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 contains a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a volume ratio of 1: 9 to 8: 2. Accordingly, the life characteristics and high voltage stability 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 same. More specifically, the present invention relates to an electrolyte for a lithium secondary battery containing a heterogeneous component and a lithium secondary battery including the same. Background Art

[0002] Secondary batteries are batteries that can be repeatedly charged and discharged. With the development of the information communications and display industries, they are widely used as power sources for portable electronic communication devices such as cameras, mobile phones, and laptop computers (PCs). Furthermore, in recent years, battery packs containing secondary batteries have been developed and used as power sources for environmentally friendly vehicles such as hybrid cars.

[0003] Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among them, lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous in charging speed and light weight, and are therefore being actively developed and used.

[0004] A lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, and a separator (a membrane); and an electrolyte impregnated with the electrode assembly. For example, when lithium metal is used as the negative electrode, lithium secondary batteries may experience uneven lithium deposition during charging, potentially leading to the growth of dendrites (dendrites), which may deteriorate battery life and stability.

[0005] Previously, lithium secondary batteries were charged at a voltage of 3.0-4.2 V, but research is underway to achieve higher energy capacity by applying a higher charging voltage (4.2-4.5 V). However, when using conventional non-aqueous carbonate-based solvents as electrolytes, charging at voltages above 4.2 V can cause electrolyte decomposition, potentially degrading battery life.

[0006] For example, Korean Patent Publication No. 10-2022-0082969 discloses an electrolyte comprising a fluorinated linear symmetrical carbonate and a fluorinated cyclic carbonate. Although the electrolyte has high voltage stability, due to its low solubility, salts may precipitate during long-term storage and a stable solid electrolyte interphase (SEI) may not form on the negative electrode surface, resulting in poor battery performance. Summary of the Invention

[0007] (1) Technical issues to be resolved

[0008] A technical problem of the present invention is to provide an electrolyte that can provide excellent high voltage stability and lifespan characteristics to a lithium secondary battery.

[0009] A technical problem of the present invention is to provide a lithium secondary battery including the electrolyte and having excellent high voltage stability and lifespan characteristics.

[0010] (2) Technical solution

[0011] The electrolyte for a lithium secondary battery according to the exemplary embodiment includes a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a volume ratio of 1:9 to 8:2.

[0012] In some embodiments, the fluorine-containing linear asymmetric carbonate may be non-polymeric.

[0013] In some embodiments, the fluorine-containing cyclic carbonate may have a structure represented by Chemical Formula 1 below.

[0014] [Chemical Formula 1]

[0015]

[0016] In Chemical Formula 1, R1 to R4 may each independently be a hydrogen atom or a fluorine atom, and at least one of R1 to R4 may be a fluorine atom.

[0017] In some embodiments, the fluorine-containing cyclic carbonate may include at least one selected from the group consisting of fluoroethylene carbonate (FEC), bisfluoroethylene carbonate (DFEC), and trifluoroethylene carbonate (TFEC).

[0018] In some embodiments, the fluorine-containing linear asymmetric carbonate may have a structure represented by the following Chemical Formula 2.

[0019] [Chemical Formula 2]

[0020]

[0021] In Chemical Formula 2, R5 may be an alkyl group having 1 to 5 carbon atoms and not containing fluorine, and R6 may be an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom is substituted with a fluorine atom.

[0022] In some embodiments, the fluorine-containing linear asymmetric carbonate may include at least one selected from the group consisting of fluorodimethyl carbonate (FDMC), fluorodiethyl carbonate (FDEC), and fluoroethylmethyl carbonate (FEMC).

[0023] In some embodiments, the content of the fluorine-containing cyclic carbonate and the fluorine-containing linear asymmetric carbonate may be 70 wt % or more of the total weight of the electrolyte.

[0024] In some embodiments, the electrolyte may further comprise a lithium salt.

