Lithium secondary battery and method for manufacturing the same

Abstract

The present invention provides a lithium secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate particles, the loading amount of the positive electrode is 450 mg / 25 cm 2 to 740 mg / 25 cm 2 , the non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive, wherein the organic solvent includes 10 to 35% by volume of ethylene carbonate, 5 to 55% by volume of dimethyl carbonate, and 20 to 70% by volume of ethyl methyl carbonate, and the volume ratio of the cyclic carbonate-based solvent to the linear carbonate-based solvent is 10:90 to 35:65, the additive includes vinylene carbonate, and the weight ratio of the vinylene carbonate to the dimethyl carbonate is greater than 0 to 0.2 or less. By improving the negative electrode reduction stability while improving the impregnation properties of the positive electrode to the non-aqueous electrolyte, the lithium secondary battery has excellent capacity performance effects, excellent life performance, and resistance reduction effects.

Description

[0001] This application is a divisional application of PCT / KR2023 / 017580, an international PCT application filed on November 3, 2023. The original application was an invention patent application, with application number 202380074710.6 entering the Chinese national phase on April 23, 2025, and entitled "Lithium Secondary Battery".

[0002] Cross-references to related applications

[0003] 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

[0004] This invention relates to a lithium secondary battery. Background Technology

[0005] With the development of the information society, personal IT devices and computer networks have also developed, and society's overall dependence on electricity has increased. Therefore, there is a need to develop technologies for the efficient storage and utilization of electrical energy.

[0006] Among the technologies that have been developed, secondary batteries are the most suitable for a variety of applications, and among these secondary batteries, there is increasing interest in lithium secondary batteries that can be manufactured small enough for use in personal IT devices and have the highest energy density.

[0007] Typically, lithium secondary batteries are manufactured by injecting or immersing a non-aqueous electrolyte into an electrode assembly consisting of a positive electrode, a negative electrode, and a porous membrane.

[0008] Carbon-based and silicon-based active materials are considered to be the negative electrode active materials for lithium secondary batteries. Meanwhile, lithium-containing cobalt oxides, layered crystal structure LiMnO2, spinel crystal structure LiMn2O4, and lithium-containing nickel oxides (LiNiO2) are used as positive electrode active materials.

[0009] Recently, lithium iron phosphate (e.g., LiFePO4) active materials, which have excellent thermal stability and are relatively inexpensive, have been used as positive electrode active materials.

[0010] However, the specific capacity of lithium iron phosphate active materials is lower than that of lithium cobalt oxide and lithium nickel oxide. Therefore, in order to improve the energy density of the cathode and the lithium secondary battery containing it, lithium iron phosphate cathode active materials should be used under high loading. However, high loading of lithium iron phosphate cathode has the problem that non-aqueous electrolyte is difficult to fully impregnate into the cathode, resulting in problems such as difficulty in expressing capacity, increased resistance and deterioration of life. Summary of the Invention

[0011] [Technical Issues]

[0012] One aspect of the present invention provides a lithium secondary battery comprising lithium iron phosphate particles as a positive electrode active material, wherein the positive electrode has a specific loading amount above a certain level, and wherein the lithium secondary battery exhibits excellent capacity performance, excellent lifespan performance, and reduced resistance by improving the impregnation properties of the positive electrode to the non-aqueous electrolyte while simultaneously improving the reduction stability of the negative electrode.

[0013] [Technical Solution]

[0014] According to one aspect of the present invention, a lithium secondary battery is provided, comprising 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 of the positive electrode is 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 The non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive. The organic solvent comprises cyclic carbonate solvents and linear carbonate solvents. The cyclic carbonate solvent contains ethylene carbonate, and the linear carbonate solvent contains dimethyl carbonate. The dimethyl carbonate content in the organic solvent is from 5% to 75% by volume. The additive contains vinylene carbonate, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 and less than 0.2.

[0015] [Beneficial Effects]

[0016] The lithium secondary battery of the present invention is characterized by comprising a positive electrode having a specific loading amount and containing lithium iron phosphate particles as the positive electrode active material, and a non-aqueous electrolyte containing ethylene carbonate and dimethyl carbonate as organic solvents and vinylene carbonate as an additive, wherein the content and ratio of dimethyl carbonate and vinylene carbonate are adjusted to a specific range. According to the lithium secondary battery of the present invention, the excellent dimethyl carbonate is used as an organic solvent component to improve the electrolyte impregnation properties of the high-loading positive electrode, while the vinylene carbonate additive having a specific content ratio relative to dimethyl carbonate is used to improve the reduction stability of the negative electrode, thereby exhibiting an excellent level of capacity in the lithium secondary battery, and improving lifespan performance and resistivity properties. Detailed Implementation

[0017] First, before describing the invention, it will be understood that the terms or words used in this specification and claims should not be construed as having the meanings defined in commonly used dictionaries, but should be construed as having meanings and concepts consistent with the technical concept of the invention, based on the principle that the inventors can appropriately define the concepts of the terms to best interpret the invention.

[0018] Furthermore, the terminology used herein is for describing exemplary embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, singular expressions include plural expressions.

