Electrolyte for lithium secondary battery, and lithium secondary battery comprising same

Abstract

The present invention relates to: an electrolyte for a lithium secondary battery, which may improve ion conductivity and lifespan characteristics of the lithium secondary battery; and a lithium secondary battery comprising same.

Description

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

[0001] Cross-citation with related application(s)

[0002] This application claims the benefit of priority to Korean Patent Application No. 10-2023-0148871, filed November 1, 2023, and Korean Patent Application No. 10-2024-0140522, filed October 15, 2024, the entire contents of which are incorporated herein by reference.

[0003] The present invention relates to an electrolyte for a lithium secondary battery capable of improving the ionic conductivity and lifespan characteristics of a lithium secondary battery, and a lithium secondary battery including the same.

[0004] Lithium metal batteries, which use lithium metal as the cathode, are attracting attention as next-generation batteries that can overcome the low capacity of existing lithium-ion batteries. However, the currently used ether-based electrolytes have limited anode stability due to their low oxidation stability. Therefore, 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB), which has high oxidation stability, has been studied as an electrolyte material. However, FDMB also has limitations due to its high viscosity, which limits its ionic conductivity. Therefore, the development of electrolytes that secure both ionic conductivity and oxidation stability through combinations of different solvents and other materials is required.

[0005] Meanwhile, in order to improve the ionic conductivity and charge / discharge speed of lithium secondary batteries such as lithium metal batteries, interest in so-called localized high concentration electrolytes (LHCEs) that contain organic non-solvents that exhibit relatively low solubility in lithium salts among electrolytes has been greatly increasing.

[0006] The more anion-derived components of lithium salts are present in the SEI layer, the more stable the membrane becomes against decomposition. However, when an organic nonsolvent is used in the electrolyte, the organic nonsolvent does not participate in the solvation structure of the Li ions, and instead, a large amount of anion-derived SEI is formed.

[0007] However, in the case of 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which is most commonly used as the organic non-solvent, it does not participate in the solvation structure, so although the SEI contains a lot of anions, the ionic conductivity is low due to high viscosity.

[0008] Accordingly, there is a continuous demand for the development of an electrolyte that can improve the ionic conductivity and lifespan characteristics of a lithium secondary battery, for example, a lithium metal secondary battery to which the LHCE is applied.

[0009] Accordingly, the present invention provides an electrolyte for a lithium secondary battery capable of improving the ionic conductivity and lifespan characteristics of a lithium secondary battery, and a lithium secondary battery including the electrolyte.

[0010] Accordingly, the present invention relates to a lithium salt;

[0011] Non-aqueous organic solvents including fluorine-substituted ether solvents; and

[0012] Contains an organic anti-solvent that exhibits a solubility in the lithium salt that is at least 10 times lower than that of the non-aqueous organic solvent,

[0013] The above organic non-solvent provides an electrolyte for a lithium secondary battery including a compound represented by the following chemical formula 1.

[0014] [Chemical Formula 1]

[0015]

[0016] In the above chemical formula 1,

[0017] Ra to Rj are the same or different, and each independently represents hydrogen or a fluoro group, and at least five of Ra to Rj are fluoro groups.

[0018] The present invention also provides a lithium secondary battery comprising a positive electrode including a positive active material; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte of the present invention.

[0019] In such a lithium secondary battery, the negative electrode may be in the form of a lithium metal secondary battery including a lithium metal layer.

[0020] The electrolyte of the present invention comprises a fluorine-substituted ether solvent such as FDMB as a non-aqueous organic solvent, and a compound represented by Chemical Formula 1 as an organic non-solvent. By combining the non-aqueous organic solvent and the organic non-solvent, it was confirmed that viscosity can be reduced and ionic conductivity can be improved in the LHCE-based electrolyte. This is expected because the compound represented by Chemical Formula 1, unlike TTE, a representative organic non-solvent, can participate in the solvation process of a lithium salt.

[0021] In addition, the electrolyte exhibits excellent oxidation stability, and due to this oxidation stability, it was confirmed that when the electrolyte is used in a lithium secondary battery, for example, a lithium metal secondary battery, the life characteristics and anode stability of the battery can be improved, and the battery performance can be improved by inducing effective SEI and CEI formation.

