Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, in which the positive electrode comprises a positive electrode active material, the positive electrode active material comprises lithium iron phosphate particles, the loading amount of the positive electrode is 450 mg / 25 cm2 to 740 mg / 25 cm2, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, the organic solvent contains a cyclic carbonate solvent and a linear carbonate solvent, the cyclic carbonate solvent contains ethylene carbonate, the linear carbonate solvent contains dimethyl carbonate, the content of the dimethyl carbonate in the organic solvent is 5-75% by volume, the additive contains vinylene carbonate, and the content of the dimethyl carbonate in the organic solvent is 5-75% by volume. The weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 and less than or equal to 0.2.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2022-0146438, filed on November 4, 2022, the disclosure of which is incorporated herein by reference. Technical Field

[0003] The invention relates to a lithium secondary battery. Background Art

[0004] As the information society has developed, personal IT devices and computer networks have also developed, and society's dependence on electric energy has increased overall, so there is a need to develop technology for efficiently storing and utilizing electric energy.

[0005] Among the developed technologies, secondary batteries are the most suitable for various uses, and among these secondary batteries, attention is increasing on lithium secondary batteries that can be made small enough to be applied to personal IT equipment and the like and also have the highest energy density.

[0006] Generally, a lithium secondary battery is manufactured by injecting or impregnating a non-aqueous electrolyte into an electrode assembly consisting of a positive electrode, a negative electrode, and a porous separator.

[0007] Carbon-based active materials, silicon-based active materials, etc. are considered to be negative electrode active materials for lithium secondary batteries. 2 , LiMn with spinel crystal structure 2 O 4 , Li-containing nickel oxide (LiNiO 2 ) etc. are used as positive electrode active materials.

[0008] Recently, lithium iron phosphate (e.g., LiFePO4) has been used, which has excellent thermal stability and is relatively inexpensive. 4 ) type active materials have been used as positive electrode active materials.

[0009] However, the specific capacity of lithium iron phosphate active materials is lower than that of lithium cobalt oxide, lithium nickel oxide, etc. Therefore, in order to improve the energy density of the positive electrode and the lithium secondary battery containing it, the lithium iron phosphate positive electrode active material should be used at a high loading amount. However, the high-load lithium iron phosphate positive electrode has the problem that the non-aqueous electrolyte is difficult to fully impregnate into the positive electrode, so there is a problem of difficulty in expressing the capacity, increased resistance and deteriorated life. Summary of the invention

[0010] [Technical issues]

[0011] One aspect of the present invention provides a lithium secondary battery comprising lithium iron phosphate particles as a positive electrode active material, and the positive electrode has a specific loading amount or more, wherein the lithium secondary battery has excellent capacity performance, excellent life performance and resistance reduction effect by improving the impregnation properties of the positive electrode for a non-aqueous electrolyte while improving the reduction stability of the negative electrode.

[0012] [Technical solution]

[0013] According to one aspect of the present invention, a lithium secondary battery is provided, which comprises a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode active material, wherein the positive electrode active material comprises lithium iron phosphate particles, and the loading amount of the positive electrode is 450 mg / 25 cm 2 Up to 740mg / 25cm 2 The non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive, wherein the organic solvent comprises a cyclic carbonate solvent and a linear carbonate solvent, wherein the cyclic carbonate solvent contains ethylene carbonate, the linear carbonate solvent contains dimethyl carbonate, the content of dimethyl carbonate in the organic solvent is 5 volume % to 75 volume %, and the additive contains vinylene carbonate, and the ratio of the weight of vinylene carbonate to the weight of dimethyl carbonate is greater than 0 to less than 0.2.

[0014] [Beneficial Effects]

[0015] The lithium secondary battery of the present invention is characterized in that it comprises a positive electrode having a specific loading amount or more and comprising lithium iron phosphate particles as a positive electrode active material, and a non-aqueous electrolyte comprising ethylene carbonate and dimethyl carbonate as organic solvents and comprising vinylene carbonate as an additive, wherein the content and content ratio of each of dimethyl carbonate and vinylene carbonate are adjusted to a specific range. According to the lithium secondary battery of the present invention, excellent dimethyl carbonate is used as an organic solvent component to improve the electrolyte impregnation properties of the positive electrode with a high loading amount, and at the same time, a vinylene carbonate additive having a specific content ratio relative to dimethyl carbonate is used to improve the negative electrode reduction stability, so that the capacity of the lithium secondary battery can be shown at an excellent level, and the life performance and resistance properties can be improved. DETAILED DESCRIPTION

[0016] First, before describing the present invention, it will be understood that the terms or words used in the present specification and claims should not be interpreted as having the meanings defined in commonly used dictionaries, but should be interpreted as having meanings and concepts consistent with the technical concept of the present invention based on the principle that the inventor can appropriately define the concept of the terms to best interpret the present invention.

[0017] Meanwhile, the terms used herein are only used to describe exemplary embodiments and are not intended to limit the present invention. Unless the context clearly indicates otherwise, a singular expression includes a plural expression.

[0018] It will also be understood that the terms “comprises,” “comprising,” or “having,” when used in this specification, specify the presence of stated features, quantities, steps, elements, or a combination thereof, but do not exclude the presence or addition of one or more other features, quantities, steps, elements, or a combination thereof.

[0019] In this specification, "%" means weight % unless otherwise specified.

[0020] Before describing the present invention, it will be understood that in the description of "a to b carbon atoms" herein, "a" and "b" refer to the number of carbon atoms contained in a particular functional group. That is, the functional group can contain "a" to "b" carbon atoms.

[0021] Furthermore, in the present specification, unless otherwise defined, "substituted" means that at least one hydrogen bonded to carbon is replaced by an element other than hydrogen, and for example, it means substitution by an alkyl group having 1 to 5 carbon atoms or a fluorine element.

[0022] In this specification, the average particle size (D 50 ) can be defined as the particle size corresponding to 50% of the volume accumulation in the particle size distribution curve. 50 ) can be measured, for example, by laser diffraction. Laser diffraction generally allows the measurement of particle sizes in the submicron range up to several millimeters, and thus highly reproducible and high-resolution results can be obtained.