[0019] It will also be understood that the terms “comprising,” “including,” or “having” as used in this specification specify the presence of the said features, quantities, steps, elements, or combinations thereof, but do not exclude the presence or addition of more than one other feature, quantity, step, element, or combination thereof.

[0020] In this specification, unless otherwise stated, “%” represents weight.

[0021] 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, a functional group may contain "a" to "b" carbon atoms.

[0022] Furthermore, in this specification, unless otherwise defined, “substitution” means that at least one hydrogen atom bonded to carbon is replaced by an element other than hydrogen, for example, it means being replaced by an alkyl or fluorine element having 1 to 5 carbon atoms.

[0023] In this specification, the average particle size (D) 50 The average particle size (D) can be defined as the particle size corresponding to the 50% volume accumulation in the particle size distribution curve. 50 Particle sizes can be measured, for example, by laser diffraction. Laser diffraction typically allows for the measurement of particle sizes ranging from submicron to several millimeters, thus providing highly reproducible and high-resolution results.

[0024] The invention will be described in more detail below.

[0025] Lithium secondary battery

[0026] This invention relates to a lithium secondary battery.

[0027] Specifically, the lithium secondary battery of the present invention comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. The positive electrode comprises a positive electrode active material, which in turn comprises lithium iron phosphate particles, and the positive electrode loading is 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2The non-aqueous electrolyte contains lithium salt, organic solvent, and additives. The organic solvent includes cyclic carbonate solvents and linear carbonate solvents. The cyclic carbonate solvent contains ethylene carbonate, and the linear carbonate solvent contains dimethyl carbonate. The content of dimethyl carbonate in the organic solvent is 5% to 75% by volume. The additive contains vinylene carbonate, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2.

[0028] The lithium secondary battery of the present invention is characterized by comprising a positive electrode having a specific loading amount and containing lithium iron phosphate particles as the positive electrode active material, and a non-aqueous electrolyte containing ethylene carbonate and dimethyl carbonate as organic solvents and vinylene carbonate as an additive, wherein the content and ratio of dimethyl carbonate and vinylene carbonate are adjusted to a specific range. In 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 high-loading positive electrode, while vinylene carbonate additive having a specific content ratio relative to dimethyl carbonate is used to improve the reduction stability of the negative electrode, thereby exhibiting excellent capacity of the lithium secondary battery and improving lifespan performance and resistivity properties.

[0029] The lithium secondary battery of the present invention can retain a capacity of 90% or more, preferably 90% to 95%, after 200 cycles; and the resistance increase rate after 200 cycles can be less than 20%, preferably less than 15%, while maintaining a cell design capacity of at least 500 mAh, more specifically at least 550 mAh and an initial discharge capacity of at least 500 mAh, more specifically at least 540 mAh. This is only if the electrolyte contains an organic solvent containing ethylene carbonate and dimethyl carbonate and an additive containing ethylene carbonate, wherein the content of dimethyl carbonate in the organic solvent is 5% to 75% by volume, and the weight ratio of ethylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2, and the battery is composed of lithium iron phosphate particles as active materials with a loading of 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 The positive electrode is provided. The cell design capacity, initial discharge capacity, capacity retention rate, and resistance increase rate are measured according to the methods described in the following examples.

[0030] A lithium secondary battery comprises a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Specifically, a lithium secondary battery comprises a positive electrode, a negative electrode opposite to the positive electrode, a separator disposed between the positive and negative electrodes, and a non-aqueous electrolyte.

[0031] Lithium secondary batteries can be manufactured by: preparing an electrode assembly comprising a positive electrode, a negative electrode, and a separator; housing the electrode assembly in a battery case; preparing a non-aqueous electrolyte comprising lithium salt, organic solvent, and additives; and injecting or impregnating the prepared non-aqueous electrolyte into the battery case.

[0032] (1) Positive electrode

[0033] The positive electrode contains positive electrode active material. The positive electrode active material contains lithium iron phosphate particles.

[0034] Lithium iron phosphate particles may contain compounds represented by the following formula A.

[0035] [Formula A]

[0036] Li 1+a Fe 1-s M s (PO 4-b )X b

[0037] 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, and X is F, S or N, where 0≤s≤0.5, -0.5≤a≤+0.5, and 0≤b≤0.1.

[0038] Equation A above can be specifically represented by LiFePO4 (a=0, s=0, b=0).

[0039] Lithium iron phosphate particles can be in the form of primary particles, or they can be in the form of secondary particles formed by the aggregation of two or more primary particles. Specifically, lithium iron phosphate particles can be in the form of primary particles.

[0040] Lithium iron phosphate particles can consist of primary particles, secondary particles formed by the agglomeration of two or more primary particles, or a mixture of primary particles and secondary particles formed by the agglomeration of two or more primary particles.