[0022] Figure 1 shows the results of NMR analysis of changes in chemical species in the electrolyte to analyze the solvation structure of the electrolytes of examples and comparative examples, and a schematic diagram of the solvation structure predicted therefrom.

[0023] Figures 2 and 3 show the results of evaluating the life characteristics under different current densities for lithium symmetric cells including the electrolytes of the examples and comparative examples.

[0024] Figure 4 shows the results of measuring the resistance of the SEI film over time for lithium symmetric cells including the electrolytes of the examples and comparative examples.

[0025] Figure 5 shows the results of measuring the amount of deactivated lithium for lithium symmetric cells including electrolytes of examples and comparative examples.

[0026] Figure 6 shows the results of evaluating the resistance of the SEI film per cycle for lithium symmetric cells including the electrolytes of the examples and comparative examples.

[0027] Figure 7 shows the results of electron microscopic analysis of the SEI film formed after 50 cycles for lithium symmetric cells including the electrolytes of the examples and comparative examples, and Figure 8 shows the thickness of the SEI film measured therefrom.

[0028] Figure 9 shows the results of component analysis of the SEI film formed after 50 cycles for lithium symmetric cells including the electrolytes of the examples and comparative examples.

[0029] Figure 10 shows the results of evaluating the oxidation stability of each electrolyte using an evaluation cell including the electrolytes of the examples and comparative examples.

[0030] Figures 11a to 11c show the results of evaluating the cycle characteristics of full cells containing the electrolytes of the examples and comparative examples.

[0031] Figure 12 shows the results of electron microscopic analysis of cracks occurring in positive electrode particles after 20 cycles for full cells including electrolytes of examples and comparative examples.

[0032] Figure 13 shows the thickness of the CEI film measured from electron microscopic analysis of the CEI film formed after 20 cycles for the full cell including the electrolyte of the examples and comparative examples.

[0033] Figure 14 shows the results of evaluating the cycle characteristics of pouch cells containing the electrolytes of the examples and comparative examples.

[0034] Figure 15 shows the results of evaluating the cycle characteristics of full cells including electrolytes and LFP cathode materials of examples and comparative examples.

[0035] Hereinafter, a lithium secondary battery electrolyte and a lithium secondary battery including the same according to a specific embodiment of the invention will be described.

[0036] Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best way.

[0037] The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0038] In this specification, “fluoro group” means -F.

[0039] In this specification, it should be understood that the terms “include,” “comprising,” or “having” are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0040]

[0041] According to one embodiment of the invention, lithium salt;

[0042] Non-aqueous organic solvents including fluorine-substituted ether solvents; and

[0043] Contains an organic anti-solvent that exhibits a solubility in the lithium salt that is at least 10 times lower than that of the non-aqueous organic solvent,

[0044] The organic non-solvent provides an electrolyte for a lithium secondary battery comprising a compound represented by the following chemical formula 1.

[0045] [Chemical Formula 1]

[0046]

[0047] In the above chemical formula 1,

[0048] Ra to Rj are the same or different, and each independently represents hydrogen or a fluoro group, and at least five of Ra to Rj are fluoro groups.

[0049] The electrolyte of this embodiment may fall into the category of localized high-concentration electrolytes (LHCE), and may exhibit a high-concentration region in which a lithium salt is dissolved in a non-aqueous organic solvent at a high concentration during charging and discharging of a battery, and a form in which the organic non-solvent is distributed around the high-concentration region. Accordingly, when the electrolyte is used, the output characteristics of the battery may be improved due to the high-concentration distribution of the lithium salt. In addition, depending on the organic non-solvent distribution region, side reactions between the lithium metal layer and the electrolyte may be suppressed, and the life characteristics of a lithium secondary battery may be improved.

[0050] Meanwhile, in the electrolyte of the above embodiment, the lithium salt is used as a medium for transferring ions within a lithium secondary battery. The lithium salt may be, for example, Li as a cation. + Including, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B10 Cl 10 - , AlCl4 - , AlO2 - , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N - , BF2C2O4 - , BC4O8 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - and SCN - It may include an anion selected from the group consisting of .

[0051] Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10, LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl) imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl) imide, LiN(SO2CF2CF3)2) and LiTFSI (lithium bis(trifluoromethanesulfonyl) imide, LiN(SO2CF3)2), and may include at least one selected from the group consisting of LiFSI or LiTFSI, taking into consideration the performance of the lithium secondary battery and the solubility in the non-aqueous organic solvent and organic non-solvent, etc.

