Electrolyte for lithium secondary battery and lithium secondary battery comprising same

Abstract

An electrolyte for a lithium secondary battery according to an embodiment of the present invention comprises: a solvent comprising a fluorine-based solvent; the content of the fluorine-based solvent is 50% by volume or more of the total volume of the solvent, and the fluorine-based solvent comprises fluorine-containing cyclic carbonate and fluorine-containing linear asymmetric carbonate. Accordingly, flame retardancy and lifespan characteristics of a lithium secondary battery including the electrolyte can be improved.

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the electrolyte for a lithium secondary battery. More specifically, the present invention relates to an electrolyte for a lithium secondary battery containing a different component and a lithium secondary battery including the electrolyte for a lithium secondary battery. Background Art

[0002] A secondary battery is a battery that can be repeatedly charged and discharged. With the development of the information communication and display industries, secondary batteries are widely used as power sources for portable electronic communication devices such as cameras, mobile phones, and laptop computers (PCs). In addition, in recent years, battery packs including secondary batteries have been developed and applied as power sources for eco-friendly vehicles such as hybrid vehicles.

[0003] Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc. Among them, lithium secondary batteries have a high working voltage and an energy density per unit weight, and are advantageous for charging speed and weight reduction. Therefore, they are being actively developed and applied.

[0004] For example, a lithium secondary battery may include: an electrode assembly including a positive electrode, a negative electrode, and a separator (separating membrane); and an electrolyte impregnating the electrode assembly. The lithium secondary battery may further include an exterior material such as a pouch type for accommodating the electrode assembly and the electrolyte.

[0005] When using lithium metal as the negative electrode, during the charging process of a lithium secondary battery, lithium may be unevenly deposited, and thus dendritic crystals (dendrites) may grow. When the dendrites penetrate the separator and contact the positive electrode, the lithium secondary battery may short-circuit. Therefore, the battery life and stability may be reduced, and the heat generated during short-circuiting may cause a fire.

[0006] For example, Korean Patent No. 10-2400731 discloses an electrolyte with a specific composition for preventing short-circuiting by suppressing the growth of dendrites, but does not disclose an electrolyte that simultaneously improves the life characteristics and flame retardancy of a lithium secondary battery. Summary of the Invention

[0007] (1) Technical Problem to be Solved

[0008] One technical problem of the present invention is to provide an electrolyte that can provide excellent flame retardancy and life characteristics to a lithium secondary battery.

[0009] One technical problem of the present invention is to provide a lithium secondary battery including the electrolyte and having excellent flame retardancy and life characteristics.

[0010] (2) Technical Solution

[0011] The electrolyte for a lithium secondary battery according to an exemplary embodiment includes: a solvent including a fluorine-based solvent; and a phosphorus-based flame retardant additive, wherein the content of the fluorine-based solvent is 50% by volume or more of the total volume of the solvent, and the fluorine-based solvent includes a fluorinated cyclic carbonate and a fluorinated linear asymmetric carbonate.

[0012] In some embodiments, the phosphorus-based flame retardant additive may include a cyclotriphosphazene having a structure represented by Chemical Formula 1 below.

[0013] [Chemical Formula 1]

[0014]

[0015] In Chemical Formula 1, R1 to R5 may each independently be a halogen atom, and R6 may be an alkyl group having 1 to 10 carbon atoms.

[0016] In some embodiments, R1 to R5 may be fluorine atoms, and R6 may be an ethyl group.

[0017] In some embodiments, the fluorinated linear asymmetric carbonate may have a non-polymerizable end group.

[0018] In some embodiments, the fluorinated cyclic carbonate may have a structure represented by Chemical Formula 2 below.

[0019] [Chemical Formula 2]

[0020]

[0021] In Chemical Formula 2, R7 to R 10 may each independently be a hydrogen atom or a fluorine atom, and at least one of R7 to R 10 may be a fluorine atom.

[0022] In some embodiments, the fluorinated linear asymmetric carbonate may have a structure represented by Chemical Formula 3 below.

[0023] [Chemical Formula 3]

[0024]

[0025] In Chemical Formula 3, R 11 may be an alkyl group having 1 to 5 carbon atoms that does not contain fluorine, and R 12 may be an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom is replaced by a fluorine atom.

[0026] In some embodiments, the content of the fluorine-based solvent may be 70% by weight or more of the total weight of the electrolyte.

[0027] In some embodiments, the fluorinated cyclic carbonate and the fluorinated linear asymmetric carbonate may be included in a volume ratio of 1:9 to 8:2.

[0028] In some embodiments, the electrolyte may further include a lithium salt.

[0029] In some embodiments, the lithium salt may include at least one selected from LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, LiSO3CF3, LiBOB, LiFOB, LiDFOB, LiDFBP, LiTFOP, LiPO2F2, LiCl, LiBr, LiI, LiB 10 Cl 10 , LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN, and LiC(CF3SO2)3.

