Lithium secondary battery

Abstract

Provided according to exemplary embodiments is a lithium secondary battery comprising: a positive electrode containing a lithium iron phosphate; and an electrolyte containing a compound represented by chemical formula 1. The lithium secondary battery can have enhanced high-temperature cycle characteristics and low resistance characteristics. The lithium secondary battery can have enhanced high-temperature cycle characteristics and low resistance characteristics.

Description

lithium secondary battery

[0001] The present disclosure relates to a lithium secondary battery.

[0002]

[0003] Secondary batteries are batteries that can be repeatedly charged and discharged, and are used as a power source for small electronic devices such as cell phones and laptop PCs.

[0004] In particular, lithium secondary batteries offer the advantages of high operating voltage, high energy density, high charging speed, and lightweight design. Accordingly, lithium secondary batteries are being used as a power source for small electronic devices as well as electric vehicles.

[0005] For example, in order for lithium secondary batteries to be used as a power source for electric vehicles, they must have better output characteristics and lifespan characteristics.

[0006] Meanwhile, a lithium secondary battery may include a negative electrode including a negative electrode active material (e.g., graphite); a positive electrode including a positive electrode active material (e.g., lithium metal oxide particles); and a non-aqueous electrolyte including a lithium salt and an organic solvent.

[0007] For example, a lithium secondary battery can be charged and discharged by repeating the process of lithium ions being inserted and removed from lithium metal oxide particles and graphite.

[0008] For example, by varying the composition of the electrolyte, the output characteristics and lifespan characteristics of a lithium secondary battery can be improved. For example, by improving lithium ion conductivity, the output characteristics of a lithium secondary battery can be improved. Furthermore, by firmly forming a solid electrolyte interface (SEI) on the negative electrode, the lifespan characteristics of a lithium secondary battery can be improved.

[0009] Korean Patent Publication No. 10-1999615 discloses an electrolyte for a lithium secondary battery that can improve the lifespan characteristics of the lithium secondary battery.

[0010]

[0011] An object of the present disclosure is to provide a lithium secondary battery having improved life characteristics, output characteristics, etc.

[0012]

[0013] A lithium secondary battery according to the present disclosure comprises: a positive electrode including a positive current collector and a positive active material layer formed on at least one surface of the positive current collector and including lithium iron phosphate; a negative electrode facing the positive electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte including a lithium salt, an organic solvent, and a compound represented by the following chemical formula 1.

[0014] [Chemical Formula 1]

[0015]

[0016] In Chemical Formula 1, R1 is a C1-C6 alkyl group; a C2-C6 alkenyl group; a C2-C6 alkynyl group; a C3-C7 cycloalkyl group; or a C3-C7 cycloalkenyl group, R2 to R5 are each independently a halogen; or a substituted or unsubstituted C1-C6 alkyl group, M is an alkali metal, and Y + is a cationic substance.

[0017] In some embodiments, the lithium iron phosphate may comprise a compound represented by the following chemical formula 2.

[0018] [Chemical Formula 2]

[0019] Li x Fe 1-y M y (PO 4-z )X z

[0020] In chemical formula 2, M includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X includes at least one element selected from the group consisting of F, S, and N, and x, y, and z are 0.5≤x≤1.5, 0≤y≤0.1, and 0≤z≤0.5, respectively.

[0021] In some embodiments, the lithium iron phosphate may comprise LiFePO4 having an olivine crystal structure.

[0022] In some embodiments, in formula 1, R1 is a C1-C6 alkyl group; or a C2-C6 alkenyl group, at least one of R2 to R5 is a halogen, M is Li, Na or K, and Y + is N + RaRbRcRd, and Ra to Rd are each independently hydrogen or a C1-C6 alkyl group, and at least one of Ra to Rd may be hydrogen.

[0023] In some embodiments, in formula 1, R1 is a C2-C6 alkenyl group, R2 to R5 are all halogen, M is Li, and Y + is N + RaRbRcRd, where Ra is hydrogen, and Rb to Rd can be independently C1-C3 alkyl groups.

[0024] In some embodiments, the content of the compound represented by the chemical formula 1 may be 0.01 to 10 wt% of the total weight of the electrolyte.

[0025] In some embodiments, the lithium salt may include a first lithium salt comprising lithium hexafluorophosphate (LiPF6) and a second lithium salt comprising lithium bisfluorosulfonylimide (LiFSI).

[0026] In some embodiments, the content of lithium bisfluorosulfonylimide may be from 0.05 M to 2 M in the electrolyte.

[0027] In some embodiments, the ratio of the molar concentration of the first lithium salt to the molar concentration of the second lithium salt in the electrolyte may be 1 to 10.