[0025] In some embodiments, the lithium salt may include a salt selected from the group consisting of LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, LiSO3CF3, LiBOB, LiFOB, LiDFOB, LiDFBP, LiTFOP, LiPO2F2, LiCl, LiBr, LiI, LiB 10 Cl 10 , at least one of LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN and LiC(CF3SO2)3.

[0026] In some embodiments, the fluorine content of the electrolyte may be 30 wt % or greater.

[0027] In some embodiments, the oxygen content of the electrolyte may be 34 wt % or less.

[0028] A lithium secondary battery according to an exemplary embodiment includes a case; an electrode assembly accommodated in the case and including a positive electrode and a negative electrode; and the above-described electrolyte for a lithium secondary battery injected into the case.

[0029] In some embodiments, the negative electrode may comprise lithium metal or graphite.

[0030] In some embodiments, the electrode assembly may further include a separator disposed between the positive electrode and the negative electrode.

[0031] (3) Beneficial effects

[0032] According to an exemplary embodiment of the present invention, the electrolyte for a lithium secondary battery may include a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a volume ratio of 1:9 to 8:2. Therefore, the high voltage stability and lifespan characteristics of the lithium secondary battery including the electrolyte may be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 and Figure 2 are a schematic plan view and a schematic cross-sectional view illustrating a lithium secondary battery according to an exemplary embodiment.

[0034] Figure 3a and Figure 3b These are photographs showing the evaluation of the deposition shapes of lithium secondary batteries manufactured according to Example 1 and Comparative Example 1. Figure 3b(A) in the graph represents deposited lithium. Figure 3b (B) in the figure represents copper foil.

[0035] Figure 4 is a graph showing capacity retention rates (2.7 V-4.3 V at 80 cycles) according to repeated charge and discharge of lithium secondary batteries manufactured according to Example 1 and Comparative Examples 1 to 3.

[0036] Figure 5 is a graph showing capacity retention rates (2.7 V-4.5 V at 55 cycles) according to repeated charge and discharge of lithium secondary batteries manufactured according to Example 1 and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0037] An exemplary embodiment of the present invention provides an electrolyte for a lithium secondary battery, comprising a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a volume ratio of 1:9 to 8:2.

[0038] Furthermore, an exemplary embodiment of the present invention provides a lithium secondary battery including the electrolyte for a lithium secondary battery.

[0039] Therefore, the high voltage stability and lifespan characteristics of the lithium secondary battery can be improved.

[0040] The following describes the embodiments of the present invention in more detail with reference to the accompanying drawings. However, the drawings in this specification are only used to illustrate exemplary embodiments of the present invention and, together with the above content, are used to better understand the technical concept of the present invention. Therefore, the present invention should not be construed as being limited to the matters described in the drawings.

[0041] <Electrolyte for lithium secondary batteries>

[0042] An electrolyte for a lithium secondary battery (hereinafter, may be simply referred to as an electrolyte) according to an exemplary embodiment may include a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a volume ratio of 1:9 to 8:2.

[0043] When lithium secondary batteries are driven at high voltages to achieve high energy density, the film on the electrode surface decomposes, potentially triggering side reactions between the electrode and the electrolyte. Consequently, the electrolyte may decompose, and decomposition products may accumulate on the electrode surface, potentially reducing ionic conductivity.

[0044] The electrolyte for a lithium secondary battery according to one embodiment includes a fluorine-containing carbonate solvent, which can form an SEI layer containing a large amount of LiF on the surface of a lithium metal electrode, thereby acting as a relatively stable film to electrochemical reactions, thereby suppressing decomposition of the electrolyte.

[0045] Furthermore, the fluorine-containing carbonate solvent contains a large amount of fluorine, thereby improving resistance to oxidative decomposition at high voltages. Therefore, the electrolyte can be used to provide a lithium secondary battery with improved high-voltage stability.