[0041] At this point, when the lithium iron phosphate particles are in the form of primary particles, the average particle size (D) of the lithium iron phosphate particles is... 50 The particle size can be from 0.2 μm to 3.0 μm, specifically from 0.2 μm to 2.0 μm, and more specifically from 0.3 μm to 1.5 μm. Additionally, when lithium iron phosphate particles are in the form of secondary particles formed by the aggregation of two or more primary particles, the average particle size (D) of the primary particles... 50 The particle size can be from 0.2 μm to 3.0 μm, specifically from 0.2 μm to 2.0 μm, more specifically from 0.3 μm to 1.5 μm, and the average particle size (D) of the secondary particles is... 50The size can range from 7 μm to 25 μm, specifically from 10 μm to 20 μm.

[0042] The positive electrode active material may also include a carbon coating layer on the surface of the lithium iron phosphate particles. This carbon coating layer can be introduced to protect the lithium iron phosphate particles, improve conductivity, and so on.

[0043] The positive electrode active material may not contain lithium nickel oxides such as lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide. Even when the positive electrode contains lithium nickel oxides, it still poses a risk. 2 The above-mentioned loading levels and the non-aqueous electrolytes described later may also fail to achieve the desired effect.

[0044] The loading capacity of the positive electrode can be 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 .

[0045] Compared to other cathode 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. However, due to their relatively low specific capacity, there is a need to increase the loading to achieve high energy density. When the cathode loading is increased (e.g., 450 mg / 25 cm⁻¹), the loading becomes more complex. 2 Up to 740 mg / 25 cm 2 When using a non-aqueous electrolyte, high-energy-density batteries can be achieved (e.g., lithium secondary batteries with a cell design capacity of at least 500 mAh, more specifically at least 550 mAh and an initial discharge capacity of at least 500 mAh, more specifically at least 540 mAh). However, non-aqueous electrolytes are difficult to fully immerse in the positive electrode, resulting in problems such as difficulty in demonstrating the capacity, increased resistance, and deterioration of lifespan performance of lithium secondary batteries.

[0046] To address this problem, the lithium secondary battery of the present invention uses a non-aqueous electrolyte comprising dimethyl carbonate as an organic solvent component and vinylene carbonate as an additive, wherein the content and ratio of these components are adjusted to a specific range. This feature improves the loading capacity to 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 The electrolyte impregnation properties of the positive electrode can improve the reduction stability of the negative electrode, thus enabling the capacity of the lithium secondary battery to be expressed at an excellent level, and improving life performance and resistivity properties.

[0047] When the loading of the positive electrode is less than 450 mg / 25 cm⁻¹ 2 When the above-mentioned problem of electrolyte immersion property deterioration does not occur, the effect of using the non-aqueous electrolyte of the present invention is not exhibited.

[0048] Meanwhile, if the loading of the positive electrode is greater than 740 mg / 25 cm⁻¹ 2 Even if the non-aqueous electrolyte of the present invention is applied to a cathode containing lithium iron phosphate particles, there is a possibility that the electrolyte impregnation properties may not be adequately guaranteed. Furthermore, when the cathode loading exceeds 740 mg / 25 cm⁻¹, [further issues may arise]. 2 For example, the average particle size (D) 50 When smaller lithium iron phosphate particles are used in the cathode, the size of the pores formed between the lithium iron phosphate particles is smaller. In this case, during the manufacturing of the cathode, the slurry solvent evaporates from the pores formed between the lithium iron phosphate particles during the drying process, which may cause the cathode to crack, making it difficult to manufacture or realize the cathode.

[0049] Specifically, the loading of the positive electrode can be 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 450 mg / 25 cm 2 Up to 730 mg / 25 cm 2 450 mg / 25 cm 2 Up to 720 mg / 25 cm 2 450 mg / 25 cm 2 Up to 710 mg / 25 cm 2 Or 450mg / 25 cm 2 Up to 700 mg / 25 cm 2 More specifically, 500 mg / 25 cm 2 Up to 680 mg / 25 cm 2 500 mg / 25 cm 2 Up to 650 mg / 25 cm 2 500 mg / 25 cm 2 Up to 625 mg / 25 cm 2 Or 500 mg / 25 cm 2 Up to 600 mg / 25 cm 2 .

[0050] 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 contain the aforementioned positive electrode active material.

[0051] There are no particular restrictions on the positive electrode current collector, as long as it is conductive and will not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel, aluminum-cadmium alloys with surface treatments of carbon, nickel, titanium, silver, etc., can be used as the positive electrode current collector.

[0052] The thickness of the positive current collector can typically range from 3 μm to 500 μm.

[0053] Positive current collectors can form fine irregularities on their surface to improve the adhesion of the positive electrode active material. For example, positive current collectors can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0054] 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 or both surfaces of the positive electrode current collector.

[0055] To fully utilize the capacity of the positive electrode active material, the content of the positive electrode active material in the positive electrode active material layer can be from 80% to 99% by weight.

[0056] The positive electrode active material layer may also contain a binder and / or a conductive material together with the aforementioned positive electrode active material.

[0057] Adhesives are components used to assist in the bonding of active materials, conductive materials, etc., and to the bonding of current collectors. Specifically, they may include at least one of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, or fluororubber, preferably polyvinylidene fluoride.

[0058] To ensure sufficient adhesion between components such as positive electrode active materials, the content of adhesive in the positive electrode active material layer can be from 1% to 20% by weight, preferably from 1.2% to 10% by weight.