[0023] Hereinafter, the present invention will be described in more detail.

[0024] Lithium secondary battery

[0025] The invention relates to a lithium secondary battery.

[0026] Specifically, the lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode active material, wherein the positive electrode active material comprises lithium iron phosphate particles, and the loading amount of the positive electrode is 450 mg / 25 cm 2 Up to 740mg / 25cm 2, wherein the non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive, wherein the organic solvent comprises a cyclic carbonate solvent and a linear carbonate solvent, wherein the cyclic carbonate solvent contains ethylene carbonate, and the linear carbonate solvent contains dimethyl carbonate, wherein the content of dimethyl carbonate in the organic solvent is 5 volume % to 75 volume %, and the additive contains vinylene carbonate, and the ratio of the weight of vinylene carbonate to the weight of dimethyl carbonate is greater than 0 to less than 0.2.

[0027] The lithium secondary battery of the present invention is characterized in that it comprises a positive electrode having a specific loading amount or more and comprising lithium iron phosphate particles as a positive electrode active material, and a non-aqueous electrolyte comprising ethylene carbonate and dimethyl carbonate as organic solvents and comprising vinylene carbonate as an additive, wherein the content and content ratio of each of dimethyl carbonate and vinylene carbonate are adjusted to a specific range. According to the lithium secondary battery of the present invention, dimethyl carbonate is used as an organic solvent component to improve the electrolyte impregnation properties of the positive electrode with a high loading amount, and at the same time, a vinylene carbonate additive having a specific content ratio relative to dimethyl carbonate is used to improve the negative electrode reduction stability, so that the capacity of the lithium secondary battery can be shown at an excellent level, and the life performance and resistance properties can be improved.

[0028] The capacity retention rate of the lithium secondary battery of the present invention at the 200th cycle can be 90% or more, preferably 90% to 95%; and the resistance increase rate at the 200th cycle can be 20% or less, preferably 15% or less, while maintaining a cell design capacity of at least 500mAh, more specifically at least 550mAh and an initial discharge capacity of at least 500mAh, more specifically at least 540mAh, only when the electrolyte contained contains an organic solvent containing ethylene carbonate and dimethyl carbonate and an additive containing vinylene carbonate, wherein the content of dimethyl carbonate in the organic solvent is 5% to 75% by volume, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2, and the lithium iron phosphate particles containing lithium iron phosphate as an active material and the loading amount is 450mg / 25cm 2 Up to 740mg / 25cm 2 The cell design capacity, initial discharge capacity, capacity retention rate and resistance increase rate were measured according to the methods described in the following examples.

[0029] The lithium secondary battery includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Specifically, the lithium secondary battery includes a positive electrode, a negative electrode opposite to the positive electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0030] A lithium secondary battery can be manufactured by: preparing an electrode assembly including a positive electrode, a negative electrode and a separator; housing the electrode assembly in a battery case; preparing a non-aqueous electrolyte including a lithium salt, an organic solvent and additives; and injecting or impregnating the prepared non-aqueous electrolyte into the battery case.

[0031] (1) Positive electrode

[0032] The positive electrode includes a positive electrode active material. The positive electrode active material includes lithium iron phosphate particles.

[0033] The lithium iron phosphate particles may include a compound represented by Formula A below.

[0034] [Formula A]

[0035] Li 1+a Fe 1-s M s (PO 4-b )X b

[0036] In the above formula A, M is one or more elements selected from the group consisting of Co, Ni, Mn, Al, Mg, Ti and V, X is F, S or N, wherein 0≤s≤0.5, -0.5≤a≤+0.5, and 0≤b≤0.1.

[0037] The above formula A can be specifically composed of LiFePO 4 (a=0, s=0, b=0) represents.

[0038] The lithium iron phosphate particles may be in the form of primary particles, or may be in the form of secondary particles formed by agglomeration of two or more primary particles. Specifically, the lithium iron phosphate particles may be in the form of primary particles.

[0039] The lithium iron phosphate particles may consist of primary particles, may consist of secondary particles formed by agglomeration of two or more primary particles, or may be a mixture of primary particles and secondary particles formed by agglomeration of two or more primary particles.

[0040] At this time, when the lithium iron phosphate particles are in the form of primary particles, the average particle size (D 50 ) may be 0.2 μm to 3.0 μm, specifically 0.2 μm to 2.0 μm, more specifically 0.3 μm to 1.5 μm. In addition, when the lithium iron phosphate particles are in the form of secondary particles formed by agglomeration of two or more primary particles, the average particle size (D 50 ) may be 0.2 μm to 3.0 μm, specifically 0.2 μm to 2.0 μm, more specifically 0.3 μm to 1.5 μm, and the average particle size (D 50) may be 7 μm to 25 μm, specifically 10 μm to 20 μm.

[0041] The positive electrode active material may further include a carbon coating layer on the surface of the lithium iron phosphate particles. The carbon coating layer may be introduced to protect the lithium iron phosphate particles, improve conductivity, and the like.

[0042] The positive electrode active material may not contain lithium nickel oxides such as lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide. In the case where the positive electrode contains lithium nickel oxide, even if 450 mg / 25 cm 2 The above loading amount and the non-aqueous electrolyte described later may also have difficulty in exerting the effect.

[0043] The positive electrode loading can be 450 mg / 25 cm 2 Up to 740mg / 25cm 2 .

[0044] Compared with other positive electrode active materials (such as lithium cobalt oxide and lithium nickel cobalt manganese oxide), lithium iron phosphate particles have the advantages of excellent thermal stability and relatively low cost, but due to their small specific capacity, there is a problem that the loading amount should be increased to achieve high energy density. When the loading amount of the positive electrode is increased (for example, 450 mg / 25 cm 2 Up to 740mg / 25cm 2 ), a high energy density battery can be achieved (for example, a lithium secondary battery having a cell design capacity of at least 500mAh, more specifically at least 550mAh and an initial discharge capacity of at least 500mAh, more specifically at least 540mAh), but it is difficult for the non-aqueous electrolyte to be fully impregnated into the positive electrode, so there is a problem of difficulty in expressing the capacity of the lithium secondary battery, increased resistance, and deterioration of life performance.