[0028] In some embodiments, the organic solvent may include at least one of a linear carbonate solvent, a cyclic carbonate solvent, a linear ester solvent, a cyclic ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, and an aprotic solvent.

[0029] In some embodiments, the electrolyte may further include an auxiliary additive comprising at least one 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 sulfonyl imide compound.

[0030] In some embodiments, the content of the auxiliary additive may be 0.01 to 10 wt% of the total weight of the electrolyte.

[0031] In some embodiments, the ratio of the content of the auxiliary additive to the content of the compound represented by the chemical formula 1 among the total weight of the electrolyte may be 0.1 to 10.

[0032] In some embodiments, the negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector and including a carbon-based active material.

[0033] In some embodiments, the carbon-based active material may include artificial graphite or natural graphite.

[0034]

[0035] A lithium secondary battery according to exemplary embodiments can have improved cycle characteristics and low resistance characteristics at high temperatures by including a positive electrode including lithium iron phosphate and an electrolyte including a compound represented by Chemical Formula 1.

[0036]

[0037] FIG. 1 and FIG. 2 are a plan perspective view and a cross-sectional view, respectively, schematically illustrating a lithium secondary battery according to exemplary embodiments.

[0038] Figure 3 shows the three-dimensional structure of the compound of Preparation Example 1 analyzed by SC-XRD (single crystal X-ray diffraction).

[0039] Figure 4 is a graph showing the capacity retention rate of lithium secondary batteries of examples and comparative examples according to the number of cycles.

[0040]

[0041] According to the present disclosure, a lithium secondary battery is provided, which includes a positive electrode comprising lithium iron phosphate and an electrolyte for a lithium secondary battery comprising a compound represented by chemical formula 1.

[0042] In this specification, “X-based compound” may mean a compound that contains an X unit in a parent group, side group, or substituent.

[0043] In this specification, "Ca-Cb" may mean "the number of carbon atoms a to b." Additionally, "5-7 membered ring" may mean "the number of atoms in the ring is 5 to 7."

[0044] 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 a lithium secondary battery according to exemplary embodiments.

[0045] 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).

[0046] Bipolar

[0047] 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). The positive electrode active material layer (110) includes a positive electrode active material. In addition, the positive electrode active material layer (110) may include a positive electrode binder and / or a conductive material.

[0048] 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).

[0049] For example, the positive electrode collector (105) may include stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

[0050] The above-mentioned positive electrode active material includes lithium iron phosphate, which enables reversible insertion and deintercalation of lithium ions. When the positive electrode active material includes lithium iron phosphate, the volume density of the positive electrode and the capacity of the battery can be increased, and a high potential can be generated.

[0051] In some embodiments, the lithium iron phosphate may comprise a compound represented by the following chemical formula 2.

[0052] [Chemical Formula 2]

[0053] Li x Fe 1-y M y (PO 4-z )X z

[0054] In chemical formula 2, M may include at least one element selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. For example, M may include Al, Mg, Co, or Mn.

[0055] In chemical formula 2, X may include at least one element selected from the group consisting of F, S, and N. For example, X may include F.

[0056] In Chemical Formula 2, x, y, and z are 0.5≤x≤1.5, 0≤y≤0.1, and 0≤z≤0.5, respectively. In some embodiments, x, y, and z are 0.8≤x≤1.2, 0≤y≤0.05, and 0≤z≤0.1, respectively.

[0057] In some embodiments, the lithium iron phosphate may comprise LiFePO4 having an olivine crystal structure.

[0058] 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, in some embodiments, the positive electrode binder may be used together with a thickener such as carboxymethyl cellulose (CMC).

[0059] 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.

[0060] 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).

[0061] 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.

[0062] 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).

[0063] 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.

[0064] 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.

[0065] For example, the lithium alloy may include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, and the like.

[0066] For example, the carbon-based active material may include crystalline carbon, amorphous carbon, carbon composite, carbon fiber, etc.

[0067] 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.

[0068] 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.

[0069] 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).

[0070] In one embodiment, a separator (140) may be interposed between the anode (100) and the cathode (130).

[0071] 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.

[0072] 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.

[0073] For example, an electrode cell can be formed including an anode (100), a cathode (130), and a separator (140).

[0074] 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).

[0075] For example, an electrode assembly (150) can be formed by winding, lamination, z-folding, etc. of a separator (140).

[0076] 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).

[0077] 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.

[0078] 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).

[0079] 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).

[0080] 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).

[0081] 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.

[0082] 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).