[0052] The concentration of the lithium salt may be included in the electrolyte at a concentration of 1.0 to 2.5 M, or 1.0 to 2.0 M, taking into account the performance of the lithium secondary battery. The concentration of the lithium salt may be defined as a molar concentration considering the total volume of the non-aqueous organic solvent and the organic non-solvent. By a high concentration of the lithium salt, the desolvation of lithium ions may be further accelerated, and the performance of the lithium secondary battery may be improved.

[0053] In addition, the fluorine-substituted ether solvent may include 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB) or 2,2,3,3-tetrafluoro-1,4-diethoxybutane (FDEB), and in addition to the fluorine-substituted ether solvent, additional ether solvents and / or carbonate solvents may be further included among the non-aqueous organic solvents.

[0054] In addition to the above fluorine-substituted ether solvent, an ether solvent that may be additionally included in the non-aqueous organic solvent may be an aliphatic ether solvent, and examples thereof include dimethyl ether, dibutyl ether, tetraglyme, diglyme, or dimethoxy ethane.

[0055] In addition, in addition to the above-mentioned fluorine-substituted ether solvent, carbonate-based solvents that may be additionally included in the non-aqueous organic solvent include dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, methyl propyl carbonate, ethyl methyl carbonate, ethyl propyl carbonate, or methyl (2,2,2-trifluoroethyl) carbonate.

[0056] The electrolyte of the above embodiment comprises a non-aqueous organic solvent that dissolves a lithium salt and acts as a migration path for lithium ions, and an organic non-solvent that exhibits a solubility in the lithium salt that is 10 times or more, or 10 to 30 times less than that of the non-aqueous organic solvent, so that the lithium salt is substantially insoluble. Such an organic non-solvent may be defined as exhibiting substantially no solubility in the lithium salt, and being capable of dissolving the lithium salt only in a concentration of, for example, 0.1 M or less, or 0 to 0.1 M, or 0 to 0.05 M.

[0057] The above organic nonsolvent includes a compound represented by the above chemical formula 1. The compound represented by the above chemical formula 1 has a structure based on diethyl ether and substituted with five or more fluoro groups. When the compound having the structure of the above chemical formula 1 is included in the organic nonsolvent, it has the effect of lowering the viscosity of the electrolyte and improving the ionic conductivity compared to 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), which is the most commonly used organic nonsolvent. The reason for this is that the TTE does not participate in the solvation structure of the lithium salt, but the compound of the above chemical formula 1, unlike TTE, can participate in the solvation process of the lithium salt.

[0058] In the above chemical formula 1, Ra to Rj each represent a substituent.

[0059] According to one embodiment of the present invention, five or six of Ra to Rj may be fluoro groups, and the remainder may be hydrogen.

[0060] In addition, in the above chemical formula 1, 5 or 6 of Ra to Rc and Rh to Rj may be fluoro groups, and the remaining substituents other than fluoro groups among Ra to Rc and Rh to Rj and Rd to Rg may be hydrogen.

[0061] According to another embodiment of the present invention, the compound represented by the chemical formula 1 may be bis(2,2,2-trifluoroethyl) ether (BTFE), and when the compound of the chemical formula 1, specifically BTFE, is included as an organic non-solvent in the electrolyte of the present invention, it has the effect of lowering the viscosity of the electrolyte and improving the ionic conductivity.

[0062] An embodiment of the electrolyte further comprising the organic nonsolvent together with the nonaqueous organic solvent may include a region in which a high concentration of lithium salt is locally present within the nonaqueous organic solvent, and an organic nonsolvent distribution region in which the lithium salt is substantially absent. In this way, since a high concentration of lithium salt is locally present in a solvated form within the electrolyte, the performance, such as the output characteristics, of a lithium secondary battery can be further improved, while an increase in electrolyte viscosity and a decrease in fluidity can be reduced due to the organic nonsolvent distribution region.

[0063] In the electrolyte of the above embodiment, the amount of the organic non-solvent used can be adjusted depending on the type of the non-aqueous organic solvent and the lithium salt, or the overall concentration of the lithium salt. For example, the organic non-solvent: the non-aqueous organic solvent can be included in a volume ratio of 1:0.5 to 1:1.5, or 1:1.