[0045] In order to solve this problem, the lithium secondary battery of the present invention uses a non-aqueous electrolyte containing dimethyl carbonate as an organic solvent component and vinylene carbonate as an additive, wherein the content and content ratio thereof are adjusted to a specific range. By this feature, the loading capacity of 450 mg / 25 cm 2 Up to 740mg / 25cm 2 The electrolyte impregnation properties of the positive electrode can be improved, and at the same time, the negative electrode reduction stability can be improved, so the capacity of the lithium secondary battery can be expressed at an excellent level, and the life performance and resistance properties can be improved.

[0046] When the positive electrode loading is less than 450 mg / 25 cm 2 When the electrolyte is impregnated with water, the above-mentioned problem of deterioration of the electrolyte impregnation property does not occur, and thus the effect of using the non-aqueous electrolyte of the present invention is not exhibited.

[0047] At the same time, if the positive electrode loading is greater than 740 mg / 25 cm 2 , even if the non-aqueous electrolyte of the present invention is applied to the positive electrode containing lithium iron phosphate particles, there is a problem that the electrolyte impregnation property may not be fully ensured. In addition, when the positive electrode loading is greater than 740 mg / 25 cm 2 When, for example, the average particle size (D 50 ) When smaller lithium iron phosphate particles are applied to the positive electrode, the size of the pores formed between the lithium iron phosphate particles is smaller. In this case, when manufacturing the positive electrode, the slurry solvent evaporates from the pores formed between the lithium iron phosphate particles during the drying process, which may cause the positive electrode to crack, making it difficult to manufacture or realize the positive electrode.

[0048] Specifically, the positive electrode loading can be 450 mg / 25 cm 2 Up to 740mg / 25cm 2 , 450mg / 25cm 2 Up to 730mg / 25cm 2 , 450mg / 25cm 2 Up to 720mg / 25cm 2 , 450mg / 25cm 2 Up to 710mg / 25cm 2 , or 450mg / 25cm 2 Up to 700mg / 25cm 2 , more specifically 500mg / 25cm 2 Up to 680mg / 25cm 2 , 500mg / 25cm 2 Up to 650mg / 25cm 2 , 500mg / 25cm 2 Up to 625mg / 25cm 2 or 500mg / 25cm 2 Up to 600mg / 25cm 2 .

[0049] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. In this case, the positive electrode active material layer may include the above-mentioned positive electrode active material.

[0050] There is no particular limitation on the positive electrode current collector as long as it has conductivity and does not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used as the positive electrode current collector.

[0051] The thickness of the positive electrode current collector may generally be 3 μm to 500 μm.

[0052] The positive electrode current collector may have fine irregularities formed on its surface to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric body.

[0053] The positive electrode active material layer is disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one surface or both surfaces of the positive electrode current collector.

[0054] In consideration of fully utilizing the capacity of the positive electrode active material, the content of the positive electrode active material in the positive electrode active material layer may be 80 wt % to 99 wt %.

[0055] The positive electrode active material layer may further include a binder and / or a conductive material together with the positive electrode active material.

[0056] The binder is a component used to assist the bonding of active materials, conductive materials, etc. and to the current collector, and specifically may include at least one of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber or fluororubber, preferably polyvinylidene fluoride.

[0057] In terms of ensuring sufficient binding force between components such as the positive electrode active material, the content of the binder in the positive electrode active material layer may be 1 wt % to 20 wt %, preferably 1.2 wt % to 10 wt %.

[0058] Conductive materials can be used to assist and improve the conductivity in secondary batteries, and there is no particular limitation as long as they have conductivity without causing chemical changes. Specifically, the positive electrode conductive material may include: graphite, such as natural graphite and artificial graphite; carbon materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal carbon black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives, and in terms of improving conductivity, carbon black may be preferably included.

[0059] In terms of ensuring sufficient conductivity, the content of the conductive material in the positive electrode active material layer may be 1 wt % to 20 wt %, preferably 1.2 wt % to 10 wt %.

[0060] The thickness of the positive electrode active material layer may be 100 μm to 300 μm, preferably 150 μm to 250 μm.

[0061] The positive electrode may be manufactured by coating a positive electrode slurry including a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming the positive electrode slurry on a positive electrode collector, followed by drying and roll-pressing.

[0062] The positive electrode slurry forming solvent may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP). The positive electrode slurry may have a solid content of 40 wt % to 90 wt %, specifically 50 wt % to 80 wt %.

[0063] (2) Negative electrode

[0064] The negative electrode may be opposite to the positive electrode.

[0065] The negative electrode includes a negative electrode active material.

[0066] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. In this case, the negative electrode active material layer may include a negative electrode active material.

[0067] The negative electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used as the negative electrode current collector.

[0068] The thickness of the negative electrode current collector may generally be 3 μm to 500 μm.

[0069] The negative electrode current collector may have fine irregularities formed on its surface to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric body.

[0070] The negative electrode active material layer is disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one surface or both surfaces of the negative electrode current collector.

[0071] The negative electrode active material layer may include a negative electrode active material.

[0072] The negative electrode active material is a material that can reversibly embed / deintercalate lithium ions, and may include at least one of a carbon-based active material, a (semi)metal-based active material or lithium metal, specifically, may include at least one of a carbon-based active material or a (semi)metal-based active material.

[0073] The carbon-based active material may include at least one of graphite, hard carbon, soft carbon, carbon black, graphene or fibrous carbon, and preferably may include graphite. The graphite may include at least one of natural graphite or artificial graphite.

[0074] In terms of ensuring structural stability during charge and discharge and reducing side reactions with the electrolyte solution, the average particle size (D 50 ) can be 10 μm to 30 μm, preferably 15 μm to 25 μm.