[0030] In some embodiments, the content of the phosphorus-based flame retardant additive may be 3-10 wt% of the total weight of the electrolyte.

[0031] In some embodiments, the fluorine content of the electrolyte may be more than 30 wt%, and the phosphorus content may be more than 1.8 wt%.

[0032] In some embodiments, the oxygen content of the electrolyte may be 32.5 wt% or less.

[0033] The lithium secondary battery according to an exemplary embodiment includes: a case; an electrode assembly accommodated in the case, and the electrode assembly includes a positive electrode and a negative electrode; and the electrolyte for the lithium secondary battery as described above, and the electrolyte is injected into the case.

[0034] In some embodiments, the negative electrode may include lithium metal.

[0035] In some embodiments, the electrode assembly may further include a separator disposed between the positive electrode and the negative electrode.

[0036] (III) Beneficial Effects

[0037] According to an exemplary embodiment of the present invention, the electrolyte for a lithium secondary battery may include: a solvent including a fluorine-based solvent having a specific composition; and a phosphorus-based flame retardant additive. Therefore, the flame retardancy and life characteristics of the lithium secondary battery including the electrolyte can be improved. Description of the Drawings

[0038] Figure 1 and Figure 2shows a schematic plan view and a schematic cross-sectional view of a lithium secondary battery according to an exemplary embodiment.

[0039] Figure 3 is a photograph for evaluating the flame retardancy of electrolytes prepared according to Example 1, Comparative Example 1, and Comparative Example 2.

[0040] Figure 4a , Figure 4b and Figure 4c are photographs for evaluating the deposition shapes of lithium secondary batteries manufactured according to Example 1, Comparative Example 1, and Comparative Example 2, respectively. Figure 4b In (A) therein represents deposited lithium. Figure 4b In (B) therein represents a copper foil.

[0041] Figure 5 is a graph showing the capacity retention rates of lithium secondary batteries manufactured according to Example 1, Comparative Example 1, and Comparative Example 2 according to repeated charge and discharge. Detailed Description

[0042] An exemplary embodiment of the present invention provides an electrolyte for a lithium secondary battery, the electrolyte comprising: a solvent containing a fluorinated solvent; and a phosphorus-based flame retardant additive, wherein the content of the fluorinated solvent is 50% by volume or more of the total volume of the solvent, and the fluorinated solvent contains a fluorinated cyclic carbonate and a fluorinated linear asymmetric carbonate.

[0043] In addition, an exemplary embodiment of the present invention provides a lithium secondary battery including the electrolyte for a lithium secondary battery.

[0044] Therefore, the flame retardancy and life characteristics of the lithium secondary battery can be improved.

[0045] Hereinafter, with reference to the accompanying drawings, embodiments of the present invention will be described in more detail. However, the drawings in this specification are only used to illustrate the exemplary embodiments of the present invention and are used together with the above content to better understand the technical idea of the present invention. Therefore, the present invention should not be construed as being limited only to the matters described in the drawings.

[0046] <Electrolyte for Lithium Secondary Battery>

[0047] An electrolyte for a lithium secondary battery according to an exemplary embodiment (hereinafter, may be simply referred to as an electrolyte) may comprise: a solvent containing a fluorinated solvent; and a phosphorus-based flame retardant additive, wherein the content of the fluorinated solvent may be 50% by volume or more of the total volume of the solvent, and the fluorinated solvent may contain a fluorinated cyclic carbonate and a fluorinated linear asymmetric carbonate.

[0048] An electrolyte for a lithium secondary battery according to an exemplary embodiment may contain a phosphorus-containing compound or a fluorine-containing compound.

[0049] When a lithium secondary battery burns, highly reactive H radicals and OH radicals are formed, and the above radicals may trigger a chain combustion reaction. Although not limited to a specific theory, the phosphorus-containing compound contained in the electrolyte for a lithium secondary battery according to an exemplary embodiment can capture the above H radicals and OH radicals by releasing P radicals, thereby having the ability to block the chain combustion reaction and prevent combustion. In addition, the phosphorus-containing compound has a high dielectric constant, so that lithium salts such as LiNO3 can be easily dissolved.

[0050] Although not limited to a specific theory, the fluorine-containing compound contained in the electrolyte for a lithium secondary battery according to an exemplary embodiment can form a solid electrolyte interphase (SEI: solid electrolyte interphase) with a large amount of LiF on the electrode surface, thereby suppressing the side reaction between the electrode and the electrolyte. In addition, the formation of dendritic crystals that cause short circuits in lithium metal batteries can be suppressed.

[0051] In the electrolyte for a lithium secondary battery according to an exemplary embodiment, the phosphorus-containing compound may be a phosphorus-based flame retardant additive, and the fluorine-containing compound may be a fluorine-based solvent. Therefore, the electrolyte containing a fluorine-based solvent and a phosphorus-based flame retardant additive with a specific composition can be used to provide a lithium secondary battery with improved flame retardancy, lifespan, and capacity retention rate.