[0064] According to one embodiment of the invention, the electrolyte may further include an additive such as lithium nitrate (LiNO3), lithium difluoro oxalate phosphate (LiDFOP), lithium tetrafluoroborate (LiBF4), lithium bis-oxalate borate (LiBOB), lithium difluorooxalate borate (LiDFOB), or fluoroethylene carbonate (FEC).

[0065] Meanwhile, according to another embodiment of the invention, a lithium secondary battery comprising the electrolyte of the above-described embodiment is provided. The lithium secondary battery comprises a positive electrode comprising a positive electrode active material; a negative electrode; a separator between the positive electrode and the negative electrode; and a lithium secondary battery comprising the electrolyte of the above-described embodiment.

[0066] According to the above embodiment, the negative electrode may be in the form of a lithium metal secondary battery including a lithium metal layer.

[0067] First, in the lithium secondary battery of the above-described other embodiment, the negative electrode may be formed with a lithium metal layer on one or both sides of a planar negative electrode collector according to the general configuration of the lithium secondary battery, and may be manufactured by depositing lithium metal on the negative electrode collector or rolling lithium foil.

[0068] The above negative electrode current collector can be formed using any metal that has been previously known to be usable as a negative electrode current collector and has high conductivity without causing chemical changes in the battery.

[0069] Specific examples thereof include metals such as stainless steel, aluminum, nickel, titanium, or copper, or surfaces of copper, aluminum, or stainless steel treated with carbon, nickel, titanium, silver, etc. These negative electrode current collectors can be formed into various forms such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

[0070] Additionally, the negative electrode current collector may have a thickness of 3 to 100 μm, and the lithium metal layer may have a thickness of, for example, 1 to 300 μm.

[0071] Meanwhile, the positive electrode may include a positive electrode current collector and a positive electrode active material layer positioned on the positive electrode current collector.

[0072] These positive electrodes can be manufactured by mixing positive electrode active materials and binders, and in some cases, conductive materials, fillers, etc. in a solvent to prepare a positive electrode slurry composition, and applying the composition to a positive electrode current collector.

[0073] The positive electrode current collector may generally have a thickness of 3 to 500 μm. In addition, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The current collector may also form fine unevenness on its surface to increase the adhesive strength of the positive electrode active material, and various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric are possible.

[0074] And, the positive electrode active material may include a lithium transition metal oxide including lithium and at least one transition metal selected from the group consisting of nickel, manganese, cobalt, and iron.

[0075] Specifically, the lithium transition metal oxide is a lithium-manganese oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt oxide (e.g., LiCoO2, etc.), a lithium-nickel oxide (e.g., LiNiO2, etc.), a lithium-nickel-manganese oxide (e.g., LiNi 1-Y Mn Y O2 (here, 0 <Y<1), LiMn 2-Z Ni Z O4 (where 0<Z<2), etc.), lithium-nickel-cobalt oxides (e.g., LiNi 1-Y1 Co Y1 O2 (here, 0 <Y1<1) 등), 리튬-망간-코발트계 산화물(예를 들면, LiCo 1-Y2 Mn Y2 O2 (here, 0 <Y2<1), LiMn 2-Z1 Co Z1 O4 (wherein, 0<Z1<2) etc.), lithium-nickel-manganese-cobalt oxides (e.g., Li(Ni p Co q Mn r)O2(wherein, 0<p<1, 0<q<1, 0<r<1, p+q+r=1) or Li(Ni p1 Co q1 Mn r1 )O4 (wherein, 0<p1<2, 0<q1<2, 0<r1<2, p1+q1+r1=2) etc.), lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )O2 (wherein, M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r2 and s2 are atomic fractions of independent elements, respectively, 0<p2<1, 0<q2<1, 0<r2<1, 0<s2<1, p2+q2+r2+s2=1), etc.), or lithium iron phosphate (e.g., Li 1+a Fe 1-x M x (PO 4-b )X b (Here, M is at least one selected from Al, Mg, and Ti, X is at least one selected from F, S, and N, -0.5≤a≤+0.5, 0≤x≤0.5, 0≤b≤0.1), and any one or two or more oxides thereof may be included.

