Electrolyte for secondary battery, and lithium secondary battery comprising same

Abstract

An electrolyte for a secondary battery, according to embodiments of the present disclosure, comprises a lithium salt, an organic solvent and an alkyl acetate comprising a C1-C9 alkyl group. The amount of the alkyl acetate is 0.1-25 wt% on the basis of the total weight of the electrolyte. The lithium secondary battery according to embodiments of the present disclosure comprises the electrolyte.

Description

Electrolyte for secondary batteries and lithium secondary batteries containing the same

[0001] The present disclosure provides an electrolyte for a secondary battery and a lithium secondary battery including the same.

[0002]

[0003] Lithium secondary batteries offer the advantages of high operating voltage, high energy density, high charging speed, and lightweight design. Consequently, they are being used as a power source for small electronic devices as well as electric vehicles.

[0004] Lithium secondary batteries, used as power sources for electric vehicles, require high output. Furthermore, to achieve stable capacity, the amount of gas generated within the battery must be minimal. This is because gas generation increases battery resistance, leading to a decrease in battery capacity.

[0005] A lithium secondary battery may include a negative electrode including a negative electrode active material (graphite, silicon), a positive electrode including a positive electrode active material (lithium metal oxide particles), and a non-aqueous electrolyte including a lithium salt and an organic solvent.

[0006] Non-aqueous electrolytes can improve the output of lithium secondary batteries while reducing gas generation by varying their composition. For example, they can enhance the output characteristics of lithium secondary batteries by improving lithium ion conductivity. Furthermore, by forming a solid film (SEI; solid electrolyte interface, CEI; cathode electrolyte interface) on the anode and cathode, they can prevent direct contact between electrolyte components and the electrodes, thereby reducing decomposition products and reducing gas generation rates.

[0007]

[0008] An object of the present disclosure is to provide an electrolyte for a secondary battery capable of improving battery characteristics.

[0009] One object of the present disclosure is to provide a lithium secondary battery including the above electrolyte.

[0010]

[0011] An electrolyte for a secondary battery according to the present disclosure comprises a lithium salt, an organic solvent, and an alkyl acetate containing a C1 to C9 alkyl group. The content of the alkyl acetate is 0.1 to 25 wt% of the total weight of the electrolyte.

[0012] According to exemplary embodiments, the alkyl acetate may be n-butyl acetate.

[0013] According to exemplary embodiments, the lithium salt may include at least one selected from the group consisting of LiPF6, Li(FSO2)2N, LiBF4, LiSbF6, LiAsF6, LiClO4, LiN(C2F5SO2)2, LiN(CF3SO2)2, CF3SO3Li, and LiC(CF3SO2)3. The concentration of the lithium salt may be from 0.01 M to 2 M with respect to the organic solvent.

[0014] According to exemplary embodiments, the lithium salt comprises LiPF6 and Li(FSO2)2N, and the ratio of the concentration (M) of Li(FSO2)2N to the concentration (M) of LiPF6 can be from 0.05 to 3.

[0015] According to exemplary embodiments, the organic solvent may include a linear carbonate and a cyclic carbonate.

[0016] According to exemplary embodiments, the linear carbonate may include at least one selected from the group consisting of dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, and dipropyl carbonate. The cyclic carbonate may include at least one selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate. The ratio of the volume of the cyclic carbonate to the volume of the linear carbonate may be from 0.1 to 1.

[0017] According to exemplary embodiments, the composition may further include at least one additive selected from the group consisting of a fluorine-containing cyclic carbonate compound, a vinyl-containing cyclic carbonate compound, a vinylene carbonate compound, a cyclic sulfate compound, a sultone compound, a fluorine-containing lithium phosphate compound, a lithium borate compound, a lactone compound, and a nitrile compound.

[0018] According to exemplary embodiments, the content of the additive may be from 0.05 wt% to 5 wt% of the total weight of the composition.

[0019] A lithium secondary battery according to the present disclosure includes a positive electrode, a negative electrode opposite to the positive electrode, and an electrolyte impregnating the positive electrode and the negative electrode.

[0020] According to exemplary embodiments, the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector and including a positive electrode active material, wherein the positive electrode active material may include lithium metal oxide.

[0021]

[0022] The electrolyte for a secondary battery according to the present disclosure can improve the low resistance characteristics and output characteristics of the battery, and gas generation inside the battery can be suppressed.

[0023] The electrolyte for a secondary battery according to the present disclosure can improve the rapid charge / discharge performance of the battery.