[0059] Conductive materials can be used to assist and improve the conductivity in secondary batteries, and there are no particular limitations, as long as they are conductive without causing chemical changes. Specifically, positive electrode conductive materials 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 pyrolysis 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 carbon black may preferably be included for improving conductivity.

[0060] To ensure sufficient conductivity, the content of conductive material in the positive electrode active material layer can be from 1% to 20% by weight, preferably from 1.2% to 10% by weight.

[0061] The thickness of the positive electrode active material layer can be from 100 μm to 300 μm, preferably from 150 μm to 250 μm.

[0062] The positive electrode can be manufactured by coating a positive electrode slurry containing positive electrode active material and optional binder, conductive material and positive electrode slurry forming solvent onto a positive electrode current collector, followed by drying and rolling.

[0063] The solvent used to form the cathode slurry may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP). The solids content of the cathode slurry may be from 40% to 90% by weight, specifically from 50% to 80% by weight.

[0064] (2) Negative electrode

[0065] The negative electrode can be opposite to the positive electrode.

[0066] The negative electrode includes the negative electrode active material.

[0067] 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 contain a negative electrode active material.

[0068] There are no particular restrictions on the negative electrode current collector, as long as it is conductive and will not cause chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or copper or stainless steel, aluminum-cadmium alloys with surface treatments of carbon, nickel, titanium, silver, etc., can be used as negative electrode current collectors.

[0069] The thickness of the negative electrode current collector can typically range from 3 μm to 500 μm.

[0070] The negative electrode current collector can form fine bumps and depressions on its surface to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as membranes, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0071] 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 or both surfaces of the negative electrode current collector.

[0072] The negative electrode active material layer may contain negative electrode active material.

[0073] The negative electrode active material is a material capable of reversibly inserting / deintercalating lithium ions, and may include at least one of carbon-based active materials, (semi-)metallic active materials, or lithium metal. Specifically, it may include at least one of carbon-based active materials or (semi-)metallic active materials.

[0074] Carbon-based active materials may include at least one of graphite, hard carbon, soft carbon, carbon black, graphene, or fibrous carbon, and preferably may include graphite. Graphite may include at least one of natural graphite or artificial graphite.

[0075] In terms of ensuring structural stability during charging and discharging and reducing side reactions with the electrolyte solution, the average particle size (D) of carbon-based active materials is crucial. 50 The diameter can be 10 μm to 30 μm, preferably 15 μm to 25 μm.

[0076] Specifically, (semi)metallic active materials may include: at least one (semi)metal selected from Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, or Sn; alloys of lithium with at least one (semi)metal selected from Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, or Sn; oxides of at least one (semi)metal selected from 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.

[0077] More specifically, (semi)metallic active materials can include silicon-based active materials.

[0078] Silicon-based active materials may include those made of SiO2 x Compounds represented by (0≤x<2). Since SiO2 does not react with lithium ions and therefore cannot store lithium, it is preferable that x is within the above range, and more preferably, the silicon-based active material can be SiO.

[0079] To ensure structural stability during charging and discharging and reduce side reactions with the electrolyte solution, the average particle size (D) of silicon-based active materials is crucial. 50 The diameter can be from 1 μm to 30 μm, preferably from 2 μm to 15 μm.

[0080] The content of negative electrode active material in the negative electrode active material layer can be from 60% to 99% by weight, preferably from 75% to 95% by weight.

[0081] In addition to the negative electrode active material, the negative electrode active material layer may also contain adhesives and / or conductive materials.

[0082] At this point, the adhesive is used to improve battery performance by improving the adhesion between the negative electrode active material layer and the negative electrode current collector, and may include, for example, at least one of the following: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0083] The content of binder in the negative electrode active material layer can be from 0.5% to 10% by weight, preferably from 1% to 5% by weight.

[0084] There are no particular restrictions on conductive materials, as long as they are conductive and do 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 pyrolysis 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 conductive materials, such as polyphenylene derivatives, etc.

[0085] The content of conductive material in the negative electrode active material layer can be from 0.5% to 10% by weight, preferably from 1% to 5% by weight.

[0086] The thickness of the negative electrode active material layer can be from 50 μm to 300 μm, preferably from 100 μm to 200 μm.

[0087] The loading of the negative electrode active material layer can be 200 mg / 25 cm. 2 Up to 500 mg / 25 cm 2 Preferred 250 mg / 25cm 2 Up to 400 mg / 25 cm 2 .

[0088] The negative electrode can be manufactured by coating a negative electrode slurry containing a negative electrode active material, a binder, a conductive material and / or a solvent for forming a negative electrode slurry onto at least one surface of a negative electrode current collector, followed by drying and rolling.

[0089] The solvent for forming the negative electrode slurry may include, for example, at least one of distilled water, N-methyl-2-pyrrolidone (NMP), ethanol, methanol, or isopropanol, with distilled water preferred for promoting the dispersion of the negative electrode active material, binder, and / or conductive material. The solids content of the negative electrode slurry may be from 30% to 80% by weight, specifically from 40% to 70% by weight.

[0090] (3) Diaphragm

[0091] The diaphragm can be placed between the positive and negative electrodes.