[0046] The lithium secondary battery electrolyte according to one embodiment comprises the fluorine-containing cyclic carbonate and the fluorine-containing straight-chain asymmetric carbonate of the above-mentioned range, so that the electrolyte contains a large amount of fluorine, thereby improving oxidation stability and being not easily decomposed in the high voltage range. In addition, a solid electrolyte interface phase (SEI) (a layer rich in LiF (LiFrich layer)) is formed on the electrode surface to form a stable electrode-electrolyte interface, thereby suppressing the side reaction of the electrode and the electrolyte. In addition, the growth of dendrites that produce a short circuit can be suppressed in lithium metal batteries. Therefore, by comprising the fluorine-containing cyclic carbonate and the fluorine-containing straight-chain asymmetric carbonate of the above-mentioned range, the electrolyte can be used to provide a lithium secondary battery with improved lifespan and capacity retention.

[0047] In some embodiments, the fluorine-containing cyclic carbonate may have a structure represented by Chemical Formula 1 below.

[0048] [Chemical Formula 1]

[0049]

[0050] In Chemical Formula 1, R1 to R4 may each independently be a hydrogen atom or a fluorine atom, and at least one of R1 to R4 may be a fluorine atom.

[0051] For example, any one of R1 to R4 may be a fluorine atom, any two of R1 to R4 may be fluorine atoms, any three of R1 to R4 may be fluorine atoms, or all of R1 to R4 may be fluorine atoms.

[0052] In some embodiments, the fluorine-containing cyclic carbonate may include at least one selected from the group consisting of fluoroethylene carbonate (FEC), bisfluoroethylene carbonate (DFEC), and trifluoroethylene carbonate (TFEC).

[0053] In some embodiments, the fluorine-containing cyclic carbonate may be fluoroethylene carbonate (FEC).

[0054] In some embodiments, the fluorine-containing linear asymmetric carbonate may have a structure represented by the following Chemical Formula 2.

[0055] [Chemical Formula 2]

[0056]

[0057] In Chemical Formula 2, R5 may be an alkyl group having 1 to 5 carbon atoms and not containing fluorine, and R6 may be an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom is substituted with a fluorine atom.

[0058] For example, R5 can be a non-fluorinated alkyl group having 1 to 5 carbon atoms, a non-fluorinated alkyl group having 1 to 4 carbon atoms, a non-fluorinated alkyl group having 1 to 3 carbon atoms, a non-fluorinated methyl group or an ethyl group, or a non-fluorinated methyl group.

[0059] For example, R6 can 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 group or an 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. 12 It may be a perfluoroalkyl group in which all hydrogen atoms are replaced by fluorine atoms.

[0060] In some embodiments, the fluorine-containing linear asymmetric carbonate may include at least one selected from the group consisting of fluorodimethyl carbonate (FDMC), fluorodiethyl carbonate (FDEC), and fluoroethylmethyl carbonate (FEMC).

[0061] 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).

[0062] In one embodiment, the fluorine-containing linear asymmetric carbonate may include fluoroethyl methyl carbonate (FEMC).

[0063] In some embodiments, the electrolyte may further comprise a lithium salt.

[0064] In some embodiments, the lithium salt may include a salt selected from the group consisting of LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, LiSO3CF3, LiBOB, LiFOB, LiDFOB, LiDFBP, LiTFOP, LiPO2F2, LiCl, LiBr, LiI, LiB 10 Cl 10 , at least one of LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN and LiC(CF3SO2)3.

[0065] In one embodiment, the lithium salt may comprise LiPF6.

[0066] 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 may migrate more smoothly.

[0067] In some embodiments, the fluorine-containing linear asymmetric carbonate may be non-polymeric, so that the electrolyte can have a more sufficient solubility for the lithium salt and will not further precipitate the lithium salt during long-term storage.

[0068] When using fluorinated linear symmetrical carbonates and fluorinated cyclic carbonates, the lithium salt may not fully dissolve. As a result, the lithium salt remains as a salt, potentially increasing resistance, causing side reactions, and leading to electrode shedding and polarization. Consequently, the power and lifespan characteristics of lithium secondary batteries may deteriorate.