[0024] The battery according to the present disclosure can have increased retention capacity and recovery capacity when stored at high temperatures.

[0025] Figures 1 and 2 are schematic plan perspective views and cross-sectional views, respectively, showing lithium secondary batteries according to exemplary embodiments.

[0026] Figure 3 is a graph of recovery capacity according to voltage of Comparative Example 6 and Example 11.

[0027] Figure 4 is a graph of 5C discharge capacity according to voltage of Comparative Example 8 and Example 16.

[0028]

[0029] According to the present disclosure, an electrolyte for a secondary battery comprising a lithium salt, an organic solvent and an alkyl acetate and a lithium secondary battery comprising the same are provided.

[0030] In this specification, “X-based compound” may mean a compound that contains an X unit in a parent group, side group, or substituent.

[0031] In this specification, "Ca to Cb" may mean "a to b carbon atoms." Additionally, "5-7 membered ring" may mean "5 to 7 atoms in the ring."

[0032] The above secondary battery electrolyte contains alkyl acetate. The alkyl acetate is an ester compound and can be represented by the following chemical formula 1.

[0033] [Chemical Formula 1]

[0034]

[0035] In the above chemical formula 1, R may be a C1 to C9 alkyl group. In some embodiments, R may be a C1 to C9 linear alkyl group or a C3 to C9 branched alkyl group.

[0036] For example, the above R can be a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an i-butyl group, a t-butyl group, an n-pentyl group, an i-pentyl group, etc.

[0037] The alkyl acetate comprises an alkyl group having a carbon number of C1 to C9. In some embodiments, the alkyl acetate may comprise an alkyl group having a carbon number of C2 to C5. For example, the alkyl acetate may comprise an n-butyl group, and the alkyl acetate may be n-butyl acetate.

[0038] In exemplary embodiments, the alkyl acetate may include two or more alkyl acetates having alkyl groups having different carbon atoms.

[0039] The above alkyl acetate may have high polarity due to the inclusion of an ester group, thereby improving the solubility of the lithium salt in the electrolyte and enabling uniform mixing with the organic solvent. In addition, the above alkyl acetate may suppress side reactions between the positive electrode active material and the electrolyte and improve the high-temperature performance of the battery.

[0040] The content of the alkyl acetate is 0.1 wt% to 25 wt% of the total weight of the electrolyte. According to exemplary embodiments, the content of the alkyl acetate is 0.1 wt% to 20 wt% of the total weight of the electrolyte. According to some embodiments, the content of the alkyl acetate is 2 wt% to 20 wt% or 5 wt% to 20 wt% of the total weight of the electrolyte.

[0041] Within the above range, gas generation inside the battery can be suppressed, a large-capacity battery can be realized, the capacity retention rate of the battery can be improved, the life characteristics can be improved, and the output can be increased.

[0042] When the content of the above alkyl acetate is less than 0.1 wt%, side reactions with the active material of the positive electrode may increase, resulting in an increase in the amount of gas generated inside the battery. Accordingly, the battery thickness may increase and the risk of explosion may increase.

[0043] If the content of the above alkyl acetate exceeds 25 wt%, the movement speed of lithium ions through the electrolyte may be inhibited, which may result in a deterioration in the output characteristics of the battery. In addition, the life characteristics and capacity of the battery may deteriorate.

[0044] According to exemplary embodiments, the electrolyte comprises a lithium salt. For example, the lithium salt may be Li + X - can be expressed as

[0045] In exemplary embodiments, the anion (X) of the lithium salt - ) is F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - These may be used singly or in combination of two or more.

[0046] In some embodiments, the lithium salt may be LiPF6, Li(FSO2)2N, LiBF4, LiSbF6, LiAsF6, LiClO4, LiN(C2F5SO2)2, LiN(CF3SO2)2, CF3SO3Li, LiC(CF3SO2)3, etc. These may be used alone or in combination of two or more.

[0047] In one embodiment, the lithium salt may include LiPF6 and Li(FSO2)2N, and a ratio of the concentration (M) of Li(FSO2)2N to the concentration (M) of LiPF6 may be from 0.05 to 3. The concentration of LiPF6 and the concentration of Li(FSO2)2N may be molar concentrations with respect to the organic solvent.

[0048] The content of the lithium salt may be 0.01 M to 2 M with respect to the organic solvent. In some embodiments, the content of the lithium salt may be 0.5 M to 1.5 M with respect to the organic solvent. Within the concentration range, lithium ions and / or electrons may move smoothly during charging and discharging of the battery.