[0092] As a separator, conventional porous polymer membranes are typically used. For example, porous polymer membranes made of polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers can be used alone, or laminates thereof can be used. Alternatively, conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can be used, but the invention is not limited thereto. Furthermore, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0093] (4) Non-aqueous electrolytes

[0094] 1) Lithium salts

[0095] First, lithium salts will be described as follows.

[0096] In the non-aqueous electrolyte solution for lithium secondary batteries according to embodiments of the present invention, any lithium salt can be used without particular limitation, as long as it is commonly used in electrolyte solutions for lithium secondary batteries, and for example, the lithium salt may contain Li. + As a cation, and may contain F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - ClO4 - AlO4 - AlCl4 - PF6 - SbF6 - AsF6 - B 10 Cl 10 - BF2C2O4 - BC4O8 - PF4C2O4 -PF2C4O8 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - C4F9SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - CH3SO3 - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - Or (CF3CF2SO2)2N - At least one of the following can be used as an anion. Specifically, the lithium salt can be selected from LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10 The lithium salt may comprise at least one of the following groups: LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2). Specifically, the lithium salt may comprise a single material or a mixture of two or more of the following: LiBF4, LiClO4, LiPF6, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), or LiBETI (LiN(SO2CF2CF3)2), and more specifically, may comprise LiPF6.

[0097] The lithium salt content can be appropriately varied within the conventional range of usable lithium salts; however, to achieve the best effect in forming an anti-corrosion film on the electrode surface, the concentration of lithium salt contained in the electrolyte solution can be from 0.8 M to 3.0 M, specifically from 1.0 M to 3.0 M. Here, the unit "M" is molar concentration, which can be specifically expressed as "mol / L".

[0098] When the concentration of lithium salt meets the above range, the viscosity of the non-aqueous electrolyte can be controlled to achieve optimal impregnation, and the mobility of lithium ions can be improved, thereby improving the capacity and cycle properties of lithium secondary batteries.

[0099] 2) Organic solvents

[0100] Organic solvents can include cyclic carbonate solvents and linear carbonate solvents. Organic solvents can be composed of cyclic carbonate solvents and linear carbonate solvents.

[0101] The volume ratio of cyclic carbonate solvents to linear carbonate solvents can be from 10:90 to 50:50, specifically from 15:85 to 50:50, and more specifically from 20:80 to 35:65. When within the above ranges, it is preferred to achieve high ion migration properties and low electrolyte viscosity.

[0102] Cyclic carbonate solvents include ethylene carbonate. Ethyl carbonate is a high-viscosity organic solvent with a high dielectric constant, and therefore can readily dissociate lithium salts in electrolytes.

[0103] Cyclic carbonate solvents may not contain fluorinated cyclic carbonates such as fluoroethylene carbonate (FEC). Specifically, cyclic carbonate solvents may be formed solely from ethylene carbonate and may not contain another cyclic carbonate solvent such as propylene carbonate.

[0104] Furthermore, the linear carbonate solvent includes dimethyl carbonate. Dimethyl carbonate is an organic solvent with low viscosity and low dielectric constant, and in particular, it is excellent in terms of electrolyte impregnation properties, thus allowing for the excellent impregnation of highly loaded positive electrodes containing the lithium iron phosphate particles of the present invention. However, as an organic solvent, dimethyl carbonate has the problem of forming an unstable negative electrode film, which leads to a deterioration in the reduction stability of the negative electrode. But as described below, when dimethyl carbonate and vinylene carbonate are used in a specific ratio, both the electrolyte impregnation properties and the reduction stability of the negative electrode can be improved simultaneously. In addition, the non-aqueous electrolyte of the present invention achieves the desired effect, especially when the loading of the positive electrode containing lithium iron phosphate particles is 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 At that time, and if the positive electrode loading is less than 450 mg / 25 cm⁻¹ 2 Or greater than 740 mg / 25 cm 2 Therefore, the use of dimethyl carbonate cannot improve the electrolyte impregnation properties.

[0105] The dimethyl carbonate content in the organic solvent is from 5 vol% to 75 vol%. In one embodiment, the dimethyl carbonate content in the organic solvent is from 5 vol% to 55 vol%, more specifically from 7 vol% to 45 vol%, and even more specifically from 35 vol% to 45 vol%. If the dimethyl carbonate content in the organic solvent is less than 5 vol%, the impregnation properties of the electrolyte to the positive electrode cannot be improved. If the dimethyl carbonate content in the organic solvent is greater than 75 vol%, an unstable SEI film is formed, and the cell performance deteriorates, which is undesirable.

[0106] Linear carbonate solvents may also contain ethyl methyl carbonate along with dimethyl carbonate. When the linear carbonate also contains ethyl methyl carbonate, it is preferable that the stability of the SEI film can be further improved.