[0052] In some embodiments, the phosphorus-based flame retardant additive may include cyclotriphosphazene having a structure represented by Chemical Formula 1 below.

[0053] [Chemical Formula 1]

[0054]

[0055] In Chemical Formula 1, R1 to R5 may each independently be a halogen atom, and R6 may be an alkyl group having 1 to 10 carbon atoms.

[0056] For example, R1 to R5 may each independently be a halogen atom, may be a fluorine atom, or may be a chlorine atom.

[0057] For example, R6 may be an alkyl group having 1 to 10 carbon atoms, an alkyl group having 1 to 8 carbon atoms, an alkyl group having 1 to 5 carbon atoms, or an alkyl group having 1 to 3 carbon atoms.

[0058] In some embodiments, R1 to R5 may be fluorine atoms, and R6 may be an ethyl group.

[0059] Therefore, the phosphorus-based flame retardant additive contains a large number of fluorine atoms, so that the electrolyte can form a solid electrolyte interface phase (SEI) and can inhibit the formation of dendrites. Therefore, the flame retardancy, lifespan, and capacity retention rate of the lithium secondary battery can be improved.

[0060] In some embodiments, the solvent may include a fluorine-based solvent. For example, the fluorine-based solvent may include a fluorinated cyclic carbonate and a fluorinated linear asymmetric carbonate. For example, the solvent may further include a solvent other than the fluorine-based solvent, and the solvent other than the fluorine-based solvent refers to a solvent without fluorine. For example, it may include a carbonate-based solvent without fluorine, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, an aprotic solvent, etc.

[0061] In some embodiments, the fluorinated cyclic carbonate may have a structure represented by the following Chemical Formula 2.

[0062] [Chemical Formula 2]

[0063]

[0064] In Chemical Formula 2, R7 to R 10 may each independently be a hydrogen atom or a fluorine atom, and at least one of R7 to R 10 may be a fluorine atom.

[0065] For example, any one of R7 to R 10 may be a fluorine atom, any two of R7 to R 10 may be fluorine atoms, any three of R7 to R 10 may be fluorine atoms, and all of R7 to R 10 may be fluorine atoms.

[0066] In some embodiments, the fluorinated cyclic carbonate may include at least one selected from fluorinated ethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and trifluoroethylene carbonate (TFEC).

[0067] In one embodiment, the fluorinated cyclic carbonate may include fluorinated ethylene carbonate (FEC).

[0068] In some embodiments, the fluorinated linear asymmetric carbonate may have a structure represented by the following Chemical Formula 3.

[0069] [Chemical Formula 3]

[0070]

[0071] In Chemical Formula 3, R 11 may be an alkyl group without fluorine having 1 to 5 carbon atoms, and R 12It may be an alkyl group with 1 to 5 carbon atoms in which at least one hydrogen atom is replaced by a fluorine atom.

[0072] For example, the R 11 may be an unfluorinated alkyl group with 1 to 5 carbon atoms, an unfluorinated alkyl group with 1 to 4 carbon atoms, an unfluorinated alkyl group with 1 to 3 carbon atoms, an unfluorinated methyl or ethyl group, or an unfluorinated methyl group.

[0073] For example, the R 12 may be an alkyl group with 1 to 5 carbon atoms in which at least one hydrogen atom is replaced by a fluorine atom, an alkyl group with 1 to 4 carbon atoms in which at least one hydrogen atom is replaced by a fluorine atom, an alkyl group with 1 to 3 carbon atoms in which at least one hydrogen atom is replaced by a fluorine atom, a methyl or ethyl group in which at least one hydrogen atom is replaced by a fluorine atom, or a methyl group in which at least one hydrogen atom is replaced by a fluorine atom. Among them, the R 12 may be a perfluoroalkyl group in which all hydrogen atoms are replaced by fluorine atoms.

[0074] In some embodiments, the fluorinated linear asymmetric carbonate may include at least one selected from fluorodimethyl carbonate (FDMC), fluorodiethyl carbonate (FDEC), and fluoroethyl methyl carbonate (FEMC).

[0075] The FDMC may refer to fluoromethyl methyl carbonate, the FDEC may refer to 2,2,2-trifluoroethyl carbonate, and the FEMC may refer to methyl (2,2,2-trifluoroethyl) carbonate (MTFEC).

[0076] In one embodiment, the fluorinated linear asymmetric carbonate may include fluoroethyl methyl carbonate (FEMC).

[0077] In some embodiments, the content of the fluorinated solvent may be 50% by volume or more of the total volume of the solvent. For example, the content of the fluorinated solvent may be 50% by volume or more, 60% by volume or more, 70% by volume or more, 80% by volume or more, or 90% by volume or more of the total volume of the solvent.