[0069] However, when using fluorinated linear asymmetric carbonates and fluorinated cyclic carbonates, the lithium salt can be uniformly dissolved. Therefore, the lithium salt does not remain in the form of salt, the lithium ions are evenly distributed in the electrolyte, and a uniform protective layer is formed on the electrode, thereby improving the capacity retention rate of the lithium secondary battery.

[0070] In some embodiments, the content of the fluorine-containing cyclic carbonate and the fluorine-containing linear asymmetric carbonate can be 70% by weight or more of the total weight of the electrolyte. For example, the content of the fluorine-based solvent can be 70-98% by weight, 75-95% by weight, or 80-90% by weight or more.

[0071] Within the above range, the content of fluorine atoms in the electrolyte is moderate, thereby allowing for a more complete formation of an SEI layer on the surface of the lithium metal electrode and further suppressing the formation of dendrites. Therefore, the high voltage stability and lifespan characteristics of the lithium secondary battery can be further improved.

[0072] In some embodiments, the fluorine content of the electrolyte may be greater than 30 wt %, for example, 30-44 wt %, 32-40 wt %, or 35-38 wt %.

[0073] Within the above range, a SEI layer containing a large amount of LiF can be formed on the surface of the lithium metal electrode, the formation of dendrites can be suppressed, and the decomposition of the electrolyte at high voltage can be further suppressed. Therefore, the high voltage stability and life characteristics of the lithium secondary battery can be further improved.

[0074] In some embodiments, the oxygen content of the electrolyte may be 34 wt % or less. For example, the oxygen content may be 24-34 wt %, 27-32 wt %, or 29-30 wt %. Within the above range, the lifespan and capacity retention of the lithium secondary battery can be further improved.

[0075] For example, the content of each element constituting the electrolyte can be measured by comprehensively using an elemental analyzer (EA), an inductively coupled plasma-optical emission spectrometer (ICP-OES), an ion chromatograph (IC), and a nuclear magnetic resonance (NMR).

[0076] In some embodiments, the electrolyte may be a non-aqueous electrolyte that does not contain water.

[0077] <Lithium Secondary Battery>

[0078] Figure 1 and Figure 2 1 and 2 are respectively a schematic plan view and a schematic cross-sectional view illustrating a lithium secondary battery according to an exemplary embodiment.

[0079] Reference Figure 1 and Figure 2 The lithium secondary battery may include an electrode assembly 150 including a positive electrode 100 and a negative electrode 130. The electrode assembly 150 may be housed in a housing 160 together with the electrolyte of the exemplary embodiment described above and impregnated with the electrolyte. The electrode assembly 150 may further include a separator 140 disposed between the positive electrode 100 and the negative electrode 130.

[0080] The positive electrode 100 may include a positive active material layer 110 formed by coating a positive active material on a positive current collector 105. The positive active material may include a compound that can reversibly intercalate and deintercalate lithium ions.

[0081] In some embodiments, the positive electrode active material may be a secondary particle formed by assembling or aggregating a plurality of primary particles, or may be in a single particle form. The single particle form may refer to, for example, excluding a secondary particle formed by assembling or aggregating a plurality of primary particles (e.g., greater than 10) that is substantially a single particle. However, the single particle form does not exclude that single particles within the range of 2 to 10 adhere to or adhere to each other and substantially have a single body form (e.g., a structure converted into a single particle). In some embodiments, the positive electrode active material may include both a secondary particle form and a single particle form.

[0082] 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-transition metal oxide such as 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, or a lithium iron phosphate (LFP)-based positive electrode active material, or a lithium-transition metal composite oxide in which a portion of these transition metals is replaced by other transition metals.

[0083] For example, the lithium-transition metal oxide may include nickel (Ni) and may further include at least one selected from cobalt (Co) or manganese (Mn). For example, the positive electrode active material may include an NCM-based positive electrode active material, a manganese-rich (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.