[0083] For example, an electrode assembly (150) and an electrolyte may be housed together in a case (160) to form a lithium secondary battery. The lithium secondary battery may be manufactured in a cylindrical, square, pouch, or coin shape, for example.

[0084] According to exemplary embodiments, an electrolyte for a lithium secondary battery is provided, comprising a compound represented by the following chemical formula 1.

[0085] [Chemical Formula 1]

[0086]

[0087] In chemical formula 1, R1 can be a C1-C6 alkyl group; a C2-C6 alkenyl group; a C2-C6 alkynyl group; a C3-C7 cycloalkyl group or a C3-C7 cycloalkenyl group.

[0088] R2 to R5 may be independently a halogen; or a C1-C6 alkyl group.

[0089] M is an alkali metal, and Y + may be a cationic substance.

[0090] For example, bonds between phosphorus (P) and oxygen (O); and between sulfur (S) and oxygen (O) " " may indicate that some electrons are delocalized.

[0091] In this specification, an alkyl group, an alkenyl group, an alkynyl group, a cycloalkyl group, and / or a cycloalkenyl group may be substituted or unsubstituted. The term "substituted" may mean that a hydrogen atom is substituted with any substituent, and that any substituent is further bonded to the substituent.

[0092] For example, the optional substituent may be a halogen, a C1-C6 alkyl group, a C2-C6 alkenyl group, a C2-C6 alkynyl group, a C1-C6 alkoxy group, a nitro group, a cyano group, etc. In some embodiments, the optional substituent may be a halogen or a C1-C6 alkyl group.

[0093] In one embodiment, R1 may be a C1-C6 alkyl group; or a C2-C6 alkenyl group.

[0094] In some embodiments, R1 can be a C2-C6 alkenyl group. In some embodiments, R1 can be an unsubstituted C2-C6 alkenyl group.

[0095] In one embodiment, at least one of R2 to R5 is a halogen (e.g., F, Cl, Br, or I). In some embodiments, at least one of R2 to R5 can be F.

[0096] In one embodiment, all of R2 to R5 can be halogen. In some embodiments, all of R2 to R5 can be F.

[0097] In one embodiment, M can be Li, Na or K.

[0098] In one embodiment, Y + may be an alkali metal ion; an ammonium ion; or a primary to quaternary ammonium ion.

[0099] In some embodiments, Y+ is N + RaRbRcRd or ReRfN + =RgRh may be.

[0100] Ra to Rd may independently be hydrogen; a C1-C6 alkyl group; or a C2-C6 alkenyl group. In addition, at least two of Ra to Rd may be combined with each other to form a 5-7 membered heterocycle.

[0101] Re to Rh may independently be hydrogen; a C1-C6 alkyl group; or a C2-C6 alkenyl group. In addition, at least two of Re to Rh may be combined with each other to form a 5-7 membered heterocycle.

[0102] In some embodiments, Y + is N + RaRbRcRd, and at least one of Ra to Rd may be hydrogen. In some embodiments, Y + is N + RaRbRcRd, where Ra is hydrogen, and Rb to Rd can be independently C1-C3 alkyl groups.

[0103] According to exemplary embodiments, the compound of formula 1 can be prepared by reacting a sulfonate salt or a sulfate salt with a phosphate alkali metal salt.

[0104] In one embodiment, the sulfonate salt or sulfate salt may be represented by the following chemical formula 3.

[0105] [Chemical Formula 3]

[0106]

[0107] In chemical formula 3, R1 may be a C1-C6 alkyl group; a C2-C6 alkenyl group; a C2-C6 alkynyl group; a C3-C7 cycloalkyl group; a C3-C7 cycloalkenyl group; or -OR1. Y + may be a cationic substance.

[0108] In one embodiment, R1 may be a C1-C6 alkyl group; or a C2-C6 alkenyl group.

[0109] In some embodiments, R1 can be a C2-C6 alkenyl group. In some embodiments, R1 can be an unsubstituted C2-C6 alkenyl group.

[0110] In one embodiment, Y + may be an alkali metal ion; an ammonium ion; or a primary to quaternary ammonium ion.

[0111] In some embodiments, Y+ is N + RaRbRcRd or ReRfN + =RgRh may be.

[0112] Ra to Rd may independently be hydrogen; a C1-C6 alkyl group; or a C2-C6 alkenyl group. In addition, at least two of Ra to Rd may be combined with each other to form a 5-7 membered heterocycle.

[0113] Re to Rh can be independently hydrogen; a C1-C6 alkyl group; or a C2-C6 alkenyl group. In addition, at least two of Re to Rh can be combined with each other to form a 5-7 membered heterocycle.