[0107] When the linear carbonate solvent further comprises ethyl methyl carbonate, the organic solvent may comprise 10 vol% to 50 vol% ethylene carbonate, 5 vol% to 55 vol% dimethyl carbonate, and 20 vol% to 70 vol% ethyl methyl carbonate, more specifically 20 vol% to 40 vol% ethylene carbonate, 7 vol% to 45 vol% dimethyl carbonate, and 25 vol% to 65 vol% ethyl methyl carbonate, and even more specifically 25 vol% to 35 vol% ethylene carbonate, 30 vol% to 45 vol% dimethyl carbonate, and 25 vol% to 50 vol% ethyl methyl carbonate, or 30 vol% to 35 vol% ethylene carbonate, 35 vol% to 45 vol% dimethyl carbonate, and 25 vol% to 50 vol% ethyl methyl carbonate. When within the above ranges, it is preferred for improving electrolyte impregnation properties and the stability of the negative electrode SEI film.

[0108] Additionally, if necessary, any organic solvent commonly used in non-aqueous electrolytes may be used without limitation. For example, at least one organic solvent selected from ester solvents, ether solvents, glycol dimethyl ether solvents, or nitrile solvents may be additionally included.

[0109] Ester solvents 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.

[0110] As an 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), or a mixture of two or more thereof, may be used, but the present invention is not limited thereto.

[0111] Glycol dimethyl ether solvents are solvents that have a higher dielectric constant and lower surface tension than linear carbonate solvents and are less reactive with metals. They may include, but are not limited to, at least one of dimethoxyethane (glycol dimethyl ether, DME), diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether or tetraethylene glycol dimethyl ether (TEGDME).

[0112] Nitrile solvents may be selected from one or more of the group consisting of acetonitrile, propionitrile, butyronitrile, valerate, octanoic acid, heptanonitrile, cyclopentanoic acid, cyclohexanoic acid, 2-fluorophenyl nitrile, 4-fluorophenyl nitrile, difluorophenyl nitrile, trifluorophenyl nitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but are not limited thereto.

[0113] Meanwhile, unless otherwise stated, the remainder of the non-aqueous electrolyte, excluding lithium salts and additives, may be an organic solvent.

[0114] 3) Additives

[0115] The non-aqueous additive of the present invention comprises additives.

[0116] The additives include vinylene carbonate.

[0117] Regarding the formation of a stable SEI film on the negative electrode, vinylene carbonate can be used as an additive in the non-aqueous electrolyte of this 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 reduction stability of the negative electrode can be improved.

[0118] In this invention, the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 and less than 0.2. If the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0.2, excessive formation of the negative electrode SEI film will occur, which may lead to problems such as increased resistance and deterioration of lifespan performance.

[0119] Specifically, the weight ratio of vinylene carbonate to dimethyl carbonate can be 0.01 to 0.18, more specifically 0.016 to 0.130, and even more specifically 0.02 to 0.08. When within the above ranges, it is preferable to simultaneously improve the electrolyte impregnation properties of the positive electrode and the reduction stability of the negative electrode.

[0120] The weight ratio of vinylene carbonate to dimethyl carbonate can be calculated using the total weight or volume of all non-aqueous electrolytes, the volumetric content of dimethyl carbonate, its weight content, density information, etc.

[0121] The content of vinylene carbonate in the non-aqueous electrolyte can be from 0.01% to 7% by weight, specifically from 0.3% to 6% by weight, more specifically from 0.4% to 3% by weight, and even more specifically from 0.6% to 2% by weight. When within the above range, it is preferable to appropriately form a negative electrode SEI film to prevent electrolyte side reactions and to prevent an increase in resistance due to excessive use of additives.

[0122] Additionally, when necessary, the additives may include other 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 the low-temperature high-rate discharge properties, high-temperature stability, overcharge protection, and the effect of suppressing battery expansion at high temperatures.

[0123] Other examples of additives may include at least one of cyclic carbonates, halogenated carbonates, sulcolepsy compounds, sulfonates / salts, sulfates / salts, phosphates / salts or phosphites / salts, borates / salts, nitriles, benzenes, amines, silanes, or lithium salts.

[0124] Cyclic carbonate compounds can be, for example, vinyl ethylene carbonate, etc.

[0125] Halogenated carbonate compounds can be, for example, fluoroethylene carbonate (FEC), etc.

[0126] The sulfonyl compounds can be, for example, at least one compound selected from the group consisting of 1,3-propanesulfonyl lactone (PS), 1,4-butanesulfonyl lactone, ethylenesulfonyl lactone, 1,3-propenesulfonyl lactone (PRS), 1,4-butenesulfonyl lactone and 1-methyl-1,3-propenesulfonyl lactone.

[0127] Sulfonate / salt compounds may contain saturated hydrocarbon groups or unsaturated hydrocarbon groups such as alkenyl or alkyneyl groups.

[0128] Sulfate / salt compounds can be, for example, ethylene sulfate (ESA), trimethylene sulfate (TMS), methyltrimethylene sulfate (MTMS), etc.

[0129] The phosphate ester / salt or phosphite / salt compound can be, for example, one or more compounds selected from the group consisting of lithium difluoro(bis(oxalato)phosphate), lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate and tris(trifluoroethyl)phosphite.

[0130] Boronate / salt compounds can be tetraphenylboronate, lithium difluoro(oxalate)borate (LiODFB), lithium bis(oxalate)borate (LiB(C2O4)2, LiBOB), etc.