[0084] For example, the lithium-transition metal oxide may have structures represented by the following Chemical Formula 3-1 to Chemical Formula 3-3.

[0085] [Chemical Formula 3-1]

[0086] Li a Ni b M 1-b O2

[0087] In the chemical formula 3-1, 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 the chemical formula 3-1, it can be 0.95 ≤ a ≤ 1.08, and b can be 0.6 or more, 0.8 or more, more than 0.8, 0.9 or more, or 0.98 or more. Specifically, in the chemical formula 3-1, M can include Co, Mn, or Al. More specifically, M can include Co and Mn, and optionally can further include Al.

[0088] [Chemical formula 3-2]

[0089] pLi2MnO3·(1-p)LiqJO2

[0090] In the chemical formula 3-2, it can be 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J can be one or more elements selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0091] [Chemical formula 3-3]

[0092] Li 1+x M 1-x O2

[0093] In the chemical formula 3-3, 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 the chemical formula 3-3, M can include Ni, Co, Mn, or Al. More specifically, it can include Ni, Co, and Mn, and optionally can further include Al.

[0094] In addition, the positive electrode active material can also be a lithium iron phosphate (LFP) - based positive electrode active material represented by the chemical formula LiFePO4.

[0095] 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.

[0096] 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.

[0097] The binder may 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.

[0098] 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 materials such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc.

[0099] The negative electrode 130 may 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.

[0100] The negative electrode current collector 125 may include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or their alloys, and may also include copper or copper alloys.

[0101] 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 comprising a carbon core and a silicon coating can be used.

[0102] In some embodiments, the negative electrode 130 may include lithium metal or graphite. 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 may include a lithium metal foil disposed on the negative electrode current collector 125. For example, the negative electrode 130 may include one or more types of graphite such as the above - mentioned artificial graphite and natural graphite.

[0103] In some embodiments, the negative electrode active material can be mixed and stirred with a binder, a conductive material, and / or a dispersing 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 manufacture the negative electrode 130. Substantially the same or similar materials as those mentioned above can be used as the conductive material.

[0104] In an exemplary embodiment, the negative electrode binder may include an organic binder such as vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, PPC-LiTFSI, or a water-based binder such as SBR, and the binder may be used together with a thickener such as CMC.

[0105] Alternatively, the lithium secondary battery may be a negative electrode-less secondary battery. That is, it may 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 negative electrode-less lithium secondary battery is initially charged or charged for the first time, the main positive electrode active material and the sacrificial positive electrode active material are delithiated, and the lithium ions generated from the positive electrode active material are reduced on the negative electrode current collector 125, thereby forming a lithium metal layer or a solid lithium layer.

[0106] A separator 140 may be provided between the positive electrode 100 and the negative electrode 130. The separator 140 may include a porous polymer film made from a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer. The separator may also include a non-woven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, or the like.

[0107] In some embodiments, the area (eg, contact area with the separator 140) and / or volume of the negative electrode 130 may be larger than that of the positive electrode 100. Therefore, lithium ions generated from the positive electrode 100 may migrate smoothly to the negative electrode 130 without being precipitated in between.

[0108] According to an exemplary embodiment, a battery cell is defined by a positive electrode 100, a negative electrode 130, and a separator 140, and a plurality of battery cells may be stacked to form, for example, an electrode assembly 150 in the form of a jelly roll. For example, the electrode assembly 150 may be formed by winding, laminating, folding, or the like the separator 140.

[0109] The electrode assembly 150 may be housed within the case 160 together with the electrolyte of the exemplary embodiment to define a lithium secondary battery.

[0110] like Figure 1 As shown, the tabs (positive tabs and negative tabs) may 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 shell 160. The tabs may be fused to the one side of the shell 160 and form electrode leads (positive lead 107 and negative lead 127) extending to the outside of the shell 160 or exposed to the outside of the shell 160.