[0075] Specifically, the (semi)metal active materials may include: at least one (semi)metal of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti or Sn; an alloy of lithium and at least one (semi)metal of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti or Sn; an oxide of at least one (semi)metal of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti or Sn, lithium titanium oxide (LTO), lithium vanadium oxide, etc.

[0076] More specifically, the (semi)metal-based active material may include a silicon-based active material.

[0077] The silicon-based active material may include SiO x (0≤x<2) represented by the compound. 2 Since it does not react with lithium ions and cannot store lithium, x is preferably within the above range. More preferably, the silicon-based active material may be SiO.

[0078] In terms of ensuring structural stability during charge and discharge and reducing side reactions with the electrolyte solution, the average particle size (D 50 ) can be 1 μm to 30 μm, preferably 2 μm to 15 μm.

[0079] The content of the negative electrode active material in the negative electrode active material layer may be 60 wt % to 99 wt %, preferably 75 wt % to 95 wt %.

[0080] The negative electrode active material layer may further include a binder and / or a conductive material in addition to the negative electrode active material.

[0081] At this time, the binder is used to improve the battery performance by improving the adhesion between the negative electrode active material layer and the negative electrode current collector, and may include, for example, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or at least any one of the materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0082] The content of the binder in the negative electrode active material layer may be 0.5 wt % to 10 wt %, preferably 1 wt % to 5 wt %.

[0083] There is no particular limitation on the conductive material as long as it has conductivity and does not cause chemical changes in the battery. For example, the following can be used: graphite, such as natural graphite and artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black and thermal black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; conductive materials, such as polyphenylene derivatives, etc.

[0084] The content of the conductive material in the negative electrode active material layer may be 0.5 wt % to 10 wt %, preferably 1 wt % to 5 wt %.

[0085] The thickness of the negative electrode active material layer may be 50 μm to 300 μm, preferably 100 μm to 200 μm.

[0086] The loading amount of the negative electrode active material layer can be 200 mg / 25 cm 2 Up to 500mg / 25cm 2 , preferably 250mg / 25cm 2 Up to 400mg / 25cm 2 .

[0087] The negative electrode may be manufactured by coating a negative electrode slurry including a negative electrode active material, a binder, a conductive material, and / or a solvent for forming a negative electrode slurry on at least one surface of a negative electrode collector, followed by drying and roll pressing.

[0088] The negative electrode slurry forming solvent may include, for example, at least one of distilled water, N-methyl-2-pyrrolidone (NMP), ethanol, methanol or isopropanol, preferably distilled water in terms of promoting the dispersion of the negative electrode active material, the binder and / or the conductive material. The solid content of the negative electrode slurry may be 30 wt % to 80 wt %, specifically 40 wt % to 70 wt %.

[0089] (3) Diaphragm

[0090] A separator may be disposed between the positive electrode and the negative electrode.

[0091] As a separator, a conventional porous polymer film is usually used as a separator, for example, a porous polymer film made of polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer can be used alone, or a laminate thereof can be used. Alternatively, a conventional porous nonwoven fabric can be used, such as a nonwoven fabric made of high melting point glass fiber or polyethylene terephthalate fiber, but the present invention is not limited thereto. In addition, a coated separator comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and can be selectively used in a single layer or multilayer structure.

[0092] (4) Non-aqueous electrolytes

[0093] 1) Lithium salt

[0094] First, the lithium salt will be described as follows.

[0095] In the non-aqueous electrolyte solution for a lithium secondary battery of the embodiment of the present invention, any lithium salt may be used as the lithium salt without particular limitation as long as it is generally used in an electrolyte solution for a lithium secondary battery, and for example, the lithium salt may include Li + as cations and may contain F - , Cl - Br - ,I - 、NO 3 - 、N(CN) 2 - , BF 4 - , ClO 4 - 、AlO 4 - 、AlCl 4 - PF 6 - , SbF 6 - , AsF 6 -、B 10 Cl 10 - 、BF 2 C 2 O 4 - 、BC 4 O 8 - 、PF 4 C 2 O 4 - 、PF 2 C 4 O 8 - 、(CF 3 ) 2 PF 4 - 、(CF 3 ) 3 PF 3 - 、(CF 3 ) 4 PF 2 - 、(CF 3 ) 5 PF - 、(CF 3 ) 6 P - 、CF 3 SO 3 - 、C 4 F 9 SO 3 - 、CF 3 CF 2 SO 3 - 、(CF 3 SO 2 ) 2 N - 、(FSO 2 ) 2 N - 、CF 3 CF 2 (CF 3 ) 2 CO - 、(CF 3 SO 2 ) 2 CH - 、CH 3 SO 3 - 、CF 3 (CF2 ) 7 SO 3 - CF 3 CO 2 - , CH 3 CO 2 - 、SCN - or (CF 3 CF 2 SO 2 ) 2 N - At least one of the following is used as an anion. Specifically, the lithium salt may be selected from LiCl, LiBr, LiI, LiBF 4 、LiClO 4 、LiAlO 4 、LiAlCl 4 、LiPF 6 、LiSbF 6 、LiAsF 6 , LiB 10 Cl 10 、LiBOB(LiB(C 2 O 4 ) 2 )、LiCF 3 SO 3 、LiTFSI(LiN(SO 2 CF 3 ) 2 )、LiFSI(LiN(SO 2 F) 2 )、LiCH 3 SO 3 、LiCF 3 CO 2 、LiCH 3 CO 2 and LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ) is at least one of the group consisting of. Specifically, the lithium salt may include LiBF 4 、LiClO 4 、LiPF 6 、LiBOB(LiB(C 2 O 4 ) 2 )、LiCF 3 SO 3 、LiTFSI(LiN(SO 2 CF 3 )2 )、LiFSI(LiN(SO 2 F) 2 ) or LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ) or a mixture of two or more thereof, and more specifically, may include LiPF 6 .

[0096] The content of the lithium salt may be appropriately changed within the conventional range of usable lithium salts, but in order to obtain the best effect of forming an anti-corrosion film on the surface of the electrode, the concentration of the lithium salt contained in the electrolyte solution may be 0.8 M to 3.0 M, specifically 1.0 M to 3.0 M. At this time, the unit "M" is the molar concentration, which may specifically represent "mol / L".