[0076] The above-described positive electrode active material may be included in an amount of 60 to 99 wt%, or 70 to 99 wt%, or 80 to 98 wt% based on the total weight of the positive electrode active material layer.

[0077] Meanwhile, the conductive material included in the positive electrode active material layer is a component for further improving the conductivity of the positive electrode active material. The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, carbon powder such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; conductive nanomaterials such as carbon nanofibers or carbon nanotubes; fluorinated carbon powder; conductive powder such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used. Among these, the conductive material can further lower the resistance of the lithium metal secondary battery and improve the output characteristics, etc. by including a conductive nanomaterial such as carbon nanotubes or carbon nanofibers.

[0078] Typically, the conductive material may be included in an amount of 1 to 20 wt%, or 1 to 15 wt%, or 1 to 10 wt% based on the total weight of the positive electrode active material layer.

[0079] The binder optionally included in the above-described positive electrode active material layer is a component that assists in the bonding of the positive electrode active material and the conductive material, and in the bonding to the current collector. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, nitrile rubber, styrene-butadiene rubber, or fluororubber, and a mixture or copolymer of two or more selected from these may also be used.

[0080] Typically, the binder may be included in an amount of 1 to 20 wt%, or 1 to 15 wt%, or 1 to 10 wt% based on the total weight of the positive electrode active material layer.

[0081] Additionally, a filler may be optionally added to the positive electrode as a component that suppresses its expansion. Such filler is not particularly limited as long as it can suppress the expansion of the electrode without causing chemical changes in the battery. Examples of fillers that can be used include olefin polymers such as polyethylene and polypropylene; fibrous materials such as glass fiber and carbon fiber; and the like.

[0082] The above-described positive electrode can be manufactured by dispersing and mixing the positive electrode active material, binder, and conductive agent in a dispersion medium (solvent) to make a slurry, applying the slurry on a metal current collector, and then drying and rolling. At this time, the dispersion medium may be NMP (N-methyl-2-pyrrolidone), DMF (Dimethyl formamide), DMSO (Dimethyl sulfoxide), ethanol, isopropanol, water, and mixtures thereof, but is not necessarily limited thereto.

[0083] Meanwhile, the lithium secondary battery described above may further include a porous separator interposed between the positive electrode and the negative electrode.

[0084] These porous membranes can be made of olefin polymers such as polyethylene (PE), polypropylene (PP), glass fibers, etc. in the form of sheets, multi-membranes, microporous films, woven fabrics, and non-woven fabrics, but are not necessarily limited thereto. However, it may be preferable to use porous polyethylene or porous glass fiber non-woven fabric (glass filter) as the membrane, and it may be more preferable to use porous glass filter (glass fiber non-woven fabric) as the membrane. The membrane may be an insulating thin film having high ion permeability and mechanical strength, and the pore diameter of the membrane may generally be in the range of 0.01 to 10 ㎛, and the thickness may generally be in the range of 5 to 300 ㎛, but is not limited thereto.

[0085] Meanwhile, the lithium secondary battery described above can be manufactured using a conventional method in the art. For example, the electrode assembly including the positive electrode, negative electrode, and separator can be housed in a case, and manufactured by injecting and impregnating the electrolyte described above.

[0086] These lithium secondary batteries are not only applicable to battery cells used as power sources for small devices, but are also particularly suitable for use as unit batteries of battery modules that serve as power sources for medium and large devices. In consideration of an appropriate discharge rate for each purpose, the batteries of the above-described embodiment or other embodiments can be selectively used.

[0087] Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

[0088]

[0089] Example 1, Comparative Example 1 and Comparative Example 2: Preparation of electrolyte

[0090] The non-aqueous organic solvent of FDMB and the organic non-solvent of BTFE were mixed in a volume ratio of 1:1, and lithium salt (LiFSI) was dissolved in a concentration of 1 M to prepare the electrolyte of Example 1.

[0091] In addition, the electrolyte of Comparative Example 1 (containing only the non-aqueous organic solvent of FDMB) without using the organic non-solvent and the electrolyte of Comparative Example 2 (FDMB+TTE) using the organic non-solvent of TTE instead of the BTFE were prepared, respectively.