[0111] The lithium secondary battery may be manufactured in a cylindrical shape using a can, a prismatic shape, a pouch shape, a coin shape, or the like.

[0112] Below, exemplary embodiments are proposed to help understand 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 changes and modifications can be made to the embodiments within the scope and technical concept of the present invention, which is obvious to those skilled in the art, and such changes and modifications obviously also fall within the scope of the claims.

[0113] Examples and Comparative Examples

[0114] 1. Example 1

[0115] (1) Preparation of electrolyte

[0116] Fluoroethylene carbonate (FEC) (manufactured by Tokyo Chemical Industry) and fluoroethyl methyl carbonate (FEMC) (manufactured by Synquest Labs) were mixed in a volume ratio of 1:4 and stirred for 12 hours to prepare a mixed solvent. 1M LiPF6 was dissolved in the mixed solvent and stirred for 12 hours to prepare an electrolyte.

[0117] (2) Manufacturing of lithium secondary batteries

[0118] As the positive electrode active material, Li[Ni 0.6 Co 0.2 Mn 0.2 O2, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 92:5:3 to prepare a slurry. The slurry was evenly coated on a 15μm thick aluminum foil and vacuum dried at 130°C. The dried slurry was rolled to produce a positive electrode for a lithium secondary battery with a density of 3.667g / cm³.

[0119] A lithium metal foil (thickness: 20 μm) (manufactured by Honzo) was used as the negative electrode.

[0120] A separator (polypropylene) was placed between the positive electrode and the negative electrode to form an electrode assembly. The electrode assembly and the electrolyte prepared in (1) were added to a casing and vacuum-sealed to manufacture a lithium secondary battery.

[0121] 2. Comparative Example 1

[0122] Ethylene carbonate (EC) (manufactured by Enchem) and ethyl methyl carbonate (EMC) (manufactured by Enchem) as organic solvents were mixed in a volume ratio of 3:7 and stirred for 12 hours to prepare a mixed solvent. 1M LiPF6 was dissolved in the mixed solvent and FEC was added, followed by stirring for 12 hours to prepare an electrolyte. Afterwards, 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.

[0123] 3. Comparative Example 2

[0124] An electrolyte was prepared by the same method as in Example 1, except that FEMC, a fluorine-containing linear asymmetric carbonate, was not used. Instead, FEC and bis(2,2,2-trifluoroethyl) carbonate (DFDEC) (manufactured by Tokyo Chemical Industry Co., Ltd.), a fluorine-containing linear asymmetric carbonate, were mixed at a volume ratio of 1:4. Subsequently, 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.

[0125] 4. Comparative Example 3

[0126] An electrolyte was prepared by the same method as in Comparative Example 2, except that 1.5 M LiPF 6 was used instead of 1 M LiPF 6. Thereafter, a lithium secondary battery was manufactured by the same method as in Example 1, except that the electrolyte was used instead of the electrolyte in Example 1.

[0127] 5. Comparative Example 4

[0128] An electrolyte was prepared by the same method as in Example 1, except that FEC and FEMC, as fluorine-based solvents, were mixed at a volume ratio of 0.8:9.2, 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.

[0129] 6. Comparative Example 5

[0130] An electrolyte was prepared by the same method as in Example 1, except that FEC and FEMC, as fluorine-based solvents, were mixed at a volume ratio of 8.2:1.8, 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.

[0131] In the examples and comparative examples, the volume ratio (v / v) of the solvent is recorded in Table 1 below, the content (weight %) of the solvent and the lithium salt relative to the total weight of the electrolyte is recorded in Table 2 below, and the content of fluorine, phosphorus, oxygen, carbon, and hydrogen contained in the electrolyte is recorded in Table 3 below.