[0049] The organic solvent may have sufficient solubility for the lithium salt, the alkyl acetate, and the additive described below. In one embodiment, the organic solvent may be a non-aqueous organic solvent.

[0050] The organic solvent may include a carbonate-based solvent. In some embodiments, the organic solvent may include a linear carbonate and a cyclic carbonate.

[0051] The linear carbonate may include dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, dipropyl carbonate, and the like. These may be used alone or in combination of two or more. In one embodiment, the linear carbonate may include dimethyl carbonate and ethylmethyl carbonate.

[0052] The cyclic carbonate may include at least one selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate. These may be used alone or in combination of two or more. In one embodiment, the cyclic carbonate may include ethylene carbonate.

[0053] In some embodiments, the organic solvent may comprise, by volume, more of the linear carbonate solvent than the cyclic carbonate solvent.

[0054] According to exemplary embodiments, the ratio of the volume of the cyclic carbonate to the volume of the linear carbonate may be from 0.1 to 1. According to some embodiments, the ratio of the volume of the cyclic carbonate to the volume of the linear carbonate may be from 0.15 to 0.5 or from 0.2 to 0.4.

[0055] Within the above range, the electrolyte can penetrate well into the electrode active material layer while securing the movement speed and movement path of lithium ions.

[0056] In one embodiment, the organic solvent may further include other solvents different from the carbonate solvent. For example, the organic solvent may include a cyclic ester solvent (carboxylate solvent), an ether solvent, a ketone solvent, an alcohol solvent, an aprotic solvent, etc.

[0057] For example, the cyclic ester solvent may include butyrolactone, caprolactone, valerolactone, etc.

[0058] For example, the ether solvent may include at least one of dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), and 2-methyltetrahydrofuran.

[0059] For example, the ketone solvent may include cyclohexanone, etc.

[0060] For example, the alcohol solvent may include at least one of ethyl alcohol and isopropyl alcohol.

[0061] For example, the aprotic solvent may include at least one of an amide solvent (e.g., dimethylformamide), a dioxolane solvent (e.g., 1,3-dioxolane), and a sulfolane solvent.

[0062] In exemplary embodiments, the additive may include a fluorine-containing cyclic carbonate compound, a vinyl-containing cyclic carbonate compound, a vinylene carbonate compound, a cyclic sulfate compound, a sultone compound, a fluorine-containing lithium phosphate compound, a lithium borate compound, a lactone compound, a nitrile compound, and the like. In one embodiment, the additive may include a vinylene carbonate compound.

[0063] For example, the fluorine-containing cyclic carbonate compound may have a 5-7 membered cyclic structure. For example, the fluorine-containing cyclic carbonate compound may have a fluorine atom directly bonded to a carbon atom, or a fluorine-substituted alkyl group (e.g., -CF3, etc.) bonded to it.

[0064] In some embodiments, the fluorine-containing cyclic carbonate compound may include fluoroethylene carbonate (FEC).

[0065] In some embodiments, the vinyl group-containing cyclic carbonate compound may include vinyl ethylene carbonate (VEC), etc. In addition, the vinylene carbonate compound may include vinylene carbonate (VC), etc.

[0066] For example, the cyclic sulfate compound may have a 5-7 membered cyclic structure. In some embodiments, the cyclic sulfate compound may include ethylene sulfate (ESA), trimethylene sulfate (TMS), methyltrimethylene sulfate (MTMS), 1,3-propanediol cyclic sulfate, and the like.

[0067] In some embodiments, the cyclic sulfate compound may include a bicyclic sulfate compound. In some embodiments, the bicyclic sulfate compound may include 2,4,8,10-tetraoxa-3,9-dithiaspiro[5.5]undecane, and the like.

[0068] For example, the sultone compound may have a 5-7 membered cyclic structure. In some embodiments, the sultone compound may include at least one of an alkyl sultone compound and an alkenyl sultone compound. For example, the alkyl sultone compound may have only saturated bonds within the ring, and the alkenyl sultone compound may include a double bond within the ring.

[0069] In some embodiments, the alkyl sultone compound may include 1,3-propane sultone (PS), 1,4-butane sultone, and the like. Additionally, the alkenyl sultone compound may include ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, 1-methyl-1,3-propene sultone, and the like.