[0114] In some embodiments, Y + is N + RaRbRcRd, and at least one of Ra to Rd may be hydrogen. In some embodiments, Y + is N + RaRbRcRd, where Ra is hydrogen, and Rb to Rd can be independently C1-C3 alkyl groups.

[0115] In one embodiment, Ra may be a C1-C6 alkyl group; or a C2-C6 alkenyl group.

[0116] In some embodiments, Ra can be a C2-C6 alkenyl group. In some embodiments, Ra can be an unsubstituted C2-C6 alkenyl group.

[0117] In one embodiment, the phosphate-based alkali metal salt may be represented by the following chemical formula 4.

[0118] [Chemical Formula 4]

[0119]

[0120] In chemical formula 4, R2 and R3 can be, independently of each other, a halogen (e.g., F, Cl, Br or I); or a C1-C6 alkyl group.

[0121] In one embodiment, at least one of R2 and R3 can be halogen. In some embodiments, at least one of R2 and R3 can be F.

[0122] In one embodiment, both R2 and R3 can be halogen. In some embodiments, both R2 and R3 can be F.

[0123] In one embodiment, the sulfonate salt or sulfate salt and the phosphate alkali metal salt are reacted to produce a crystal, which is filtered and dried to obtain a compound represented by the chemical formula 1.

[0124] In one embodiment, the reaction temperature may be 10 to 50°C.

[0125] In one embodiment, the molar ratio of the sulfonate salt or sulfate salt and the phosphate alkali metal salt may be 1:0.9 to 1:1.1.

[0126] In one embodiment, the compound represented by the chemical formula 1 can be provided as an additive for an electrolyte for a lithium secondary battery.

[0127] A lithium secondary battery according to exemplary embodiments may have improved life characteristics (e.g., repeated charge / discharge capacity retention rate), output characteristics, etc., by including the electrolyte for the lithium secondary battery.

[0128] For example, the compound represented by the above chemical formula 1 can be reduced and decomposed at a voltage of about 2.1 V to form a solid electrolyte interface (SEI) film on the negative electrode surface. The SEI film can suppress the decomposition of organic solvents (e.g., ethylene carbonate, etc.) that occur at a voltage of 2.9 V. Accordingly, the life characteristics of a lithium secondary battery can be improved.

[0129] For example, the compound represented by the above chemical formula 1 can reduce the positive electrode interfacial resistance. Accordingly, the output characteristics of a lithium secondary battery can be improved.

[0130] An electrolyte for a lithium secondary battery according to exemplary embodiments may include a lithium salt (excluding a lithium salt form of a compound represented by the above chemical formula 1); an organic solvent; and a compound represented by the above chemical formula 1.

[0131] In one embodiment, the content of the compound represented by the chemical formula 1 may be 0.01 to 10 wt%, 0.01 to 5 wt%, 0.01 to 3 wt%, 0.05 to 1 wt%, 0.05 to 0.7 wt%, or 0.05 to 0.4 wt% of the total weight of the electrolyte. Within this range, the life characteristics and output characteristics of the lithium secondary battery can be further improved.

[0132] For example, the compound represented by the above chemical formula 1 may exist as a single unit in the electrolyte, or may exist in the form of a dimer (e.g., see chemical formula 5 below, description of substituents omitted), a trimer, a tetramer, or a multimer of a higher unit.

[0133] [Chemical Formula 5]

[0134]

[0135] In one embodiment, the electrolyte for the lithium secondary battery may further include an auxiliary additive to further improve the life characteristics and output characteristics of the lithium secondary battery, or to improve high-temperature storage characteristics, etc.

[0136] For example, the auxiliary 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 sulfonyl imide compound, and the like.

[0137] In some embodiments, the content of the auxiliary additive may be 0.01 to 10 wt%, 0.1 to 7.5 wt%, or 0.3 to 5 wt% of the total weight of the electrolyte. Within this range, the life characteristics, output characteristics, high-temperature storage characteristics, etc. of the lithium secondary battery may be further improved.

[0138] In some embodiments, the ratio of the content of the auxiliary additive to the content of the compound represented by the chemical formula 1 among the total weight of the electrolyte may be 0.1 to 10, preferably 0.1 to 7, more preferably 0.5 to 5.

[0139] 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.

[0140] In some embodiments, the fluorine-containing cyclic carbonate compound may include fluoroethylene carbonate (FEC).

[0141] In some embodiments, the content of the fluorine-containing cyclic carbonate compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0142] 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.

[0143] In some embodiments, the content of the vinyl group-containing cyclic carbonate compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte. Additionally, the content of the vinylene carbonate compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0144] 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.