[0132] [Table 1]

[0133]

[0134] [Table 2]

[0135]

[0136] [Table 3]

[0137]

[0138] evaluate

[0139] Evaluation 1: Evaluation of lithium deposition

[0140] At room temperature (25° C.), the lithium secondary batteries according to Examples and Comparative Examples were charged at a constant current of 0.1 C to 4.3 V, and then charged at a constant voltage to a current of 0.01 C while maintaining 4.3 V. Thereafter, the shape of lithium deposited on the surface of the copper foil was evaluated using a scanning electron microscope (SEM).

[0141] Figure 3a and Figure 3b These are photographs showing the evaluation of the deposition shapes of lithium secondary batteries manufactured according to Example 1 and Comparative Example 1. Figure 3b (A) in the graph represents deposited lithium. Figure 3b (B) in the figure represents copper foil.

[0142] Reference Figure 3a and Figure 3b In Example 1, lithium is densely formed on the surface of the copper foil as a whole, and the formation of dendrites is also suppressed. However, in Comparative Example 1, lithium is unevenly deposited on the surface of the copper foil, and the growth of dendrites is not suppressed.

[0143] The evaluation results are shown in Table 4 below.

[0144] Evaluation 2: Evaluation of normal voltage life

[0145] At room temperature (25° C.), the lithium secondary batteries according to Examples and Comparative Examples were charged to 4.3 V at a constant current of 0.5 C, and then charged to a current of 0.05 C while maintaining 4.3 V. Thereafter, they were discharged to 2.7 V at 0.5 C to measure the initial discharge capacity.

[0146] The above charge and discharge were repeated 80 times, and the discharge capacity of each cycle was divided by the initial capacity to calculate the capacity retention ratio (%).

[0147] Figure 4 is a graph showing capacity retention rates (2.7 V-4.3 V at 80 cycles) according to repeated charge and discharge of lithium secondary batteries manufactured according to Example 1 and Comparative Examples 1 to 3.

[0148] Reference Figure 4 The capacity retention rate of Example 1 decreases slightly after 60 cycles, but the capacity retention rates of Comparative Examples 1 and 3 decrease sharply after 30 cycles. The capacity retention rate of Comparative Example 2 decreases sharply at the beginning of repeated charge and discharge and becomes 0% after 18 cycles.

[0149] The evaluation results of the capacity retention rate after 80 cycles are shown in Table 4 below.

[0150] Evaluation 3: Evaluation of high voltage life

[0151] At room temperature (25° C.), the lithium secondary batteries according to Examples and Comparative Examples were charged to 4.5 V at a constant current of 0.5 C, and then charged to a current of 0.05 C while maintaining 4.5 V. Thereafter, they were discharged to 2.7 V at 0.5 C to measure the initial discharge capacity.

[0152] The above charge and discharge were repeated 55 times, and the discharge capacity of each cycle was divided by the initial capacity to calculate the capacity retention rate (%).

[0153] Figure 5 is a graph showing capacity retention rates (2.7 V-4.5 V at 55 cycles) according to repeated charge and discharge of lithium secondary batteries manufactured according to Example 1 and Comparative Examples 1 to 3.

[0154] Reference Figure 5 The capacity retention rate of Example 1 decreases slightly after 30 cycles, but the capacity retention rate of Comparative Example 1 decreases sharply after 22 cycles. The capacity retention rates of Comparative Examples 2 and 3 decrease sharply at the beginning of repeated charge and discharge, and become 0% after 11 cycles and 17 cycles, respectively.

[0155] The evaluation results of the capacity retention rate after 55 cycles are shown in Table 4 below.

[0156] [Table 4]

[0157]

[0158] Referring to Tables 3 and 4, the electrolyte of Example 1, which contains a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a predetermined volume ratio, has a fluorine content of 30 wt % or more, and the dendrite suppression capability, the capacity retention rate at conventional voltage, and the capacity retention rate at high voltage are all improved.

[0159] In Comparative Example 1, in which FEC was added but the EC / EMC mixed solvent was mainly used, the dendrite suppression capability, the capacity retention ratio at normal voltage, and the capacity retention ratio at high voltage were all deteriorated.