[0092]

[0093] Experimental Example 1: Analysis of the solvation structure and ionic conductivity of electrolytes

[0094] A symmetrical cell was manufactured by combining 50 μL of the electrolyte of the example or comparative example, 40 μm of working electrode Li, 150 μm of counter electrode Li, and PP as separator, and the current density was 1 mA / cm. 2 , Capacity: 1 mAh / cm 2 An electrochemical reaction was performed under the conditions.

[0095] During this process, changes in chemical species in the electrolyte were analyzed using NMR, and the solvation structure of the electrolyte was analyzed based on the analysis results. The analysis results and a schematic diagram of the solvation structure predicted through this analysis are illustrated in Figure 1.

[0096] Referring to Figure 1, the electrolyte of Example 1 mixed FDMB / BTFE in a volume ratio of 1:1, which increased the ion pairing of Li and FSI anions compared to Comparative Example 1, which used FDMB alone, and this was predicted to affect effective interface formation. In addition, BTFE participates in the solvation structure, whereas TTE does not. Accordingly, Example 1, which added BTFE, showed improved ion conductivity compared to Comparative Example 2, which added TTE.

[0097] For more clear confirmation of this, the ionic conductivity was measured in the above symmetric cell and is shown in Table 1 below.

[0098] Sample ionic conductivity σ ion [mS cm -1 ]Overvoltageη[mV]Cycles[number]Comparative Example 1(FDMB)0.41127.7162Example 1(FDMB:BTFE 1:1(v / v))0.4686.0175Comparative Example 2(FDMB:TTE 1:1 (v / v))0.24132.3145

[0099] Referring to Table 1 above, when the electrolyte of Example 1 was used, higher ionic conductivity and lower overvoltage were observed compared to Comparative Examples 1 and 2, and thus the cell life was improved.

[0100]

[0101] Experimental Example 2: Life Characteristics Evaluation

[0102] Using the same lithium symmetric cell used in Experimental Example 1 above, the life characteristics were evaluated when the electrolytes of the examples and comparative examples were used, and the evaluation results are shown in Fig. 2. At this time, the current density applied to the symmetric cell: 1 mA / cm 2 , Capacity: 1 mAh / cm 2 It was done as follows.

[0103] Additionally, the current density applied to each symmetric cell was 2 mA / cm 2 The life characteristics were evaluated using the same method by increasing the value, and the evaluation results are shown in Fig. 3.

[0104] Referring to FIGS. 2 and 3, it can be observed that the lithium symmetric cell lifespan is improved and the overvoltage is improved when the electrolyte of Example 1 is used, and it can be confirmed that it contributes to the formation of a more effective interface for lithium ion transfer.

[0105]

[0106] Experimental Example 3: Lithium Corrosion Evaluation

[0107] A symmetrical cell was manufactured by combining 50 μL of the electrolyte of the example or comparative example, 150 μm of working electrode Li, 150 μm of counter electrode Li, and PP as a separator.

[0108] The SEI film interface resistance of the manufactured symmetric cell was measured using the EIS (Electroscopy Impedance Spectroscopy) analysis method at room temperature (25°C) for 1 hour, 2 hours, 5 hours, and then at 5-hour intervals up to 60 hours. The results are shown in Fig. 4.

[0109] Referring to Fig. 4, it was confirmed that when the electrolyte of Example 1 was used, the resistance of the SEI film became the lowest over time. This is because when the electrolyte of Example 1 was used, an SEI film was formed that well protected the surface of lithium, thereby suppressing further reaction between the electrolyte and lithium, and accordingly, the amount of lithium corrosion is also expected to be the lowest.

[0110]

[0111] Experimental Example 4: Measurement of the amount of inactive lithium

[0112] A symmetrical cell was manufactured by combining 50 μL of the electrolyte of the example or comparative example, 150 μm of Li as a working electrode, 150 μm of Cu as a counter electrode, and PP as a separator.

[0113] Ten charge and discharge cycles were performed on the manufactured symmetrical cell, and the Cu electrode obtained by disassembling the symmetrical cell on which 10 cycles were performed was placed in a vial. Then, water was injected and the amount of H2 generated by reflection with Li on the surface of the Cu electrode was measured to calculate the amount of deactivated lithium, and the measured amount of lithium is shown in Fig. 5. The following reaction formula was referred to in order to calculate the amount of deactivated lithium.