[0131] The nitrile compound can be, for example, at least one compound selected from the group consisting of succinic anion, adiponitrile, acetonitrile, propionitrile, butyric anion, valerate, octanoic anion, heptanonitrile, cyclovalerate, cyclohexanoic anion, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0132] Benzene compounds can be, for example, fluorobenzene; amine compounds can be, for example, triethanolamine, ethylenediamine; and silane compounds can be, for example, tetravinylsilane.

[0133] Lithium salts are compounds that are different from lithium salts contained in non-aqueous electrolyte solutions, and can be lithium difluorophosphate (LiPO2F2), LiBF4, etc.

[0134] Additional additives may be used in combination of two or more compounds, and based on the total weight of the non-aqueous electrolyte, the total content of the aforementioned vinylene carbonate and additional additives may be from 0.05% by weight to 20% by weight, more specifically from 0.05% by weight to 10% by weight. When the total content of additives meets the above range, high-temperature storage properties and high-temperature life properties can be improved more effectively, and battery side reactions caused by residual additives after the reaction can be prevented.

[0135] The non-aqueous electrolyte can be prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and optionally ethyl methyl carbonate (EMC) in the amounts described above to prepare an organic solvent, particularly wherein the content of dimethyl carbonate (DMC) in the organic solvent is 5 vol% to 75 vol%, more specifically 5 vol% to 55 vol%, 7 vol% to 45 vol%, or 35 vol% to 45 vol%. Then, the above-described lithium salt is dissolved in the organic solvent at a concentration of 0.8 M to 3.0 M, specifically 1.0 M to 3.0 M. Next, vinylene carbonate (VC) is added to an organic solvent in which lithium salt is dissolved, wherein the content of vinylene carbonate (VC) relative to the weight of the non-aqueous electrolyte is from 0.01 wt% to 7 wt%, specifically from 0.3 wt% to 6 wt%, more specifically from 0.4 wt% to 3 wt%, and even more specifically from 0.6 wt% to 2 wt%, and the weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 to less than 0.2, specifically from 0.01 to 0.18, specifically from 0.016 to 0.130, and even more specifically from 0.02 to 0.08. The non-aqueous electrolyte may contain the additional solvents and / or additives described above.

[0136] The lithium secondary battery of the present invention, as described above, can be effectively used in portable devices such as mobile phones, laptops and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0137] Therefore, according to another embodiment of the present invention, a battery module comprising the above-mentioned lithium secondary battery as a unit cell is provided, and a battery pack comprising the battery module is provided.

[0138] Battery modules or battery packs can be used as power sources for one or more medium to large-sized devices, such as power tools, electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), or power storage systems.

[0139] The shape of the lithium secondary battery of the present invention is not particularly limited, but it can be cylindrical, square, bag-shaped, coin-shaped, etc.

[0140] The lithium secondary battery of the present invention can be used as a battery cell for powering small devices, and can also preferably be used as a unit cell of a medium or large battery module containing multiple battery cells.

[0141] The present invention will be described in detail below with reference to embodiments.

[0142] In this context, embodiments of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as limited to the embodiments described below. Embodiments of the present invention are provided to provide a more complete description of the invention to those skilled in the art.

[0143] The present invention will be described in detail below with reference to specific embodiments.

[0144] Examples

[0145] Example 1

[0146] (Preparation of non-aqueous electrolytes)

[0147] 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.

[0148] LiPF6, as a lithium salt, was dissolved in an organic solvent at a molar concentration of 1.0 M.

[0149] Furthermore, a non-aqueous electrolyte was prepared by adding vinylene carbonate (VC) to an organic solvent in which lithium salt was dissolved. The non-aqueous electrolyte contained 1% by weight of vinylene carbonate.

[0150] (Manufacturing of secondary batteries)

[0151] Lithium iron phosphate (LiFePO4) particles with a carbon coating, used as the positive electrode active material, carbon black as the conductive material, and polyvinylidene fluoride (PVdF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 94:3:3 to prepare a positive electrode slurry. The positive electrode slurry was prepared at a concentration of 600 mg / 25 cm⁻¹. 2 The loading was coated onto a 15 μm thick positive electrode current collector (Al film) and dried, then rolled to fabricate the positive electrode (thickness of the positive electrode active material: 220 μm). The average particle size (D) of the positive electrode active material... 50 The lithium iron phosphate (LiFePO4) particles, which are 1.1 μm in size and have a carbon coating, are in the form of primary particles.

[0152] A negative electrode slurry was prepared by adding artificial graphite (as the negative electrode active material), SBR-CMC (as the binder), and carbon black (as the conductive material) to water (as the solvent) in a weight ratio of 97:2:1. The negative electrode slurry was then prepared at a concentration of 300 mg / 25 cm⁻¹. 2 The loading amount was coated onto a 15 μm thick copper (Cu) film used as the negative electrode current collector and dried, and then rolled to manufacture the negative electrode (thickness of the negative electrode active material: 170 μm).

[0153] An electrode assembly is prepared by sequentially stacking a positive electrode, a polyolefin porous membrane, and a negative electrode.