[0078] When the content of the fluorinated solvent is less than 50% by volume of the total volume of the solvent, the SEI layer is insufficiently formed, which may cause side reactions between the electrode and the electrolyte, and the growth of dendritic crystals is not inhibited, thus possibly causing a short circuit. Therefore, the flame retardancy and life characteristics of the lithium secondary battery may be reduced.

[0079] In some embodiments, the fluorine-based solvent may contain the fluorinated cyclic carbonate and the fluorinated linear asymmetric carbonate in a volume ratio of 1:9 to 8:2. Within the above range, the electrolyte can form a stable electrode-electrolyte interface and can inhibit the growth of dendrites. When the above volume ratio is not within the above range, the life characteristics of the lithium secondary battery may be reduced.

[0080] In some embodiments, the fluorinated linear asymmetric carbonate may have non-polymerizable end groups. Therefore, the electrolyte can have a more sufficient solubility for the lithium salt and the phosphorus-based flame retardant additive.

[0081] In some embodiments, the content of the fluorine-based solvent may be 70% by weight or more of the total weight of the electrolyte. For example, the content of the fluorine-based solvent may be 70-95% by weight, 75-90% by weight, or 80-85% by weight.

[0082] Within the above range, the content of fluorine atoms contained in the electrolyte is appropriate, so that a sufficient SEI layer can be formed on the surface of the lithium metal electrode, and the formation of dendrites can be further inhibited. Therefore, the flame retardancy and life of the lithium secondary battery can be further improved.

[0083] In some embodiments, the electrolyte may further contain a lithium salt.

[0084] In some embodiments, the lithium salt may include at least one selected from LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, LiSO3CF3, LiBOB, LiFOB, LiDFOB, LiDFBP, LiTFOP, LiPO2F2, LiCl, LiBr, LiI, LiB 10 Cl 10 , LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN, and LiC(CF3SO2)3.

[0085] In one embodiment, the lithium salt may include LiPF6.

[0086] The concentration of the lithium salt dissolved in the solvent may be 0.5 M to 3 M. For example, the concentration of the lithium salt may be 0.5 M to 3 M, 0.8 M to 1.5 M, or 0.8 M to 1.2 M. Within the above range, lithium ions and / or electrons can migrate more smoothly.

[0087] In some embodiments, the content of the phosphorus-based flame retardant additive can be 3-10 wt% of the total weight of the electrolyte. For example, the content of the phosphorus-based flame retardant additive can be 3-8 wt%, 4-7 wt% or 5-6 wt%.

[0088] Within the above range, the contents of phosphorus atoms and fluorine atoms contained in the electrolyte are appropriate, so that the flame retardant ability, SEI formation ability and dendrite inhibition ability can be further improved. Therefore, the flame retardancy, lifespan and capacity retention rate of the lithium secondary battery can be further improved.

[0089] In some embodiments, the fluorine content of the electrolyte can be 30 wt% or more, and the phosphorus content can be 1.8 wt% or more. For example, the fluorine content can be 30-40 wt%, 32-38 wt% or 34-37 wt%. The phosphorus content can be 1.8-6.0 wt%, 2.8-4.0 wt% or 3.3-3.8 wt%.

[0090] Within the above range, the electrolyte can have higher flame retardant ability, and at the same time, the formation of the SEI layer and the inhibition of dendrites can be more sufficient. Therefore, the flame retardancy, lifespan and capacity retention rate of the lithium secondary battery can be further improved.

[0091] In some embodiments, the oxygen content of the electrolyte can be 32.5 wt% or less. For example, the oxygen content can be 26-32.5 wt%, 27-31 wt% or 28-29 wt%. Within the above range, the flame retardancy, lifespan and capacity retention rate of the lithium secondary battery can be further improved.

[0092] For example, the contents of the respective elements constituting the electrolyte, such as fluorine, phosphorus, oxygen, carbon, hydrogen and nitrogen, can be measured by comprehensively using an Elemental Analyzer (EA), an Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), an Ion Chromatography (IC) and a Nuclear Magnetic Resonance (NMR).

[0093] In some embodiments, the electrolyte can be a non-aqueous electrolyte without water.

[0094] <Lithium secondary battery>

[0095] Figure 1 and Figure 2 are respectively a schematic plan view and a schematic cross-sectional view showing a lithium secondary battery according to an exemplary embodiment.

[0096] Referring to Figure 1 and Figure 2 Figure 2

[0097] The positive electrode 100 may include a positive electrode active material layer 110, which is formed by coating a positive electrode active material on a positive electrode current collector 105. The positive electrode active material may include a compound that allows lithium ions to be reversibly inserted and extracted.

[0098] In some embodiments, the positive electrode active material may be secondary particles substantially in the form of a single particle formed by assembling or aggregating a plurality of primary particles, or may be in the form of single particles. The single particle form may mean, for example, excluding secondary particles substantially in the form of a single particle formed by assembling or aggregating a plurality of primary particles (e.g., more than 10). However, the single particle form does not exclude single particles within the range of 2 to 10 being attached or adhered to each other to substantially have a single body form (e.g., converted into a single particle structure). In some embodiments, the positive electrode active material may contain both secondary particle form and single particle form.