[0114] [Reaction formula: 2Li + 2H2O → H2+ 2LiOH]

[0115] Referring to Figure 5, it can be confirmed that the smallest amount of inactive lithium is generated when the electrolyte of Example 1 is used. This is interpreted to be because the fast electrolyte of the present invention has fast ionic conductivity, enabling uniform deposition / deposition of lithium.

[0116]

[0117] Experimental Example 5: SEI Film Thickness, Resistance Evaluation, and Component Analysis

[0118] For the lithium symmetric cell used in Experimental Example 1, the resistance of the SEI film formed on the negative electrode was evaluated while performing charge and discharge cycles, and the evaluation results are shown in Fig. 6. The table described in Fig. 6 shows the resistance values ​​of the SEI film according to the charge and discharge cycles of the examples and comparative examples. Referring to Fig. 6, it was confirmed that the SEI film resistance after 50 cycles in Example 1 was the lowest.

[0119] Additionally, after the 50 cycles, each SEI film was analyzed by electron microscopy and shown in Fig. 7, and the thickness of the SEI film measured therefrom is shown in Fig. 8.

[0120] Referring to Figures 7 and 8, it was confirmed that the thickness of the SEI film was the thinnest when the electrolyte of Example 1 was used.

[0121] In addition, after 50 cycles, the composition of the SEI film was analyzed using XPS (X-ray Photoelectron Spectroscopy), and the results are shown in Fig. 9. From Fig. 9, it can be confirmed that when the electrolyte of Example 1 was used, the SEI film contained a lot of inorganic components (Li2O, Li3N, Li2S, Li2S2). The SEI film containing a lot of these inorganic components protects the surface of lithium well and enables uniform deposition / deposition of lithium.

[0122] From the above resistance evaluation results, SEI film thickness evaluation results, and SEI film component analysis results, it can be predicted that overvoltage is hardly generated in Example 1.

[0123]

[0124] Experimental Example 6: Evaluation of Oxidation Stability and Cycle Characteristics

[0125] An evaluation cell was manufactured by combining 75 μL of the electrolyte of the example or comparative example, carbon as a working electrode, Li 150 μm as a counter electrode, and PP as a separator, and the oxidation stability of each electrolyte was evaluated, and is shown in Fig. 10.

[0126] Referring to Fig. 10, when the electrolyte of Example 1 was used, a higher oxidation potential was exhibited than that of Comparative Example 1, similar to Comparative Example 2, and it was confirmed from this that the electrolyte of the Example exhibited high oxidation stability and enabled the use of a high-voltage positive electrode.

[0127] Meanwhile, 200 μL of the electrolyte of the example or comparative example, the cathode material NCM811 (LiNi 0·8 Co 0·1 Mn 0·1 A full cell was manufactured by combining O2), a negative lithium metal layer Li 150 μm, and a separator PP, and after activation for 2 cycles at 0.1 C, charge and discharge were performed at a rate of 0.33 C.

[0128] From the results of these charge and discharge tests, the results of evaluating the cycle characteristics of Comparative Example 1 (Fig. 11a), Example 1 (Fig. 11b), and Comparative Example 2 (Fig. 11c) are shown in Figs. 10a to 10c, respectively.

[0129] Through this, it was confirmed that the capacity reduction was the lowest when Example 1 was used.

[0130]

[0131] Experimental Example 7: Evaluation of CEI film thickness and crack occurrence within anode particles

[0132] For the full cell manufactured in Experimental Example 6, 20 charge and discharge cycles were performed, and the cross-section of the positive electrode and the CEI film obtained by disassembling the cell were analyzed using an electron microscope (SEM, TEM), and the results are shown in Figures 12 and 13, respectively.

[0133] From Fig. 12, it can be confirmed that the least amount of cracks occurred when the electrolyte of Example 1 was used, and it is predicted that this is because the deposition / deposition reaction occurs uniformly due to the rapid movement of lithium ions in the positive electrode, and thus the occurrence of cracks due to volume shrinkage / expansion is small.

[0134] In addition, it can be confirmed from Figure 13 that the thickness of the CEI film becomes the thinnest when the electrolyte of Example 1 is used. This is expected to be because the amount of side reactions occurring from the cracks is small.