[0145] 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 3,3,9,9-tetraoxide, 4,4'-bi(1,3,2-dioxathiolane)] 2,2,2',2'-tetraoxide, and the like.

[0146] In some embodiments, the content of the cyclic sulfate compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0147] 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.

[0148] 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.

[0149] In some embodiments, the content of the sultone compound may be 0.01 to 2 wt% or 0.1 to 1 wt% based on the total weight of the electrolyte.

[0150] 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.

[0151] In some embodiments, the fluorine-containing lithium phosphate compound may include lithium difluorophosphate, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalato)phosphate, and the like.

[0152] In some embodiments, the content of the fluorine-containing lithium phosphate compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0153] In some embodiments, the lithium borate compound may include lithium tetraphenylborate, lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiFOB), or the like.

[0154] In some embodiments, the content of the lithium borate compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0155] 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.

[0156] In some embodiments, the content of the lactone compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0157] In some embodiments, the sulfonyl imide compound may include lithium bis(fluorosulfonyl)imide, etc.

[0158] In some embodiments, the content of the sulfonyl imide compound may be 0.01 to 2 wt% or 0.1 to 1 wt% of the total weight of the electrolyte.

[0159] For example, the organic solvent may have sufficient solubility for the lithium salt, the compound represented by the chemical formula 1, and the auxiliary additive. In one embodiment, the organic solvent may be a non-aqueous organic solvent.

[0160] In one embodiment, the organic solvent may include a carbonate solvent, an ester solvent (carboxylate solvent), an ether solvent, a ketone solvent, an alcohol solvent, an aprotic solvent, and the like.

[0161] In some embodiments, the carbonate solvent may include a linear carbonate solvent and a cyclic carbonate solvent.

[0162] For example, the linear carbonate solvent may include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, etc.

[0163] For example, the cyclic carbonate solvent may include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, etc.

[0164] In some embodiments, the organic solvent may comprise, by volume, more of the linear carbonate solvent than the cyclic carbonate solvent.

[0165] For example, among the organic solvents, the volume ratio of the cyclic carbonate solvent to the linear carbonate solvent may be 1:1 to 9:1, preferably 1.5:1 to 4:1.

[0166] In some embodiments, the ester solvent may include a linear ester solvent and a cyclic ester solvent.

[0167] For example, the linear ester solvent may include methyl propionate, ethyl propionate, propyl acetate, butyl acetate, ethyl acetate, and the like.

[0168] For example, the cyclic ester solvent may include butyrolactone, caprolactone, valerolactone, etc.

[0169] 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.

[0170] For example, the ketone solvent may include cyclohexanone, etc.

[0171] For example, the alcohol solvent may include at least one of ethyl alcohol and isopropyl alcohol.

[0172] For example, the aprotic solvent may include at least one of a nitrile solvent, an amide solvent (e.g., dimethylformamide), a dioxolane solvent (e.g., 1,3-dioxolane), and a sulfolane solvent.

[0173] In some embodiments, the organic solvent may include the carbonate solvent, and the carbonate solvent may include at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0174] The above lithium salt is Li + X - can be expressed as. In one embodiment, 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 - It could be the back.

[0175] In some embodiments, the lithium salt may include a first lithium salt comprising lithium hexafluorophosphate (LiPF6) and a second lithium salt comprising lithium bisfluorosulfonylimide (LiFSI). In addition to LiPF6, the first lithium salt may further include LiBF4, LiSbF6, LiAsF6, LiClO4, LiN(C2F5SO2)2, LiN(CF3SO2)2, CF3SO3Li, LiC(CF3SO2)3, etc.

[0176] LiFePO4 has slightly lower lithium ion mobility than lithium-nickel metal oxide, which may deteriorate the output characteristics of lithium secondary batteries. To compensate for this, when the lithium salt includes LiFSI and LiPF6, the lithium ion mobility in the electrolyte can be increased. In addition, the compound of the above chemical formula 1 can be decomposed during charging and discharging of a lithium secondary battery to form a CEI (cathode electrolyte interphase) film on the surface of the positive electrode including LiFePO4, and the Fe of LiFePO4 caused by HF generated by the decomposition of LiPF6 during charging and discharging of the battery 2+ It can prevent the elution of the form.

[0177] Accordingly, a lithium secondary battery including a cathode containing LiFePO4 and a non-aqueous electrolyte combining LiPF6, LiFSI, and the compound of chemical formula 1 as an additive can secure both high-temperature cycle characteristics and low-resistance characteristics.