[0099] In an exemplary embodiment, the positive electrode active material may include a lithium-transition metal oxide. For example, the positive electrode active material may include a lithium cobalt oxide (LiCoO2)-based positive electrode active material, a lithium manganese oxide (LiMn2O4)-based positive electrode active material, a lithium nickel oxide (LiNiO2)-based positive electrode active material, a lithium iron phosphate (LFP)-based positive electrode active material, etc., or a lithium-transition metal composite oxide in which a part of these transition metals is replaced by other transition metals.

[0100] For example, the lithium-transition metal oxide may include nickel (Ni), and may further include at least one selected from cobalt (Co) and manganese (Mn). For example, the positive electrode active material may include an NCM-based positive electrode active material, a Mn-rich-based positive electrode active material, or an LLO (Li rich layered oxide, Over Lithiated Oxide, Over-lithiated layered oxide, OLO, LLOs)-based positive electrode active material.

[0101] For example, the lithium-transition metal oxide may have a structure represented by the following Chemical Formula 6-1 to Chemical Formula 6-3.

[0102] [Chemical Formula 6-1]

[0103] Li a Ni b M 1-b O2

[0104] In Chemical Formula 6-1, 0.9 ≤ a ≤ 1.2, b ≥ 0.5, and M is at least one of Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Ba, and Zr. Specifically, in Chemical Formula 6-1, it may be 0.95 ≤ a ≤ 1.08, b may be 0.6 or more, 0.8 or more, more than 0.8, 0.9 or more, or 0.98 or more. Specifically, in Chemical Formula 6-1, M may include Co, Mn, or Al, and more specifically, M may include Co and Mn, and optionally may further include Al.

[0105] [Chemical Formula 6-2]

[0106] pLi2MnO3·(1-p)LiqJO2

[0107] In Chemical Formula 6-2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may be one or more elements selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0108] [Chemical Formula 6-3]

[0109] Li 1+x M 1-x O2

[0110] In Chemical Formula 6-3, 0 ≤ x ≤ 0.4, and M is at least one of Na, Mg, Ca, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Ni, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Ba, and Zr. Specifically, in Chemical Formula 6-3, M may include Ni, Co, Mn, or Al, and more specifically, it may include Ni, Co, and Mn, and optionally may further include Al.

[0111] In addition, the positive electrode active material may also be a lithium iron phosphate (LFP)-based positive electrode active material represented by the chemical formula LiFePO4.

[0112] The positive electrode active material can be mixed and stirred with a binder, a conductive material, and / or a dispersion material, etc. in a solvent to prepare a slurry. The slurry can be coated on the positive electrode current collector 105 and then pressed and dried to manufacture the positive electrode 100.

[0113] The positive electrode current collector 105 can include, for example, stainless steel, nickel, aluminum, titanium, copper, or their alloys, and can also include aluminum or aluminum alloys.

[0114] The binder can include, for example, organic binders such as vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc.; or water - based binders such as styrene - butadiene rubber (SBR). The binder can be used together with thickeners such as carboxymethyl cellulose (CMC). For example, a PPC - LiTFSI - based binder can be used as the positive electrode binder.

[0115] The conductive material can be included to promote electron migration between the active material particles. For example, the conductive material can include carbon - based conductive materials such as graphite, carbon black, graphene, carbon nanotubes, etc. and / or metal - based conductive materials including perovskite substances such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc.

[0116] The negative electrode 130 can include a negative electrode current collector 125 and a negative electrode active material layer 120, and the negative electrode active material layer 120 is formed by coating a negative electrode active material on the negative electrode current collector 125.

[0117] The negative electrode current collector 125 can include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or their alloys, and can also include copper or copper alloys.

[0118] In some embodiments, carbon - based compounds such as artificial graphite, natural graphite, amorphous carbon, etc. or silicon (Si) - based compounds can be used as the negative electrode active material. For example, pure silicon (PureSi), silicon oxide (SiO x ; 0 < x < 2), silicon carbide (SiC), or silicon - carbon particles including a carbon core and a silicon coating can be used.

[0119] In some embodiments, the negative electrode 130 can include lithium metal. For example, lithium metal or a lithium - metal alloy can be used as the negative electrode active material, and the negative electrode active material layer 120 can include a lithium metal foil disposed on the negative electrode current collector 125.

[0120] In some embodiments, the negative electrode active material can be mixed and stirred with a binder, a conductive material, and / or a dispersion material, etc. in a solvent to prepare a slurry. The slurry can be coated on the negative electrode current collector 125, and then pressed and dried to fabricate the negative electrode 130. As the conductive material, a substance substantially the same as or similar to the above substances can be used.

[0121] In an exemplary embodiment, the binder for the negative electrode can include organic binders such as vinylidene fluoride - hexafluoropropylene copolymer (PVDF - HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, PPC - LiTFSI, etc. or water - based binders such as SBR. The binder can be used together with a thickener such as CMC.