[0160] Comparative Examples 2 and 3, which contain a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate, have poor dendrite suppression capabilities, capacity retention ratios at normal voltages, and capacity retention ratios at high voltages.

[0161] The capacity retention rates at normal voltage and high voltage of Comparative Examples 4 and 5, in which the volume ratio of the fluorinated cyclic carbonate to the fluorinated linear asymmetric carbonate is not within a specific range, are improved compared with Comparative Example 1, but the capacity retention rates at normal voltage and high voltage are worse than those in Example 1.

[0162] Therefore, it can be seen that the electrolyte of Example 1 satisfying the predetermined composition can be used to provide a lithium secondary battery having improved high-voltage stability and lifespan characteristics.

Claims

1. An electrolyte for a lithium secondary battery, comprising a fluorine-containing cyclic carbonate and a fluorine-containing linear asymmetric carbonate in a volume ratio of 1:9 to 8:

2.

2. The electrolyte for lithium secondary batteries according to claim 1, wherein The fluorine-containing straight-chain asymmetric carbonate is non-polymerizable.

3. The electrolyte for lithium secondary batteries according to claim 1, wherein The fluorine-containing cyclic carbonate has a structure represented by the following Chemical Formula 1: [Chemical Formula 1] In Chemical Formula 1, R1 to R4 are each independently a hydrogen atom or a fluorine atom, and at least one of R1 to R4 is a fluorine atom.

4. The electrolyte for lithium secondary batteries according to claim 1, wherein The fluorine-containing cyclic carbonate includes at least one selected from fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC) and trifluoroethylene carbonate (TFEC).

5. The electrolyte for lithium secondary batteries according to claim 1, wherein The fluorine-containing linear asymmetric carbonate has a structure represented by the following Chemical Formula 2: [Chemical Formula 2] In Chemical Formula 2, R5 is an alkyl group having 1 to 5 carbon atoms and not containing fluorine, and R6 is an alkyl group having 1 to 5 carbon atoms in which at least one hydrogen atom is substituted with a fluorine atom.

6. The electrolyte for lithium secondary batteries according to claim 1, wherein The fluorine-containing linear asymmetric carbonate includes at least one selected from fluorodimethyl carbonate (FDMC), fluorodiethyl carbonate (FDEC) and fluoroethylmethyl carbonate (FEMC).

7. The electrolyte for lithium secondary batteries according to claim 1, wherein The content of the fluorine-containing cyclic carbonate and the fluorine-containing linear asymmetric carbonate is 70 weight % or more of the total weight of the electrolyte.

8. The electrolyte for lithium secondary batteries according to claim 1, wherein The electrolyte for a lithium secondary battery further includes a lithium salt.

9. The electrolyte for lithium secondary batteries according to claim 8, wherein The lithium salt comprises a salt selected from the group consisting of LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, LiSO3CF3, LiBOB, LiFOB, LiDFOB, LiDFBP, LiTFOP, LiPO2F2, LiCl, LiBr, LiI, LiB 10 Cl 10 , at least one of LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, LiSCN and LiC(CF3SO2)3.

10. The electrolyte for lithium secondary batteries according to claim 1, wherein The lithium secondary battery electrolyte has a fluorine content of 30% by weight or more.

11. The electrolyte for lithium secondary batteries according to claim 1, wherein The electrolyte for a lithium secondary battery has an oxygen content of 34 wt % or less.

12. A lithium secondary battery comprising: case; an electrode assembly housed in the housing and comprising a positive electrode and a negative electrode; as well as The lithium secondary battery electrolyte according to any one of claims 1 to 11, wherein the electrolyte is injected into the case.

13. The lithium secondary battery according to claim 12, wherein The negative electrode includes lithium metal or graphite.

14. The lithium secondary battery according to claim 12, wherein The electrode assembly further includes a separator disposed between the positive electrode and the negative electrode.

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