[0097] When the concentration of the lithium salt satisfies the above range, the viscosity of the nonaqueous electrolyte can be controlled to achieve optimal impregnation, and the mobility of lithium ions can be improved to obtain the effect of improving the capacity properties and cycle properties of the lithium secondary battery.

[0098] 2) Organic solvents

[0099] The organic solvent may include a cyclic carbonate-based solvent and a linear carbonate-based solvent. The organic solvent may consist of a cyclic carbonate-based solvent and a linear carbonate-based solvent.

[0100] The volume ratio of the cyclic carbonate solvent to the linear carbonate solvent may be 10:90 to 50:50, specifically 15:85 to 50:50, more specifically 20:80 to 35:65, and within the above range is preferred in terms of achieving high ion transport properties and low electrolyte viscosity.

[0101] The cyclic carbonate-based solvent includes ethylene carbonate. Ethylene carbonate is a high-viscosity organic solvent with a high dielectric constant, and thus can dissociate lithium salts well in the electrolyte.

[0102] The cyclic carbonate-based solvent may not contain a fluorinated cyclic carbonate such as fluoroethylene carbonate (FEC). Specifically, the cyclic carbonate-based solvent may be formed of only ethylene carbonate, and may not contain another cyclic carbonate-based solvent such as propylene carbonate.

[0103] In addition, the linear carbonate solvent contains dimethyl carbonate. Dimethyl carbonate is an organic solvent with low viscosity and low dielectric constant, and is particularly excellent in electrolyte impregnation properties, so a high-load positive electrode containing the lithium iron phosphate particles of the present invention can be impregnated at an excellent level. At the same time, as an organic solvent, dimethyl carbonate has the problem of forming an unstable negative electrode film, which leads to the problem of deterioration of the reduction stability of the negative electrode, but as described below, when dimethyl carbonate and vinylene carbonate are used in a specific content ratio, the electrolyte impregnation properties and the reduction stability of the negative electrode can be improved at the same time. In addition, the non-aqueous electrolyte of the present invention can achieve the desired effect, especially when the loading amount of the positive electrode containing the lithium iron phosphate particles is 450mg / 25cm 2 Up to 740mg / 25cm 2 If the positive electrode loading is less than 450 mg / 25 cm 2 or greater than 740mg / 25cm 2 , the effect of improving the electrolyte impregnation property cannot be obtained by using dimethyl carbonate.

[0104] The content of dimethyl carbonate in the organic solvent is 5 volume % to 75 volume %. In one embodiment, the content of dimethyl carbonate in the organic solvent is 5 volume % to 55 volume %, more specifically 7 volume % to 45 volume %, and more specifically 35 volume % to 45 volume %. If the content of dimethyl carbonate in the organic solvent is less than 5 volume %, the impregnation property of the electrolyte to the positive electrode cannot be improved. If the content of dimethyl carbonate in the organic solvent is greater than 75 volume %, an unstable SEI film is formed, and the performance of the battery cell is degraded, which is not preferred.

[0105] The linear carbonate solvent may further include ethyl methyl carbonate together with dimethyl carbonate. When the linear carbonate further includes ethyl methyl carbonate, preferably, the stability of the SEI film may be further improved.

[0106] When the linear carbonate solvent also includes ethyl methyl carbonate, the organic solvent may include 10% to 50% by volume of ethylene carbonate, 5% to 55% by volume of dimethyl carbonate, and 20% to 70% by volume of ethyl methyl carbonate, more specifically 20% to 40% by volume of ethylene carbonate, 7% to 45% by volume of dimethyl carbonate, and 25% to 65% by volume of ethyl methyl carbonate, and more specifically 25% to 35% by volume of ethylene carbonate, 30% to 45% by volume of dimethyl carbonate, and 25% to 50% by volume of ethyl methyl carbonate, or 30% to 35% by volume of ethylene carbonate, 35% to 45% by volume of dimethyl carbonate, and 25% to 50% by volume of ethyl methyl carbonate. When within the above range, it is preferred in terms of improving electrolyte impregnation properties and improving the stability of the negative electrode SEI film.

[0107] Meanwhile, if necessary, the organic solvent may further use any organic solvent generally used in non-aqueous electrolytes without limitation. For example, at least one organic solvent selected from ester solvents, ether solvents, glyme solvents, or nitrile solvents may be further included.

[0108] The ester solvent may include at least one of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate and butyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone or ε-caprolactone.

[0109] As the ether solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL) and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL) can be used, or a mixture of two or more thereof can be used, but the present invention is not limited thereto.

[0110] Glyme solvents are solvents having a higher dielectric constant and lower surface tension than linear carbonate solvents and having less reactivity with metals, and may include at least one of dimethoxyethane (glyme dimethyl ether, DME), diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether (TEGDME), but are not limited thereto.

[0111] The nitrile solvent can be one or more selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, octanonitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, benzyl cyanide, 2-fluorobenzoyl cyanide, and 4-fluorobenzoyl cyanide, but is not limited thereto.

[0112] Meanwhile, unless otherwise specified, the rest of the non-aqueous electrolyte except the lithium salt and the additive may be an organic solvent.

[0113] 3) Additives

[0114] The non-aqueous additive of the present invention comprises additives.

[0115] The additive includes vinylene carbonate.

[0116] In terms of forming a stable SEI film on the negative electrode, vinylene carbonate can be used as an additive for the non-aqueous electrolyte of the present invention. In particular, when dimethyl carbonate is used as an organic solvent, there is a problem that the stability of the negative electrode SEI film deteriorates when exposed to high temperatures, but by using vinylene carbonate as an additive, the negative electrode reduction stability can be improved.

[0117] In the present invention, the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2. If the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0.2, the negative electrode SEI film is excessively formed, which may cause problems such as increased resistance and deterioration of life performance.