[0178] In one embodiment, the lithium salt may be included in the organic solvent at a concentration of 0.01 to 5 M, preferably 0.01 to 2 M. Within the concentration range, lithium ions and / or electrons can move smoothly during charging and discharging of the battery.

[0179] In some embodiments, the content of lithium bisfluorosulfonylimide in the electrolyte may be 0.05 M to 2 M, 0.1 M to 2 M, 0.1 M to 1 M, or 0.2 M to 0.6 M. Within this range, the cycle characteristics of the battery may be improved.

[0180] In some embodiments, the ratio of the molar concentration of the first lithium salt to the molar concentration of the second lithium salt in the electrolyte may be 1 to 10, 1 to 9, 1 to 5, or 1 to 2. Within this range, the capacity of the battery can be further increased, and the capacity retention rate during repeated charge and discharge of the battery at high temperatures can be further increased.

[0181] Hereinafter, preferred embodiments and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention, and the present invention is not limited to the following examples.

[0182]

[0183] Manufacturing Example 1

[0184] 1,000 g of a solvent containing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 3:7 and 50 g of lithium difluorophosphate (LiPO2F2) were added to the reactor and stirred, and 97 g of triethylammonium ethenesulfonate was additionally added.

[0185] The mixture in the reactor was stirred at room temperature for approximately 7 hours to allow the reaction to proceed. After the reaction was completed, the resulting crystals were filtered, washed with DMC, and vacuum-dried to obtain a white target product (yield approximately 90%).

[0186] A three-dimensional structural analysis using SC-XRD was performed on the above target object and is described in Fig. 3. The SC-XRD analysis data is described in Table 1 below.

[0187] Through SC-XRD analysis, it was confirmed that the target product had the desired structure (chemical formula 6 below).

[0188] [Chemical Formula 6]

[0189]

[0190]

[0191] Empirical formula C8H19F4Li2NO7P2S Chemical formula 425.12 Å Temperature 223(2) K Wavelength 0.71073 Å Crystal system Monoclinic Space group P21 / c Unit lattice dimensions a = 13.4915(13) Å b = 12.5771(13) Å c = 11.0764(11) Å α = 90° β = 96.338(3)° γ = 90° Volume 1868.0(3) Å 3Z4 density (theoretical value) 1.512 mg / m 3 Absorption coefficient 0.410 mm -1 F(000)872 crystal size 0.413 × 0.207 × 0.144 mm 3 Data collection theta range 2.220 to 28.389°Index range -18<=h<=15, -16<=k<=16, -14<=l<=14Reflections collected 25372Independent reflections 4581[R(int) = 0.0655]Completeness to theta = 25.242° 98.10%Absorptivity correction Semi-empirical from equivalentsMaximum and minimum transmission 0.7457 and 0.5236Segmentation methodAll-matrix least-squares for F2Data / Suppression / Variables 4581 / 0 / 233Goodness of fit for F2 1.078Final R index [I>2sigma(I)] R1 = 0.0587, wR2 = 0.1633R index (all data) R1 = 0.0762, wR2 = 0.1797 occlusion coefficient n / a maximum diffraction peak and hole 1.097 and -0.660 e.Å -3

[0192]

[0193] Examples and Comparative Examples

[0194] (1) Preparation of electrolyte

[0195] A solution was prepared by dissolving lithium hexafluorophosphate (LiPF6) and / or lithium bisfluorosulfonylimide (LiFSI) in a molar concentration according to Table 2 below using a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a ratio of 25:75 (v / v).

[0196] In the above solution, the electrolytes of the examples and comparative examples were prepared by adding additives according to Table 2 below based on the total weight of the electrolyte (100 wt%).

[0197] (2) Manufacturing of lithium secondary batteries

[0198] A cathode slurry was prepared by dispersing LiFePO4, polyvinylidene fluoride (PVdF), and carbon black in a weight ratio of 90:5:5 in N-methyl-2-pyrrolidone (NMP).

[0199] 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.

[0200] A cathode slurry was prepared by dispersing crystalline artificial graphite, acetylene black, and PVDF in NMP at a weight ratio of 92:1:7.

[0201] 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.

[0202] 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.

[0203] The cell was housed inside a pouch so that some areas of the positive lead and the negative lead were exposed to the outside.

[0204] An electrolyte was injected into the pouch, and the pouch was sealed and sealed to manufacture a lithium secondary battery.