[0070] For example, the fluorine-containing lithium phosphate compound may have a fluorine atom directly bonded to a phosphorus atom, or a fluorine-substituted alkyl group (e.g., -CF3) bonded to it.

[0071] In some embodiments, the fluorine-containing lithium phosphate compound may include lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalato)phosphate, and the like.

[0072] In some embodiments, the lithium borate compound may include lithium tetraphenylborate, lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiFOB), or the like.

[0073] In some embodiments, the lactone compound may include at least one of a bicyclolactone compound and a lactone compound including a double bond in a ring. In some embodiments, the lactone compound may include a muconic lactone, etc.

[0074] In some embodiments, the nitrile compound may include succinonitrile, 1,3,6-hexanetricarbonitrile, and the like.

[0075] According to exemplary embodiments, the content of the additive may be 0.05 wt% to 5 wt% of the total weight of the composition. According to some embodiments, the content of the additive may be 0.1 wt% to 3 wt% of the total weight of the composition. Within the above range, the content may be adjusted depending on the type of additive for improving the performance of the electrolyte.

[0076] According to exemplary embodiments of the present disclosure, a lithium secondary battery including the electrolyte for the secondary battery is provided.

[0077] Hereinafter, a lithium secondary battery according to exemplary embodiments will be described in more detail with reference to the drawings. Figures 1 and 2 are schematic plan perspective views and cross-sectional views, respectively, showing lithium secondary batteries according to exemplary embodiments.

[0078] Referring to FIGS. 1 and 2, a lithium secondary battery may include a positive electrode (100) and a negative electrode (130) opposite to the positive electrode (100).

[0079] The positive electrode (100) may include a positive electrode current collector (105) and a positive electrode active material layer (110) on the positive electrode current collector (105).

[0080] For example, the positive electrode active material layer (110) may include a positive electrode active material, a positive electrode binder, and a conductive material, if necessary.

[0081] For example, the positive electrode (100) can be manufactured by mixing and stirring a positive electrode active material, a positive electrode binder, a conductive agent, a dispersion medium, etc. to manufacture a positive electrode slurry, and then applying, drying, and rolling the positive electrode slurry onto a positive electrode current collector (105).

[0082] For example, the positive electrode collector (105) may include stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0083] For example, the positive electrode active material may include lithium metal oxide particles capable of reversible insertion and de-insertion of lithium ions.

[0084] In one embodiment, the lithium metal oxide particles can contain nickel, cobalt, manganese, aluminum, and the like.

[0085] In some embodiments, the lithium metal oxide particles contain nickel, and the content of nickel in the lithium metal oxide particles may be 80 mol% or more of the total elements excluding lithium and oxygen.

[0086] In some embodiments, the lithium metal oxide particles may include LiNiO2, LiCoO2, LiMnO2, LiMn2O4, or a compound represented by the following formula 2.

[0087] [Chemical Formula 2]

[0088] Li x Ni (1-a-b) Co a M b O y

[0089] In chemical formula 2, M is at least one of Al, Zr, Ti, Cr, B, Mg, Mn, Ba, Si, Y, W, and Sr, and 0.9≤x≤1.2, 1.9≤y≤2.1, 0≤a+b≤0.5.

[0090] In some embodiments, in formula 2, 0 <a+b≤0.4, 0<a+b≤0.3, 0<a+b≤0.2 또는 0<a+b≤0.1을 만족할 수 있다.

[0091] For example, the positive electrode binder may include an organic binder such as polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile, polymethylmethacrylate, or an aqueous binder such as styrene-butadiene rubber (SBR). In addition, for example, the positive electrode binder may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0092] For example, the conductive material may include a carbon-based conductive material such as graphite, carbon black, graphene, carbon nanotubes, etc.; a metal-based conductive material such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc.

[0093] The negative electrode (130) may include a negative electrode current collector (125) and a negative electrode active material layer (120) on the negative electrode current collector (125).

[0094] For example, the negative electrode active material layer (120) may include a negative electrode active material, a negative electrode binder, and a conductive material, if necessary.

[0095] For example, the negative electrode (130) can be manufactured by mixing and stirring a negative electrode active material, a negative electrode binder, a conductive material, a solvent, etc. to manufacture a negative electrode slurry, and then applying, drying, and rolling the negative electrode slurry onto a negative electrode current collector (125).

[0096] For example, the negative electrode current collector (125) may include gold, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, and preferably, may include copper or a copper alloy.

[0097] For example, the negative electrode active material may be a material capable of absorbing and desorbing lithium ions. For example, the negative electrode active material may include a lithium alloy, a carbon-based active material, a silicon-based active material, and the like.