[0122] In addition, the lithium secondary battery can be a non - negative electrode secondary battery. That is, it can be a battery in which the negative electrode active material layer 120 is not formed on the negative electrode current collector 125 during the battery assembly process. When the non - negative electrode lithium secondary battery is initially charged or first charged, the main positive electrode active material and the sacrificial positive electrode active material are de - lithiated, and the lithium ions generated from the positive electrode active material are reduced on the negative electrode current collector 125, thereby forming a lithium layer of a lithium metal layer or a solid lithium layer.

[0123] A separator 140 can be disposed between the positive electrode 100 and the negative electrode 130. The separator 140 can include a porous polymer membrane prepared from polyolefin - based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer, etc. The separator can also include a non - woven fabric formed from glass fibers with a high melting point, polyethylene terephthalate fibers, etc.

[0124] In some embodiments, the area (e.g., the contact area with the separator 140) and / or volume of the negative electrode 130 can be larger than that of the positive electrode 100. Therefore, the lithium ions generated from the positive electrode 100 can migrate smoothly to the negative electrode 130 without precipitation in the middle.

[0125] According to an exemplary embodiment, a battery cell is defined by the positive electrode 100, the negative electrode 130, and the separator 140, and a plurality of battery cells can be stacked to form an electrode assembly 150 in a form such as a jelly roll. For example, the electrode assembly 150 can be formed by winding, laminating, folding, etc. of the separator 140.

[0126] The electrode assembly 150 can be accommodated in a housing 160 together with the electrolyte of the exemplary embodiment to define a lithium secondary battery.

[0127] AsFigure 1 As shown, the tabs (the positive tab and the negative tab) can respectively protrude from the positive current collector 105 and the negative current collector 125 belonging to each battery cell and extend to one side of the housing 160. The tabs can be fused to the one side of the housing 160 and form electrode leads (the positive lead 107 and the negative lead 127) extending to the outside of the housing 160 or exposed to the outside of the housing 160.

[0128] The lithium secondary battery can be made, for example, in a cylindrical shape using a can, a prismatic shape, a pouch type, a coin shape, or the like.

[0129] Hereinafter, exemplary embodiments are presented to assist in understanding the present invention, but these embodiments are only used to illustrate the present invention and are not used to limit the scope of the claims. Various modifications and changes can be made to the embodiments within the scope and technical concept of the present invention, which are obvious to those skilled in the art, and such modifications and changes clearly also fall within the scope of the claims.

[0130] Examples and Comparative Examples

[0131] 1. Example 1

[0132] (1) Preparation of the electrolyte

[0133] Fluoroethylene carbonate (FEC) (manufactured by Tokyo Chemical Industry Co., Ltd.) and fluoroethyl methyl carbonate (FEMC) (manufactured by Synquest labs) as fluorine-based solvents were mixed at a volume ratio of 1:4 and stirred for 12 hours to prepare a mixed solvent. 1M LiPF6 and ethoxy(pentafluoro)cyclotriphosphazene (EPC) (manufactured by Tokyo Chemical Industry Co., Ltd.) as a phosphorus-based flame retardant additive were dissolved in the mixed solvent and stirred for 12 hours to prepare the electrolyte.

[0134] The contents of the fluorine-based solvent, the lithium salt, and EPC relative to the total weight of the electrolyte are shown in Table 2 below. The structure of EPC is represented by Chemical Formula 5 below.

[0135] [Chemical Formula 5]

[0136]

[0137] Ethoxy(pentafluoro)cyclotriphosphazene (EPC)

[0138] (2) Manufacture of the lithium secondary battery

[0139] Li[Ni 0.6 Co 0.2 Mn 0.2O2, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder are mixed at a weight ratio of 92:5:3 to prepare a slurry. The slurry is uniformly coated on an aluminum foil with a thickness of 15 μm and vacuum dried at 130 °C. The dried slurry is calendered to manufacture a positive electrode for a lithium secondary battery with a density of 3.667 g / cm³.

[0140] Lithium metal foil (thickness 20 μm, manufactured by Honzo Co., Ltd.) is used as the negative electrode.

[0141] A separator (polypropylene) is disposed between the manufactured positive electrode and negative electrode to form an electrode assembly. The electrode assembly and the electrolyte prepared in (1) above are added to a case and vacuum sealed to manufacture a lithium secondary battery.

[0142] 2. Example 2

[0143] An electrolyte is prepared by the same method as in Example 1, except that FEC and FEMC, which are fluorine-based solvents, are mixed at a volume ratio of 0.8:9.2. Thereafter, a lithium secondary battery is manufactured by the same method as in Example 1, except that the above electrolyte is used instead of the electrolyte of Example 1.