[0118] Specifically, the ratio of the weight of vinylene carbonate to the weight of dimethyl carbonate can be 0.01 to 0.18, more specifically 0.016 to 0.130, and still more specifically 0.02 to 0.08, and when within the above range, the effect of simultaneously improving the electrolyte impregnation properties of the positive electrode and the reduction stability of the negative electrode can be preferably achieved.

[0119] The ratio of the weight of vinylene carbonate to the weight of dimethyl carbonate can be calculated from the weight or volume of the entire non-aqueous electrolyte, the volume content, weight content, density information, and the like of dimethyl carbonate.

[0120] The content of vinylene carbonate in the non-aqueous electrolyte can be 0.01 wt % to 7 wt %, specifically 0.3 wt % to 6 wt %, more specifically 0.4 wt % to 3 wt %, and even more specifically 0.6 wt % to 2 wt %. When within the above range, it is preferred to appropriately form a negative electrode SEI film to prevent electrolyte side reactions and prevent an increase in resistance due to excessive use of additives.

[0121] At the same time, if necessary, the additives may also include additional additives besides vinylene carbonate to prevent the non-aqueous electrolyte solution from decomposing under high output conditions and causing the negative electrode to disintegrate, or to further improve low-temperature high-rate discharge properties, high-temperature stability, overcharge protection, and the effect of inhibiting battery expansion at high temperatures, etc.

[0122] Examples of other additives may include at least one of cyclic carbonate compounds, halogenated carbonate compounds, sultone compounds, sulfonate compounds, sulfate compounds, phosphate compounds or phosphite compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds or lithium salt compounds.

[0123] The cyclic carbonate compound may be, for example, vinyl ethylene carbonate or the like.

[0124] The halogenated carbonate compound may be, for example, fluoroethylene carbonate (FEC) or the like.

[0125] The sultone compound may be, for example, at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethylene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone.

[0126] The sulfonate compounds may contain a saturated hydrocarbon group or an unsaturated hydrocarbon group such as an alkenylene group or an alkynylene group.

[0127] The sulfate compound may be, for example, ethylene sulfate (ESA), trimethylene sulfate (TMS), methyl trimethylene sulfate (MTMS), or the like.

[0128] The phosphate or phosphite compound can be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate and tris(trifluoroethyl)phosphite.

[0129] The borate / salt compound can be tetraphenylborate, lithium difluoro(oxalato)borate (LiODFB), lithium bis(oxalato)borate (LiB(C 2 O 4 ) 2 , LiBOB), etc.

[0130] The nitrile compound can be, for example, at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, octanonitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, benzyl cyanide, 2-fluorobenzoyl cyanide and 4-fluorobenzoyl cyanide.

[0131] The benzene compound may be, for example, fluorobenzene, etc., the amine compound may be triethanolamine, ethylenediamine, etc., and the silane compound may be tetravinylsilane, etc.

[0132] The lithium salt compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution and may be lithium difluorophosphate (LiPO 2 F 2 )、LiBF 4 wait.

[0133] The additional additives may be used in combination of two or more compounds, and the total content of the above vinylene carbonate and the additional additives may be 0.05 wt % to 20 wt %, more specifically 0.05 wt % to 10 wt %, based on the total weight of the non-aqueous electrolyte. When the total content of the additives satisfies the above range, high temperature storage properties and high temperature life properties may be more effectively improved, and battery side reactions caused by the additives remaining after the reaction may be prevented.

[0134] The non-aqueous electrolyte can be manufactured in the following manner: ethylene carbonate (EC), dimethyl carbonate (DMC) and optional ethyl methyl carbonate (EMC) are mixed in the amounts as described above to prepare an organic solvent, in particular, wherein the content of dimethyl carbonate (DMC) in the organic solvent is 5 volume % to 75 volume %, more specifically 5 volume % to 55 volume %, 7 volume % to 45 volume % or 35 volume % to 45 volume %. Then, the above lithium salt is dissolved in the organic solvent at a concentration of 0.8M to 3.0M, specifically 1.0M to 3.0M. Next, vinylene carbonate (VC) is added to the organic solvent in which the lithium salt is dissolved, wherein the content of vinylene carbonate (VC) is 0.01 wt % to 7 wt %, specifically 0.3 wt % to 6 wt %, more specifically 0.4 wt % to 3 wt %, and more specifically 0.6 wt % to 2 wt %, relative to the weight of the non-aqueous electrolyte, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2, specifically 0.01 to 0.18, specifically 0.016 to 0.130, and more specifically 0.02 to 0.08. The above-mentioned additional solvents and / or additives may be included in the non-aqueous electrolyte.

[0135] The lithium secondary battery of the present invention as described above can be effectively used in portable devices such as mobile phones, notebook computers, and digital cameras, in electric vehicles such as hybrid electric vehicles (HEVs), and the like.

[0136] Therefore, according to another embodiment of the present invention, a battery module including the above-mentioned lithium secondary battery as a unit cell is provided, and a battery pack including the battery module is provided.

[0137] The battery module or battery pack can be used as a power source for one or more medium or large-sized devices such as power tools, electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEV) and plug-in hybrid electric vehicles (PHEV), or power storage systems.

[0138] The outer shape of the lithium secondary battery of the present invention is not particularly limited, but may be cylindrical using a can, square, pouch, coin, or the like.

[0139] The lithium secondary battery of the present invention can be used for a battery cell used as a power source for small devices, and can also be preferably used as a unit cell of a medium or large battery module including a plurality of battery cells.

[0140] Hereinafter, the present invention will be described in detail with reference to Examples.

[0141] At this time, the embodiments of the present invention may be modified into other various forms, and the scope of the present invention should not be interpreted as being limited to the embodiments described below. The embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

[0142] Hereinafter, the present invention will be described in detail with reference to specific examples.

[0143] Example

[0144] Example 1

[0145] (Preparation of non-aqueous electrolyte)

[0146] An organic solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:30:40.

[0147] LiPF as a lithium salt 6 Dissolved in organic solvents at a molar concentration of 1.0 M.

[0148] In addition, a non-aqueous electrolyte was prepared by adding vinylene carbonate (VC) to an organic solvent in which a lithium salt was dissolved. The content of vinylene carbonate in the non-aqueous electrolyte was 1 wt %.