[0098] For example, the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, and the like.

[0099] For example, the carbon-based active material may include crystalline carbon, amorphous carbon, carbon composite, carbon fiber, etc.

[0100] For example, the amorphous carbon may include hard carbon, coke, mesocarbon microbeads (MCMB) calcined at 1500°C or lower, mesophase pitch-based carbon fibers (MPCF), etc. For example, the crystalline carbon may include natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0101] In one embodiment, the negative active material may include a silicon-based active material. For example, the silicon-based active material may include Si, SiO. x (0 <x<2), Si / C, SiO / C, Si-Metal 등을 포함할 수 있다.

[0102] The above-described negative electrode binder and conductive material may be substantially the same or similar materials as the above-described positive electrode binder and conductive material. For example, the negative electrode binder may be an aqueous binder such as styrene-butadiene rubber (SBR). Furthermore, for example, the negative electrode binder may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0103] In one embodiment, a separator (140) may be interposed between the anode (100) and the cathode (130).

[0104] In some embodiments, the area of ​​the negative electrode (130) may be larger than the area of ​​the positive electrode (100). In this case, lithium ions generated from the positive electrode (100) may smoothly move to the negative electrode (130) without being precipitated in the middle.

[0105] For example, the separator (140) may include a porous polymer film made of a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, etc. In addition, for example, the separator (140) may include a nonwoven fabric formed of high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0106] For example, an electrode cell can be formed including an anode (100), a cathode (130), and a separator (140).

[0107] For example, a plurality of electrode cells can be stacked to form an electrode assembly (150) (however, for convenience, one electrode cell is illustrated in FIG. 2).

[0108] For example, an electrode assembly (150) can be formed by winding, lamination, z-folding, etc. of a separator (140).

[0109] A lithium secondary battery according to exemplary embodiments may include a positive electrode lead (107) connected to a positive electrode (100) and protruding outward from a case (160); and a negative electrode lead (127) connected to a negative electrode (130) and protruding outward from a case (160).

[0110] For example, the positive electrode (100) and the positive electrode lead (107) may be electrically connected. Similarly, the negative electrode (130) and the negative electrode lead (127) may be electrically connected.

[0111] For example, the positive electrode lead (107) may be electrically connected to the positive electrode collector (105). Additionally, the negative electrode lead (130) may be electrically connected to the negative electrode collector (125).

[0112] For example, the positive electrode current collector (105) may include a protrusion (positive electrode tab, 106) on one side. The positive electrode active material layer (110) may not be formed on the positive electrode tab (106). The positive electrode tab (106) may be integral with the positive electrode current collector (105) or may be connected by welding or the like. The positive electrode current collector (105) and the positive electrode lead (107) may be electrically connected through the positive electrode tab (106).

[0113] Likewise, the negative electrode collector (125) may include a protrusion (negative electrode tab, 126) on one side. The negative electrode active material layer (120) may not be formed on the negative electrode tab. The negative electrode tab (126) may be integral with the negative electrode collector (125) or may be connected by welding or the like. The negative electrode collector (125) and the negative electrode lead (127) may be electrically connected through the negative electrode tab (126).

[0114] In one embodiment, the electrode assembly (150) may include a plurality of positive electrodes and a plurality of negative electrodes. For example, the plurality of positive electrodes and the plurality of negative electrodes may be alternately arranged, and a separator may be interposed between the positive electrodes and the negative electrodes. Accordingly, a lithium secondary battery according to one embodiment of the present invention may include a plurality of positive electrode tabs and a plurality of negative electrode tabs protruding from each of the plurality of positive electrodes and the plurality of negative electrodes.

[0115] In one embodiment, the positive tabs (or negative tabs) may be laminated, pressed, and welded to form a positive tab laminate (or negative tab laminate). The positive tab laminate may be electrically connected to the positive lead (107). Additionally, the negative tab laminate may be electrically connected to the negative lead (127).

[0116] For example, the electrode assembly (150) and the above-described electrolyte may be housed together in a case (160) to form a lithium secondary battery.

[0117] The above lithium secondary battery can be manufactured in, for example, a cylindrical shape, a square shape, a pouch shape, or a coin shape.

[0118] Hereinafter, embodiments of the present invention will be further described with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are merely illustrative of the present invention and do not limit the scope of the appended claims. It will be apparent to those skilled in the art that various modifications and variations of the examples are possible within the scope and technical spirit of the present invention, and it is also natural that such modifications and variations fall within the scope of the appended claims.

