material, 3 wt % of PVdF as a binder, and 1 wt % of carbon black as a conductive agent to NMP as a solvent.
  • a 10 ⁇ m thick copper (Cu) thin film as a negative electrode collector was coated with the negative electrode mixture slurry and dried, and the coated Cu thin film was then roll-pressed to prepare a negative electrode.
  • a polymer type battery was prepared by a typical method using a separator formed of three layers of polypropylene/polyethylene/polypropylene (PP/PE/PP) with the positive electrode and negative electrode thus prepared, and a lithium secondary battery was then completed by injecting the prepared non-aqueous electrolyte solution.
  • PP/PE/PP polypropylene/polyethylene/polypropylene
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1 except that 0.7 mol/L of LiPF 6 and 0.3 mol/L of lithium bis(fluorosulfonyl)imide were used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1 except that 0.5 mol/L of LiPF 6 and 0.5 mol/L of lithium bis(fluorosulfonyi)imide were used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that 5 wt % of the 2,3-difluorobiphenyl was used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that 10 wt % of the 2,3-difluorobiphenyl was used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that 0.5 wt % of the 2,3-difluorobiphenyl was used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that 2-fluorobiphenyl was used instead of the 2,3-difluorobiphenyl.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that Li(Ni 0.5 Co 0.3 Mn 0.2 )O 2 was used as the positive electrode active material.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 was used as the positive electrode active material.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that LiCoO 2 was used as the positive electrode active material.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that Li(Ni 0.5 Co 0.3 Mn 0.2 )O 2 was used as the positive electrode active material and 2,3-difluorobiphenyl was not used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 2 except that 2,3-difluorobiphenyl was not used.
  • a non-aqueous electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1 except that 0.3 mol/L of LiPF 6 and 0.7 mol/L of lithium bis(fluorosulfonyl)imide were used.
  • the secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 6 were charged at 1 C to 4.2 V/38 mA under a constant current/constant voltage (CC/CV) condition and then discharged at a constant current (CC) of 2 C to a voltage of 2.5 V to measure discharge capacities.
  • CC/CV constant current/constant voltage
  • the secondary batteries were again charged at 1 C to 4.2 V/38 mA under a constant current/constant voltage (CC/CV) condition at room temperature and then discharged at a constant current (CC) of 2 C to a voltage of 2.5 V to measure discharge capacities.
  • the discharge capacity after 20 weeks was calculated as a percentage based on the initial discharge capacity (discharge capacity after 20 weeks/initial discharge capacity ⁇ 100(%)), and the results thereof are presented in Table 1 below.
  • NMC622 represents Li(Ni 0.6 Co 0.2 Mn 0.2 )O 2
  • NMC532 represents Li(Ni 0.5 Co 0.3 Mn 0.2 )O 2
  • NMC811 represents Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2
  • DEBT represents 2,3-difluorobiphenyl
  • FBP represents 2-fluorobiphenyl.
  • the lithium secondary batteries of Examples 1 to 7 exhibited high capacity and output even after the high-temperature storage by including the non-aqueous electrolyte solution which included both of the lithium bis(fluorosulfonyl)imide and the fluorobiphenyl compound.
  • the lithium secondary batteries of Examples 1 to 5 including 2,3-difluorobiphenyl, as the fluorobiphenyl compound exhibited better high-temperature storage characteristics than the lithium secondary battery including 2-fluorobiphenyl as the fluorobiphenyl compound.
  • the lithium secondary batteries of Examples 1 to 4 included the non-aqueous electrolyte solution including both of the lithium bis(fluorosulfonyl)imide and the fluorobiphenyl compound, the lithium secondary batteries of Examples 1 to 4 exhibited better high-temperature storage characteristics than the lithium secondary batteries of Comparative Examples 4 and 5 which did not include the fluorobiphenyl compound.
  • Example 2 including Li(Ni 0.6 Co 0.2 Mn 0.2 )O 2 as the positive electrode active material exhibited better high-temperature storage characteristics than Comparative Examples 1 to 3 respectively including Li(Ni 0.6 Co 0.3 Mn 0.2 )O 2 , Li(Ni 0.8 Co 0.1 Mn 0.1 )O 2 , and LiCoO 2 as the positive electrode active material.
  • the lithium secondary batteries prepared in Example 1 and Comparative Example 5 were overcharged to 8.3 V under a constant current/constant voltage (CC/CV) condition of 1 C (775 mAh)/12 V from a charged state at 25° C., changes in temperature and voltage of the battery at that time were measured, and the results thereof are presented in FIG. 1 .
  • CC/CV constant current/constant voltage
  • the lithium secondary battery of Example 1 which included the non-aqueous electrolyte solution including the 2,3-difluorobiphenyl additive had a lower SOC at the time of short circuit caused by overcharge and a lower temperature at the center of the battery than the lithium secondary battery of Comparative Example 5 which did not include the 2,3-difluorobiphenyl additive.

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