[0135]

[0136] Experimental Example 8: Pouch Cell Performance Evaluation

[0137] Cathode material NCM811 (LiNi 0·8 Co 0·1 Mn 0·1 O2), Li 40 μm, and PP separator were combined to manufacture a bi-cell, and then the electrode assembly was placed in a pouch-type case and filled with the electrolyte of the example or comparative example to manufacture a pouch cell (areal capacity of NCM811 3.8 mAhcm -2 , cathode capacity / anode capacity ratio (N / P) 2.16, electrolyte / cathode capacity ratio (E / C) 2.5 g Ah -1 ) The manufactured pouch cells were charged / discharged at 3.0-4.3 V 0.1 C / 0.5 C, and the results of the charge / discharge tests, evaluating the cycle characteristics of Example 1, Comparative Example 1, and Comparative Example 2, are shown in Fig. 14.

[0138] Through this, it was confirmed that when the electrolyte of Example 1 was used, a high discharge capacity was maintained for 100 cycles and the highest capacity retention rate was achieved. This is expected to be because the electrolyte of Example 1 has high ionic conductivity, forms structurally / chemically stable SEI and CEI, suppresses unnecessary side reactions, and exhibits almost no overvoltage.

[0139]

[0140] Experimental Example 9: Cycle Characteristics Evaluation

[0141] Except that the cathode material in Experimental Example 6 was LFP (LiFePO4), a full cell was manufactured in the same manner as in Experimental Example 6, and cycle characteristics were evaluated, and the results are shown in Fig. 15.

[0142] Referring to Figure 15, the discharge capacity was measured while increasing the cell operation speed for 55 cycles, and it can be confirmed that the highest discharge capacity was shown at all speeds when the electrolyte of Example 1 was used. Through this, it can be confirmed that the performance of the battery using the electrolyte of Example 1 is the best even when the type of positive electrode material is changed.

Claims

1. Lithium salt; Non-aqueous organic solvents including fluorine-substituted ether solvents; and Contains an organic anti-solvent that exhibits a solubility in the lithium salt that is at least 10 times lower than that of the non-aqueous organic solvent, The organic non-solvent is an electrolyte for a lithium secondary battery comprising a compound represented by the following chemical formula 1: [Chemical Formula 1] In the above chemical formula 1, Ra to Rj are the same or different, and each independently represents hydrogen or a fluoro group, and at least five of Ra to Rj are fluoro groups.

2. In the first paragraph, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiB 10 Cl 10 An electrolyte for a lithium secondary battery comprising at least one selected from the group consisting of LiAlCl4, LiAlO2, LiPF6, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsF6, LiSbF6, LiCH3SO3, LiFSI (lithium bis(fluorosulfonyl) imide, LiN(SO2F)2), LiBETI (lithium bis(perfluoroethanesulfonyl) imide, LiN(SO2CF2CF3)2), and LiTFSI (lithium bis(trifluoromethanesulfonyl) imide, LiN(SO2CF3)2).

3. An electrolyte for a lithium secondary battery, wherein the lithium salt is included in the electrolyte at a concentration of 1.0 to 2.5 M in the first paragraph.

4. In the first paragraph, the fluorine-substituted ether solvent is an electrolyte for a lithium secondary battery comprising 2,2,3,3-tetrafluoro-1,4-dimethoxybutane (FDMB) or 2,2,3,3-tetrafluoro-1,4-diethoxybutane (FDEB).

5. In paragraph 1, An electrolyte for a lithium secondary battery, wherein five or six of the above Ra to Rj are fluoro groups and the remainder are hydrogen.

6. In paragraph 1, The compound represented by the above chemical formula 1 is an electrolyte for a lithium secondary battery, which is bis(2,2,2-trifluoroethyl) ether (BTFE).

7. An electrolyte for a lithium secondary battery, wherein the organic non-solvent and the non-aqueous organic solvent are included in a volume ratio of 1:0.5 to 1:1.5 in the first paragraph.

8. A cathode containing a cathode active material; cathode; a separator between the anode and cathode; and A lithium secondary battery comprising the electrolyte of any one of claims 1 to 7.

9. A lithium secondary battery comprising a lithium transition metal oxide including lithium and at least one transition metal selected from the group consisting of nickel, manganese, cobalt, and iron, in the 8th paragraph, wherein the positive electrode active material is lithium; 10. In the 8th paragraph, a lithium secondary battery wherein the negative electrode includes a lithium metal layer.

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