[0144] 3. Example 3

[0145] An electrolyte is prepared by the same method as in Example 1, except that FEC and FEMC, which are fluorine-based solvents, are mixed at a volume ratio of 8.2:1.8. Thereafter, a lithium secondary battery is manufactured by the same method as in Example 1, except that the above electrolyte is used instead of the electrolyte of Example 1.

[0146] 4. Comparative Example 1

[0147] Ethylene carbonate (EC, manufactured by Enchem Co., Ltd.) and ethyl methyl carbonate (EMC, manufactured by Enchem Co., Ltd.), which are organic solvents, are mixed at a volume ratio of 3:7 respectively and stirred for 12 hours to prepare a mixed solvent. 1 M LiPF6 is dissolved in the mixed solvent and FEC is added, and then stirred for 12 hours to prepare an electrolyte. Thereafter, a lithium secondary battery is manufactured by the same method as in Example 1, except that the above electrolyte is used instead of the electrolyte of Example 1.

[0148] 5. Comparative Example 2

[0149] An electrolyte was prepared by the same method as in Example 1, except that EPC as a phosphorus-based flame retardant additive was not used. Thereafter, a lithium secondary battery was manufactured by the same method as in Example 1, except that the electrolyte described above was used instead of the electrolyte of Example 1.

[0150] 6. Comparative Example 3

[0151] An electrolyte was prepared by the same method as in Example 1, except that FEC, FEMC, EC, and EMC were mixed in a volume ratio of 0.8:3.2:1.2:4.8, respectively. A lithium secondary battery was then manufactured by the same method as in Example 1, except that the electrolyte described above was used instead of the electrolyte in Example 1.

[0152] 7. Comparative Example 4

[0153] An electrolyte was prepared by the same method as in Example 1, except that the contents of the fluorine-based solvent, lithium salt, and EPC relative to the total weight of the electrolyte were changed to the contents described in Table 2 below (using an excess amount of EPC). Thereafter, a lithium secondary battery was manufactured by the same method as in Example 1, except that the above electrolyte was used instead of the electrolyte in Example 1.

[0154] In the Examples and Comparative Examples, the content (volume %) of the fluorine-based solvent relative to the total weight of the solvent and the volume ratio (v / v) of the solvent are recorded in Table 1 below, the content (weight %) of the solvent, lithium salt and phosphorus-based flame retardant additive relative to the total weight of the electrolyte are recorded in Table 2 below, and the contents of fluorine, phosphorus, oxygen, carbon, hydrogen and nitrogen contained in the above-mentioned electrolyte are recorded in Table 3 below.

[0155] [Table 1]

[0156]

[0157] [Table 2]

[0158]

[0159] [Table 3]

[0160]

[0161] evaluate

[0162] Evaluation 1: Electrolyte combustion test

[0163] The electrolytes according to Examples and Comparative Examples were respectively immersed in glass filters, ignited with a blowtorch for 2 seconds, and then the flame retardancy was evaluated by observing the burning conditions.

[0164] Figure 3It is a photograph for evaluating the flame retardancy of electrolytes prepared according to Example 1, Comparative Example 1, and Comparative Example 2.

[0165] Referring to Figure 3 , when the ignition source is removed, the fire in Example 1 goes out. Although the size of the flame in Comparative Example 2 is small, it still burns briefly. The combustion duration of Comparative Example 1 is the longest.

[0166] In addition, according to the following formula 1, the reduction rate of the combustion time of the examples and comparative examples compared to the combustion time of Comparative Example 1 is calculated.

[0167] [Formula 1]

[0168] Combustion time reduction rate (%) = ((Combustion time of other examples or comparative examples) - (Combustion time of Comparative Example 1)) ÷ (Combustion time of Comparative Example 1) × 100

[0169] The evaluation results are shown in Table 4 below.

[0170] Evaluation 2: Evaluation of lithium deposition

[0171] At room temperature (25 °C), the lithium secondary batteries according to the examples and comparative examples are charged at a constant current of 0.1C to 4.3V and then charged at a constant voltage until the current reaches 0.01C while maintaining 4.3V. After that, the shape of the lithium deposited on the surface of the copper foil is evaluated by scanning electron microscopy (SEM).

[0172] Figure 4a , Figure 4b and Figure 4c are photographs for evaluating the deposition shapes of the lithium secondary batteries manufactured according to Example 1, Comparative Example 1, and Comparative Example 2 respectively. Figure 4b In (A) represents the deposited lithium. Figure 4b In (B) represents the copper foil.

[0173] Referring to Figure 4a , Figure 4b and Figure 4c , in Example 1 and Comparative Example 2, lithium is formed densely on the surface of the copper foil as a whole, and the formation of dendritic crystals is also suppressed. However, in Comparative Example 1, lithium is deposited unevenly on the surface of the copper foil, and the growth of dendritic crystals is not suppressed.

[0174] The evaluation results are shown in Table 4 below.