[0149] (Manufacturing of Secondary Batteries)

[0150] Lithium iron phosphate (LiFePO4) with a carbon coating layer formed thereon as a positive electrode active material 4 ) particles, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 94:3:3 to prepare a positive electrode slurry. The positive electrode slurry was mixed at 600 mg / 25 cm 2 The positive electrode active material was coated on a positive electrode current collector (Al film) with a thickness of 15 μm and dried, and then roll-pressed to produce a positive electrode (thickness of the positive electrode active material: 220 μm). The average particle size (D 50 ) is 1.1 μm, and a lithium iron phosphate (LiFePO 4 ) particles are in the form of primary particles.

[0151] Artificial graphite as a negative electrode active material, SBR-CMC as a binder, and carbon black as a conductive material were added to water as a solvent at a weight ratio of 97:2:1 to prepare a negative electrode slurry. 2 A loading amount of 500 Å was coated on a copper (Cu) thin film having a thickness of 15 μm as a negative electrode current collector and dried, and then roll-pressed to manufacture a negative electrode (thickness of the negative electrode active material: 170 μm).

[0152] The positive electrode, the polyolefin-based porous separator, and the negative electrode are sequentially stacked to prepare an electrode assembly.

[0153] The assembled electrode assembly was housed in a battery case, and the prepared nonaqueous electrolyte solution was then injected therein to manufacture a lithium secondary battery.

[0154] Example 2

[0155] A lithium secondary battery was manufactured in the same manner as in Example 1, except that a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:60:10 was used as an organic solvent to prepare a nonaqueous electrolyte.

[0156] Example 3

[0157] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the nonaqueous electrolyte was prepared by using a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 30:70 as an organic solvent.

[0158] Example 4

[0159] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was prepared by adding 0.5 wt % instead of 1 wt % of vinylene carbonate as an additive to the non-aqueous electrolyte.

[0160] Example 5

[0161] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was prepared by adding 5 wt % instead of 1 wt % of vinylene carbonate as an additive to the non-aqueous electrolyte.

[0162] Example 6

[0163] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 500mg / 25cm 2 to make the positive electrode.

[0164] Example 7

[0165] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 700mg / 25cm 2 to make the positive electrode.

[0166] Example 8

[0167] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 450mg / 25cm 2 to make the positive electrode.

[0168] Comparative Example 1

[0169] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the nonaqueous electrolyte was prepared by using a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 as an organic solvent.

[0170] Comparative Example 2

[0171] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was prepared by not adding vinylene carbonate as an additive.

[0172] Comparative Example 3

[0173] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was prepared by adding 8 wt % instead of 1 wt % of vinylene carbonate as an additive to the non-aqueous electrolyte.

[0174] Comparative Example 4

[0175] A lithium secondary battery was manufactured in the same manner as in Example 6, except that the nonaqueous electrolyte was prepared by using a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 as an organic solvent.

[0176] Comparative Example 5

[0177] A lithium secondary battery was manufactured in the same manner as in Example 1, except that a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 30:30:40 was used as an organic solvent to prepare a non-aqueous electrolyte, and the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2Change to 500mg / 25cm 2 to prepare the positive electrode.

[0178] Comparative Example 6

[0179] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 750mg / 25cm 2 to make the positive electrode.

[0180] Comparative Example 7

[0181] A lithium secondary battery was manufactured in the same manner as in Example 1, except that a mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70 was used as an organic solvent to prepare a nonaqueous electrolyte, and the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 400mg / 25cm 2 to prepare the positive electrode.

[0182] Comparative Example 8

[0183] A lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 400mg / 25cm 2 to make the positive electrode.

[0184] Comparative Example 9

[0185] A lithium secondary battery was manufactured in the same manner as in Example 1, except that a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 30:30:40 was used as an organic solvent to prepare a non-aqueous electrolyte, and the loading amount of the positive electrode slurry was increased from 600 mg / 25 cm 2 Change to 400mg / 25cm 2 to prepare the positive electrode.

[0186] [Table 1]

[0187]

[0188]

[0189] Experimental example

[0190] Experimental Example 1: Initial Capacity Performance Rate Measurement

[0191] Initial charge and discharge were performed by charging the lithium secondary batteries of Examples 1 to 8 and Comparative Examples 1 to 9 manufactured above to 3.65 V under CC / CV and 0.33 C at 25° C. and discharging them to 2.5 V at 0.33 C to measure their initial discharge capacity (unit: mAh).

[0192] The initial discharge capacity was divided by the cell design capacity (based on 0.33C) and then multiplied by 100 to evaluate the capacity performance rate (%). The results are shown in Table 2 below.

[0193] Experimental Example 2: Evaluation of Cyclic Charge and Discharge Capacity Retention Rate

[0194] The lithium secondary batteries of Examples 1 to 8 and Comparative Examples 1 to 9 manufactured above were charged to 3.65V at 25°C under CC / CV and 0.33C, and discharged to 2.5V at 0.33C, and the above was set as 1 cycle, and the discharge capacity and resistance after 1 cycle were measured. At this time, by detecting the capacity at room temperature, the battery was charged to SOC50% based on the discharge capacity, and then the battery was discharged at a current of 2.5C for 10 seconds, and the resistance was measured using the voltage drop difference obtained thereby.

[0195] Thereafter, 200 charge / discharge cycles were performed under the above-mentioned charge and discharge conditions, and then the capacity retention rate (%) and the resistance increase rate (%) were measured. The capacity retention rate (%) was calculated according to the following [Equation 1], and the resistance increase rate (%) was calculated according to the following [Equation 2]. The measurement results are shown in Table 2.

[0196] [Equation 1]

[0197] Capacity retention rate (%) = (discharge capacity after 200 cycles / discharge capacity after 1 cycle) × 100

[0198] [Equation 2]

[0199] Resistance increase rate (%) = {(resistance after 200 cycles - resistance after 1 cycle) / resistance after 1 cycle} × 100

[0200] [Table 2]

[0201]

[0202] Referring to the above Table 2, it can be confirmed that the lithium secondary batteries of Examples 1 to 8 of the present invention exhibit excellent capacity performance effects, as well as excellent life performance and low resistance increase rate, compared with Comparative Examples 1 to 6.