[0205] Lithium salt additive LiPF6(M)LiFSI(M) Manufacturing example 1 (wt%) Other examples 11.00.20.3-Example 20.80.40.3-Example 30.60.60.3-Example 40.60.60.3VC 1.0PS 1.0FEC 1.0Example 50.60.60.2-Example 60.60.60.01-Example 70.60.61.0-Comparative example 11.2---Comparative example 21.00.2--Comparative example 30.80.4--VC: Vinylene carbonate PS: Propane sultone FEC: Fluoroethylene carbonate

[0206]

[0207] Comparative Example 4

[0208] LiNi instead of LiFePO4 as the cathode active material 0.6 Co 0.2 Mn 0.2 A lithium secondary battery was manufactured in the same manner as in Example 1 except that O2 was used.

[0209]

[0210] Evaluation method

[0211] 1. 45℃ lifespan characteristics evaluation

[0212] (1) Measurement of initial discharge capacity

[0213] The lithium secondary batteries of the examples and comparative examples were charged at 1C CC / CV (3.9V CUT-OFF) at a temperature of 45°C and discharged at 1C CC (2.5V CUT-OFF) to measure the initial discharge capacity C1.

[0214] (2) Capacity retention rate after 300 cycles of charge and discharge

[0215] For the lithium secondary batteries of the examples and comparative examples, the above charging and discharging process was repeated 300 times at a temperature of 45°C, and the 300th discharge capacity C2 was measured.

[0216] The capacity retention rate was calculated as a percentage of C2 compared to C1, as follows.

[0217] The initial discharge capacity, 300-cycle discharge capacity, and capacity retention rate are shown in Table 3, and the capacity retention rate graph of the lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 4 according to the number of cycles is shown in Fig. 4.

[0218] Capacity retention rate (%) = C2 / C1 × 100 (%)

[0219] In addition, the capacity retention rate value (R1) of Comparative Example 1 is used as a reference for the capacity retention rate value (R) of each embodiment and comparative example. X ) was calculated as follows and recorded in Table 3.

[0220] Increase (%) = R X -R1 /

[0221]

[0222] 2. Resistance Characteristics Evaluation

[0223] The lithium secondary batteries of the examples and comparative examples were stored at 70°C for 2 weeks after being charged at 0.33C CC / CV (3.9V CUT-OFF), and then charged at 0.33C CC / CV (3.9V CUT-OFF) at room temperature (25°C), and then discharged at 0.33C CC until the SOC reached 50%. Thereafter, the C-rate was varied at 0.33C, 0.66C, 1C, 2C, and 3C, and the discharge and recharge were performed for 10 seconds each, and the DCIR was measured. The measured DCIR values ​​are shown in Table 3 below.

[0224] In addition, the DCIR value (D1) of each embodiment and comparative example is based on the DCIR value (D) of comparative example 1. X ) was calculated as follows and recorded in Table 3.

[0225] Decrease rate (%) = (D1-D X / D1) × 100(%)

[0226]

[0227] 45℃ Life Characteristics Resistance Characteristics Initial Capacity (mAh) Capacity after 300 cycles (mAh) Capacity Retention Rate (%) (Increase, %) DCIR (mΩ) (Decrease, %) Example 1 991.5 799.1 80.6 (6.1) 374.2 (13.1) Example 2 992.8 816.1 82.2 (7.7) 337.2 (21.7) Example 3 995.4 853.1 85.7 (11.2) 298.7 (30.6) Example 4 10 10.7 897.5 88.8 (14.3) 293.6 (31.8) Example 5 993.6 841.1 84.7 (10.2) 312.5 (27.4) Example 6988.5817.182.7(8.2)341.2(20.8)Example 7998.7862.486.4(11.9)295.5(31.4)Comparative Example 1980.5730.574.5(-)430.7(-)Comparative Example 2982.1760.177.4(2.9)416.9(3.2)Comparative Example 3983.3782.779.6(5.1)388.4(9.8)Comparative Example 4995.4791.679.5(5.0)380.4(11.7)

[0228] As can be seen in Table 3 above, the lithium secondary batteries of Examples 1 to 7 exhibited improved high-temperature life characteristics compared to the lithium secondary batteries of the comparative examples.

[0229] Referring to FIG. 4, it can be confirmed that the batteries of Examples 1 to 4 have particularly improved high-temperature life characteristics compared to the batteries of Comparative Examples 1 to 4.

[0230] All of the batteries of the examples had a capacity retention rate value that was 6.1% or more higher than that of the battery of Comparative Example 1, indicating improved high-temperature life characteristics.

[0231] Therefore, it was confirmed that the high-temperature life characteristics of a lithium secondary battery were significantly improved when LiPF6 and LiFSI were mixed as lithium salts and the compound of chemical formula 1 was used in combination as an additive.