[0175] Evaluation 3: Evaluation of capacity retention rate

[0176] At room temperature (25 °C), the lithium secondary batteries according to the examples and comparative examples were charged at a constant current of 0.5 C to 4.3 V and then charged at a constant voltage to a current of 0.05 C while maintaining 4.3 V. Thereafter, they were discharged at 0.5 C to 2.7 V to measure the initial discharge capacity.

[0177] The above charging and discharging were repeated 60 times, and the discharge capacity of each cycle was divided by the initial capacity to calculate the capacity retention ratio (%).

[0178] Figure 5 It is a graph showing the capacity retention ratio according to repeated charge and discharge of the lithium secondary batteries manufactured according to Example 1, Comparative Example 1, and Comparative Example 2.

[0179] Refer to Figure 5 , when charging and discharging were carried out 40 times or more, the capacity retention ratio of Comparative Example 1 decreased sharply compared to Example 1 and Comparative Example 2.

[0180] The evaluation results of the capacity retention ratio after 60 cycles are shown in Table 4 below.

[0181] [Table 4]

[0182]

[0183] Referring to Table 3 and Table 4, it can be confirmed that, compared with the electrolytes of Comparative Examples 1 to 4, the electrolytes of Examples 1 to 3 containing a fluorine-based solvent and a phosphorus-based flame retardant additive with a specific composition have excellent flame retardancy, dendrite inhibition ability, and capacity retention ratio at the same time.

Claims

1. An electrolyte for a lithium secondary battery, comprising: A solvent containing a fluorine-based solvent; and A phosphorus-based flame retardant additive, wherein the content of the fluorine-based solvent is 50% by volume or more of the total volume of the solvent, The fluorine-based solvent contains a fluorinated cyclic carbonate and a fluorinated linear asymmetric carbonate.

2. The electrolyte for a lithium secondary battery according to claim 1, wherein, The phosphorus-based flame retardant additive contains a cyclotriphosphazene having a structure represented by the following Chemical Formula 1, [Chemical Formula 1] In Chemical Formula 1, R1 to R5 are each independently a halogen atom, and R6 is an alkyl group having 1 to 10 carbon atoms.

3. The electrolyte for a lithium secondary battery according to claim 2, wherein, R1 to R5 are fluorine atoms, and R6 is an ethyl group.

4. The electrolyte for a lithium secondary battery according to claim 1, wherein The fluorinated linear asymmetric carbonate has a non-polymerizable end group.

5. The electrolyte for a lithium secondary battery according to claim 1, wherein, The fluorinated cyclic carbonate has a structure represented by the following Chemical Formula 2, [Chemical Formula 2] In Chemical Formula 2, R7 to R 10 are each independently a hydrogen atom or a fluorine atom, and at least one of R7 to R 10 is a fluorine atom.

6. The electrolyte for a lithium secondary battery according to claim 1, wherein, The fluorinated linear asymmetric carbonate has a structure represented by the following Chemical Formula 3, [Chemical Formula 3] In Chemical Formula 3, R 11 is a non-fluorinated alkyl group having 1 to 5 carbon atoms, and R 12 is an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom is replaced by a fluorine atom.

7. The electrolyte for a lithium secondary battery according to claim 1, wherein, The content of the fluorine-based solvent is 70% by weight or more of the total weight of the electrolyte.

8. The electrolyte for a lithium secondary battery according to claim 1, wherein, The fluorinated cyclic carbonate and the fluorinated linear asymmetric carbonate are contained in a volume ratio of 1:9 to 8:

2.

9. The electrolyte for a lithium secondary battery according to claim 1, wherein, The electrolyte for the lithium secondary battery further contains a lithium salt.

10. The electrolyte for a lithium secondary battery according to claim 9, wherein, The lithium salt includes at least one selected from LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, LiSO3CF3, LiBOB, LiFOB, LiDFOB, LiDFBP, LiTFOP, LiPO2F2, LiCl, LiBr, LiI, LiB 10 Cl 10 , LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN and LiC(CF3SO2)3.

11. The electrolyte for a lithium secondary battery according to claim 1, wherein, The content of the phosphorus-based flame retardant additive is 3-10% by weight of the total weight of the electrolyte.

12. The electrolyte for a lithium secondary battery according to claim 1, wherein, The fluorine content of the electrolyte for the lithium secondary battery is 30% by weight or more, and the phosphorus content is 1.8% by weight or more.

13. The electrolyte for a lithium secondary battery according to claim 1, wherein, The oxygen content of the electrolyte for the lithium secondary battery is 32.5% by weight or less.

14. A lithium secondary battery, comprising: A case; An electrode assembly accommodated in the case, and the electrode assembly includes a positive electrode and a negative electrode; And The electrolyte for the lithium secondary battery according to any one of claims 1 to 13, and the electrolyte is injected into the case.

15. The lithium secondary battery according to claim 14, wherein, The negative electrode contains lithium metal.

16. The lithium secondary battery according to claim 14, wherein The electrode assembly further includes a separator disposed between the positive electrode and the negative electrode.

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