[0203] In addition, compared with Comparative Example 4 in which dimethyl carbonate was not used, the positive electrode loading was designed to be 500 mg / 25 cm 2Example 6 shows excellent capacity performance effect, excellent life performance and low resistance increase rate.

[0204] At the same time, the reference positive electrode loading is designed to be 400 mg / 25 cm 2 In Comparative Examples 7 to 9, the loading is adjusted to be lower, and electrolyte impregnation is not a major problem, so it can be seen that it is not significantly affected by the components and content of the non-aqueous electrolyte. Specifically, by comparing Comparative Examples 7 and 8, it can be confirmed that even if dimethyl carbonate is used as an organic solvent component, the improvement effect in capacity performance, life performance and resistance increase rate is not significant. In addition, Comparative Example 9 using a linear carbonate other than dimethyl carbonate exhibits the same or similar performance level as Comparative Examples 7 and 8. As a result, it can be confirmed that the non-aqueous electrolyte of the present invention has a specific loading (e.g., greater than 400 mg / 25 cm 2 And less than 750mg / 25cm 2 , more specifically 450mg / 25cm 2 Up to 740mg / 25cm 2 ) shows particularly good effects in positive electrodes containing lithium iron phosphate.

Claims

1. A lithium secondary battery, comprising: Positive electrode, negative electrode, separator and non-aqueous electrolyte, in, The positive electrode comprises a positive electrode active material, wherein the positive electrode active material comprises lithium iron phosphate particles, and the loading amount of the positive electrode is 450 mg / 25 cm 2 Up to 740mg / 25cm 2 ;and The non-aqueous electrolyte comprises a lithium salt, an organic solvent and an additive, The organic solvent comprises a cyclic carbonate solvent and a linear carbonate solvent, wherein the cyclic carbonate solvent comprises ethylene carbonate, and the linear carbonate solvent comprises dimethyl carbonate. The additive comprises vinylene carbonate, and In the organic solvent, the content of dimethyl carbonate is 5 volume % to 75 volume %, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.

2.

2. The lithium secondary battery according to claim 1, in, The positive electrode loading is 450 mg / 25 cm 2 Up to 700mg / 25cm 2 .

3. The lithium secondary battery according to claim 1, in, The positive electrode loading is 500 mg / 25 cm 2 Up to 600mg / 25cm 2 .

4. The lithium secondary battery according to claim 1, in, In the non-aqueous electrolyte, the content of vinylene carbonate is 0.01 wt % to 7 wt %.

5. The lithium secondary battery according to claim 1, in, The volume ratio of the cyclic carbonate solvent to the linear carbonate solvent is 10:90 to 50:

50.

6. The lithium secondary battery according to claim 1, in, The linear carbonate solvent further comprises ethyl methyl carbonate.

7. The lithium secondary battery according to claim 6, in, The organic solvent includes 10 to 50 volume % of ethylene carbonate, 5 to 55 volume % of dimethyl carbonate, and 20 to 70 volume % of ethyl methyl carbonate.

8. The lithium secondary battery according to claim 1, in, The lithium salt includes LiCl, LiBr, LiI, LiBF 4 、LiClO 4 、LiAlO 4 、LiAlCl 4 、LiPF 6 、LiSbF 6 、LiAsF 6 , LiB 10 Cl 10 、LiBOB(LiB(C 2 O 4 ) 2 )、LiCF 3 SO 3 、LiTFSI(LiN(SO 2 CF 3 ) 2 )、LiFSI(LiN(SO 2 F) 2 )、LiCH 3 SO 3 、LiCF 3 CO 2 、LiCH 3 CO 2 or LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ) at least one of.

9. The lithium secondary battery according to claim 1, in, In the non-aqueous electrolyte, the lithium salt is included in a concentration of 0.8M to 3.0M.

10. The lithium secondary battery according to claim 1, in, The lithium iron phosphate particles include a compound represented by formula A: [Formula A] Li 1+a Fe 1-s M s (PO 4-b )X b Wherein, in formula A, M is one or more elements selected from the group consisting of Co, Ni, Mn, Al, Mg, Ti and V, and X is F, S or N, wherein 0≤s≤0.5; -0.5≤a≤+0.5; 0≤b≤0.

1.

11. The lithium secondary battery according to claim 1, in, The lithium iron phosphate particles include LiFePO 4 .

12. The lithium secondary battery according to claim 1, in, The lithium iron phosphate particles are in the form of primary particles; and The average particle size D of the lithium iron phosphate particles 50 0.2μm to 3.0μm.

13. The lithium secondary battery according to claim 1, in, The lithium iron phosphate particles have a carbon coating layer on the surface.

14. The lithium secondary battery according to claim 1, in, The positive electrode active material does not contain lithium nickel-based oxides.

15. The lithium secondary battery according to claim 1, in, The negative electrode includes a carbon-based active material.

16. The lithium secondary battery according to claim 15, in, The carbon-based active material includes at least one of natural graphite and artificial graphite.

17. A method for preparing a lithium secondary battery, the method include: An electrode assembly comprising a positive electrode, a negative electrode and a separator is prepared, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprises lithium iron phosphate particles, and the loading amount of the positive electrode is 450 mg / 25 cm 2 Up to 740mg / 25cm 2 ; housing the electrode assembly in a battery case; A non-aqueous electrolyte comprising a lithium salt, an organic solvent and an additive is prepared, wherein the organic solvent comprises a cyclic carbonate solvent and a linear carbonate solvent, the cyclic carbonate solvent comprises ethylene carbonate, the linear carbonate solvent comprises dimethyl carbonate, wherein the additive comprises vinylene carbonate, wherein the content of dimethyl carbonate in the organic solvent is 5 volume % to 75 volume %, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2; and The prepared nonaqueous electrolyte is injected or impregnated into the battery case.

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