[0119] Example 1

[0120] (1) Electrolyte production

[0121] An electrolyte was prepared by adding butyl acetate (BA) to a solution containing 1 M LiPF6 in a mixed solvent (EC:EMC:DMC=2:4:4, volume ratio) containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) so that the content of BA was 0.1 wt% of the total weight of the electrolyte (100 wt%).

[0122] (2) Secondary battery manufacturing

[0123] Li(Ni) 0.8 Co 0.1 Mn 0.1 )O2, polyvinylidene fluoride (PVdF), and carbon black were dispersed in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 92:4:4 to prepare a cathode slurry.

[0124] The above positive electrode slurry was applied onto an aluminum foil (thickness: 20 μm) having a protrusion (hereinafter, positive electrode tab) on one side (excluding the protrusion portion), dried, and rolled to manufacture a positive electrode.

[0125] A cathode slurry was prepared by dispersing crystalline artificial graphite, acetylene black, and PVDF in NMP at a weight ratio of 92:1:7.

[0126] The above cathode slurry was applied onto a copper foil (thickness: 15 μm) having a protrusion (hereinafter, cathode tab) on one side (excluding the protrusion portion), dried, and rolled to manufacture a cathode.

[0127] A cell was formed by interposing a polyethylene separator (thickness: 20 μm) between the positive and negative electrodes. The positive and negative electrode leads were connected by welding to the positive and negative tabs, respectively.

[0128] The cell was housed inside a pouch so that some areas of the positive lead and the negative lead were exposed to the outside.

[0129] An electrolyte was injected into the pouch, and the pouch was sealed and sealed to manufacture an 880 mAh-class lithium secondary battery.

[0130]

[0131] Examples 2 to 18 and Comparative Examples 1 to 10

[0132] An electrolyte and a secondary battery were manufactured in the same manner as in Example 1, except that the electrolyte was manufactured with the lithium salt, butyl acetate, and additive compositions shown in Table 1 below. The content of the lithium salt was expressed as molar concentration (M), and the contents of the butyl acetate and additives were expressed as weight percent of the total weight of the electrolyte.

[0133]

[0134] Lithium salt (content) BA content (wt%) Additive (content) Example 1 LiPF6 (1 M) 0.1 - Example 2 LiPF6 (1 M) 2 - Example 3 LiPF6 (1 M) 5 - Example 4 LiPF6 (1 M) 10 - Example 5 LiPF6 (1 M) 20 - Comparative Example 1 LiPF6 (1 M) -- Comparative Example 2 LiPF6 (1 M) 0.05 - Comparative Example 3 LiPF6 (1 M) 30 - Example 6 LiPF6 (1 M), LIFSI (0.2 M) 0.1 - Example 7 LiPF6 (1 M), LIFSI (0.2 M) 2 - Example 8 LiPF6 (1 M), LIFSI (0.2 M) 5 - Example 9 LiPF6 (1 M), LIFSI (0.2 M) 20 - Comparative Example 4 LiPF6 (1 M), LIFSI (0.2 M)--Comparative Example 5LiPF6(1 M), LIFSI (0.2 M)0.5-Example 10LiPF6(1 M)0.1VC (1 wt%)Example 11LiPF6(1 M)2VC (1 wt%)Example 12LiPF6(1 M)5VC (1 wt%)Example 13LiPF6(1 M)10VC (1 wt%)Example 14LiPF6(1 M)20VC (1 wt%)Comparative Example 6LiPF6(1 M)-VC (1 wt%)Comparative Example 7LiPF6(1 M)0.05VC (1 wt%)Example 15LiPF6(1 M), LIFSI (0.2 M)0.1VC (1 wt%)Example 16LiPF6(1 M), LIFSI (0.2 M)2VC (1 wt%)Example 17LiPF6(1 M), LIFSI (0.2 M) 5 VC (1 wt%)Example 18 LiPF6 (1 M), LIFSI (0.2 M) 20 VC (1 wt%)Comparative Example 8 LiPF6 (1 M), LIFSI (0.2 M) - VC (1 wt%)Comparative Example 9 LiPF6 (1 M), LIFSI (0.2 M) 0.05 VC (1 wt%)Comparative Example 10 LiPF6 (1 M), LIFSI (0.2 M) 30 VC (1 wt%)

[0135]

[0136] In Table 1 above, LiFSI is lithium bis(fluorosulfonyl)imide (Li(FSO2)2N) and VC is vinylene carbonate.