[0232] In addition, when VC, PS, and FEC additives were used together as in Example 4, the high-temperature life characteristics of the lithium secondary battery could be further improved. As can be further confirmed in Table 3 above, the lithium secondary batteries of the examples exhibited improved low-resistance characteristics compared to the lithium secondary batteries of the comparative examples.

[0233] All examples had values ​​that were reduced by 13.1% or more compared to the DCIR value of Comparative Example 1, indicating that the low resistance characteristics of the battery were improved by using a mixture of LiPF6 and LiFSI as lithium salts.

[0234] In particular, the battery of Comparative Example 4 including the NCM-based positive electrode active material had other characteristics similar to those of the examples, but had low high-temperature capacity retention and thus inferior high-temperature life characteristics.

[0235] Therefore, it was confirmed that the low-resistance characteristics of lithium secondary batteries were significantly improved when LiPF6 and LiFSI were mixed as lithium salts and the compound of chemical formula 1 was used in combination as an additive. In addition, it was confirmed that the low-resistance characteristics of lithium secondary batteries were further improved when VC, PS, and FEC additives were used together.

Claims

1. A positive electrode including a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector and including lithium iron phosphate; A cathode opposite to the anode; A separator interposed between the positive electrode and the negative electrode; and A lithium secondary battery comprising an electrolyte comprising a lithium salt, an organic solvent, and a compound represented by the following chemical formula 1: [Chemical Formula 1] (In chemical formula 1, R1 is a C1-C6 alkyl group; a C2-C6 alkenyl group; a C2-C6 alkynyl group; a C3-C7 cycloalkyl group; or a C3-C7 cycloalkenyl group, R2 to R5 are each independently a halogen; or a C1-C6 alkyl group, M is an alkali metal, Y + is a cationic substance).

2. In claim 1, a lithium secondary battery, wherein the lithium iron phosphate comprises a compound represented by the following chemical formula 2: [Chemical Formula 2] Li x Fe 1-y M y (PO 4-z )X z (In the chemical formula 2, M includes at least one element selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, X includes at least one element selected from the group consisting of F, S, and N, and x, y, and z are 0.5≤x≤1.5, 0≤y≤0.1, and 0≤z≤0.5, respectively.) 3. A lithium secondary battery according to claim 1, wherein the lithium iron phosphate comprises LiFePO4 having an olivine crystal structure.

4. In claim 1, R1 is a C1-C6 alkyl group; or a C2-C6 alkenyl group, At least one of R2 to R5 is a halogen, M is Li, Na or K, Y + is N + A lithium secondary battery, wherein RaRbRcRd, Ra to Rd are each independently hydrogen or a C1-C6 alkyl group, and at least one of Ra to Rd is hydrogen.

5. In claim 1, R1 is a C2-C6 alkenyl group, R2 to R5 are all halogens, M is Li, Y + is N + A lithium secondary battery, wherein RaRbRcRd, Ra is hydrogen, and Rb to Rd are each independently a C1-C3 alkyl group.

6. A lithium secondary battery according to claim 1, wherein the content of the compound represented by the chemical formula 1 is 0.01 to 10 wt% of the total weight of the electrolyte.

7. A lithium secondary battery according to claim 1, wherein the lithium salt comprises a first lithium salt including lithium hexafluorophosphate (LiPF6) and a second lithium salt including lithium bisfluorosulfonylimide (LiFSI).

8. A lithium secondary battery according to claim 7, wherein the content of lithium bisfluorosulfonylimide in the electrolyte is 0.05 M to 2 M.

9. A lithium secondary battery according to claim 7, wherein the ratio of the molar concentration of the first lithium salt to the molar concentration of the second lithium salt in the electrolyte is 1 to 10.

10. A lithium secondary battery according to claim 1, wherein the organic solvent comprises at least one of a linear carbonate solvent, a cyclic carbonate solvent, a linear ester solvent, a cyclic ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, and an aprotic solvent.

11. A lithium secondary battery according to claim 1, wherein the electrolyte further comprises an auxiliary additive comprising at least one 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 sulfonyl imide compound.

12. A lithium secondary battery according to claim 11, wherein the content of the auxiliary additive is 0.01 to 10 wt% of the total weight of the electrolyte.

13. A lithium secondary battery according to claim 11, wherein the ratio of the content of the auxiliary additive to the content of the compound represented by the chemical formula 1 among the total weight of the electrolyte is 0.1 to 10.

14. A lithium secondary battery according to claim 1, wherein the negative electrode comprises a negative electrode current collector and a negative electrode active material layer formed on at least one surface of the negative electrode current collector and including a carbon-based active material.

15. A lithium secondary battery according to claim 14, wherein the carbon-based active material comprises artificial graphite or natural graphite.

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