[0137]

[0138] Experimental example

[0139] (1) Output characteristics evaluation

[0140] The lithium secondary batteries of the examples and comparative examples were subjected to a chemical charge / discharge (charge / discharge conditions: charged in 0.2C CC / CV mode (4.2 V, 0.05C cut-off) and discharged in 0.2C CC mode (2.5 V cut-off)) and a standard charge / discharge (charge / discharge conditions: charged in 0.5C CC / CV mode (4.2 V, 0.05C cut-off) and discharged in 0.5C CC mode (2.5 V cut-off) and then charged in 0.5C CC / CV mode (4.2 V, 0.05C cut-off)), followed by a 1C CC / CV charge (4.2 V cut-off) and a 1C CC discharge to SOC 50. After that, the C-rate was changed to 0.5C, 1C, 2C, and 4C, and the discharge and recharge were performed for 10 seconds each, and the DCIR was measured.

[0141] The output (pulse power) value (W) was calculated using the measured DCIR value and is listed in Table 2 below.

[0142]

[0143] (2) Evaluation of thickness increase rate

[0144] The lithium secondary batteries of the examples and comparative examples were subjected to a chemical charge / discharge (charge / discharge conditions: charge in 0.2C CC / CV mode (4.2 V, 0.05 C cut-off) and then discharged in 0.2C CC mode (2.5 V cut-off)) and a standard charge / discharge (charge / discharge conditions: charge in 0.5C CC / CV mode (4.2 V, 0.05 C cut-off) and then discharged in 0.5C CC mode (2.5 V cut-off) and then charged in 0.5C CC / CV mode (4.2 V, 0.05 C cut-off)) process, and then left in an oven at 70°C for one week, and the thickness increase after leaving was calculated as a percentage of the initial thickness.

[0145]

[0146] (3) Capacity measurement

[0147] The lithium secondary batteries of the examples and comparative examples were subjected to a chemical charge / discharge process (charge / discharge condition: 0.2C CC / CV mode (4.2V, 0.05C cut-off) charged, then discharged at 0.2C CC mode (2.5V cut-off)) and a standard charge / discharge process (charge / discharge condition: 0.5C CC / CV mode (4.2V, 0.05C cut-off) charged, then discharged at 0.5C CC mode (2.5V cut-off) charged, then charged at 0.5C CC / CV mode (4.2V, 0.05C cut-off)). After being left in an oven at 70°C for one week, the maintenance capacity and recovery capacity after being left were measured.

[0148] Figure 3 shows a graph of the recovery capacity according to voltage of Comparative Example 6 and Example 11.

[0149]

[0150] (4) High-rate discharge capacity evaluation

[0151] The lithium secondary batteries of the examples and comparative examples were subjected to a chemical charge / discharge process (charge / discharge condition: 0.2C CC / CV mode (4.2V, 0.05C cut-off) charged, then 0.2C CC mode (2.5V cut-off) discharged) and a standard charge / discharge process (charge / discharge condition: 0.5C CC / CV mode (4.2V, 0.05C cut-off) charged, then 0.5C CC mode (2.5V cut-off) discharged, then 0.5C CC / CV mode (4.2V, 0.05C cut-off) charged), then 1C CC / CV mode (4.2V, 0.05C cut-off) discharged, then 5C CC mode (2.5V cut-off) discharged, to calculate the discharge capacity.

[0152] Figure 4 shows a graph of 5C discharge capacity according to voltage of Comparative Example 8 and Example 16.

[0153] Output (W) Thickness Increase Rate (%) Maintenance Capacity (mAh) Recovery Capacity (mAh) 5C Discharge Capacity (mAh) Example 166.275 1.127 34.45 78 4.27 32 6.94 Example 264.08 49.24 73 0.32 78 5.51 32 5.1 Example 364.56 31.17 76 0.75 8 10.12 35 5.47 Example 463.11 40.55 738.18 79 2.46 33 0.84 Example 562.77 41.35 74 2.83 78 1.01 34 4.52 Comparative Example 161.27 54.27 710.15 77 1.54 30 4.31 Comparative Example 262.4953.15724.32770.25305.36Comparative Example 361.5642.71712.44775.39311.84Example 667.4850.68744.36789.24348.57Example 764.7745.51770.97830.63332.18Example 863.442.64762.18816.25340.12Example 959.4238.77756.42804.17337.07Comparative Example 463.3949.79721.15778.59309.16Comparative Example 563.2948.52731.77777.25305.36 Example 1063.1247.88746.87787.14320.33 Example 1166.9744.56750.38809.76337.78 Example 1265.4446.45756.37797.86342.97 Example 1362.0743.66746.12801.91343.03 Example 1461.8240.43748.97780.49327.61 Comparative Example 660.3956.11711.24763.05297.38 Comparative Example 760.1452.64739.16795.62302.44 Example 1570.8342.47751.58795.87344.31 Example 1671.0238.23768.44832.24367.35 Example 1768.9735.12772.67840.12359.66 Example 1859.8834.98744.35802.55335.83 Comparative Example 862.7247.84732.57791.41310.49 Comparative Example 964.5640.54723.14792.49323.72 Comparative Example 1052.3731.48758.44824.65351.52

[0154]

[0155] Referring to Table 2, the batteries of the examples exhibited reduced volumetric increase rates due to suppressed gas generation within the batteries even when stored at high temperatures for extended periods of time. Furthermore, the batteries of the examples exhibited high retention and recovery capacities even when stored at high temperatures for extended periods of time, resulting in high-capacity batteries with improved high-temperature characteristics. Furthermore, the batteries exhibited high output and high-rate discharge capacities, enabling high-speed charging and discharging.

[0156] Referring to FIGS. 3 and 4, the batteries of Examples 11 and 16, each having an electrolyte containing more alkyl acetate in the same composition, had a higher recovery capacity or high-rate discharge capacity than the batteries of Comparative Examples 6 and 8, each having an electrolyte containing no alkyl acetate.

[0157] The batteries of the comparative examples did not contain alkyl acetate, or contained alkyl acetate in a very small or excessive amount, which resulted in increased gas generation inside the battery or deterioration of capacity characteristics and output characteristics.

[0158] The above description is merely an example of applying the principles of the present disclosure, and other configurations may be included without departing from the scope of the present invention.

Claims

1. Containing a lithium salt, an organic solvent and an alkyl acetate containing a C1 to C9 alkyl group, An electrolyte for a secondary battery, wherein the content of the alkyl acetate is 0.1 wt% to 25 wt% of the total weight of the electrolyte.

2. An electrolyte for a secondary battery, wherein the alkyl acetate in paragraph 1 is n-butyl acetate.

3. In the first paragraph, the lithium salt comprises at least one selected from the group consisting of LiPF6, Li(FSO2)2N, LiBF4, LiSbF6, LiAsF6, LiClO4, LiN(C2F5SO2)2, LiN(CF3SO2)2, CF3SO3Li and LiC(CF3SO2)3, An electrolyte for a secondary battery, wherein the concentration of the lithium salt is 0.01 M to 2 M with respect to the organic solvent.

4. In the first paragraph, the lithium salt comprises LiPF6 and Li(FSO2)2N, An electrolyte for a secondary battery, wherein the ratio of the concentration (M) of Li(FSO2)2N to the concentration (M) of LiPF6 is 0.05 to 3.

5. In the first paragraph, the organic solvent comprises a linear carbonate and a cyclic carbonate, The linear carbonate comprises at least one selected from the group consisting of dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate and dipropyl carbonate, An electrolyte for a secondary battery, wherein the cyclic carbonate comprises at least one selected from the group consisting of ethylene carbonate, propylene carbonate, and butylene carbonate.

6. An electrolyte for a secondary battery, wherein in clause 5, the ratio of the volume of the cyclic carbonate to the volume of the linear carbonate is 0.1 to 1.

7. An electrolyte for a secondary battery, according to claim 1, further comprising at least one additive selected from the group consisting of a fluorine-containing cyclic carbonate compound, a vinyl group-containing cyclic carbonate compound, a vinylene carbonate compound, a cyclic sulfate compound, a sultone compound, a fluorine-containing lithium phosphate compound, a lithium borate compound, a lactone compound, and a nitrile compound.

8. An electrolyte for a secondary battery, wherein the content of the additive in paragraph 7 is 0.05 wt% to 5 wt% of the total weight of the composition.

9. A lithium secondary battery comprising a positive electrode, a negative electrode opposite to the positive electrode, and an electrolyte according to claim 1 that impregnates the positive electrode and the negative electrode.

10. In the 9th paragraph, the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector and including a positive electrode active material, A lithium secondary battery, wherein the positive electrode active material comprises lithium metal oxide.

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