[0154] The assembled electrode assembly is housed in a battery case, and then a prepared non-aqueous electrolyte solution is injected into it to manufacture a lithium secondary battery.

[0155] Example 2

[0156] The lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous electrolyte was prepared by using a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:60:10 as an organic solvent.

[0157] Example 3

[0158] The lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous 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.

[0159] Example 4

[0160] The 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% by weight instead of 1% by weight of vinylene carbonate as an additive to the non-aqueous electrolyte.

[0161] Example 5

[0162] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was prepared by adding 5% by weight instead of 1% by weight of vinylene carbonate as an additive to the non-aqueous electrolyte.

[0163] Example 6

[0164] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 500 mg / 25 cm 2 To create the positive electrode.

[0165] Example 7

[0166] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 700 mg / 25 cm 2 To create the positive electrode.

[0167] Example 8

[0168] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 450 mg / 25 cm 2 To create the positive electrode.

[0169] Comparative Example 1

[0170] The lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous 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.

[0171] Comparative Example 2

[0172] The 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.

[0173] Comparative Example 3

[0174] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was prepared by adding 8% by weight of vinylene carbonate as an additive instead of 1% by weight to the non-aqueous electrolyte.

[0175] Comparative Example 4

[0176] The lithium secondary battery was manufactured in the same manner as in Example 6, except that a non-aqueous 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.

[0177] Comparative Example 5

[0178] The lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous electrolyte was prepared by using a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 30:30:40 as the organic solvent, and the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 500mg / 25 cm 2 To prepare the positive electrode.

[0179] Comparative Example 6

[0180] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 750 mg / 25 cm 2 To create the positive electrode.

[0181] Comparative Example 7

[0182] The 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) at a volume ratio of 30:70 was used as the organic solvent to prepare the non-aqueous electrolyte, and the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 400 mg / 25 cm 2 To prepare the positive electrode.

[0183] Comparative Example 8

[0184] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 400 mg / 25 cm 2 To create the positive electrode.

[0185] Comparative Example 9

[0186] The lithium secondary battery was manufactured in the same manner as in Example 1, except that a non-aqueous electrolyte was prepared by using a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 30:30:40 as the organic solvent, and the loading of the positive electrode slurry was increased from 600 mg / 25 cm⁻¹. 2 Change to 400mg / 25 cm 2 To prepare the positive electrode.

[0187] [Table 1]

[0188] Experimental examples

[0189] Experiment Example 1: Initial Capacity Performance Measurement

[0190] The initial charge and discharge were performed as follows: 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 conditions, and discharged to 2.5V at 0.33C to measure their initial discharge capacity (in mAh).

[0191] 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 (%). The results are shown in Table 2 below.

[0192] Experimental Example 2: Evaluation of Capacity Retention During Cyclic Charge-Discharge Cycles

[0193] 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 conditions, and then discharged to 2.5V at 0.33C. This was set as one cycle, and the discharge capacity and resistance after one cycle were measured. At this time, the battery was charged to 50% SOC based on the discharge capacity by detecting the capacity at room temperature, and then the battery was discharged at a current of 2.5C for 10 seconds. The resistance was measured using the voltage drop difference obtained therefrom.

[0194] Subsequently, 200 charge / discharge cycles were performed under the above charge / discharge conditions, and then the capacity retention rate (%) and 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.

[0195] [Equation 1]

[0196] Capacity retention (%) = (Discharge capacity after 200 cycles / Discharge capacity after 1 cycle) × 100

[0197] [Equation 2]

[0198] Resistance increase rate (%) = {(resistance after 200 cycles - resistance after 1 cycle) / resistance after 1 cycle} × 100

[0199] [Table 2]

[0200] Referring to Table 2 above, it can be confirmed that, compared with Comparative Examples 1 to 6, the lithium secondary batteries of Examples 1 to 8 of the present invention exhibit excellent capacity performance, as well as excellent life performance and low resistance increase rate.

[0201] Furthermore, compared to Comparative Example 4, which did not use dimethyl carbonate, the positive electrode loading was designed to be 500 mg / 25 cm⁻¹. 2 Example 6 exhibits excellent capacitance performance, excellent lifetime performance, and low resistance increase rate.

[0202] Meanwhile, the cathode loading was designed to be 400 mg / 25 cm⁻¹. 2 Comparative Examples 7 to 9, with lower loading levels, showed that electrolyte impregnation was not a major issue, thus demonstrating that they were not significantly affected by the composition and content of the non-aqueous electrolyte. Specifically, comparing Comparative Examples 7 and 8, it was confirmed that even using dimethyl carbonate as the organic solvent component did not significantly improve capacity performance, lifetime performance, or resistivity increase rate. Furthermore, Comparative Example 9, using a straight-chain carbonate other than dimethyl carbonate, exhibited the same or similar performance levels as Comparative Examples 7 and 8. As a result, it was confirmed that the non-aqueous electrolyte of the present invention, even with a specific loading level (e.g., greater than 400 mg / 25 cm⁻¹), exhibited good performance. 2 And less than 750 mg / 25 cm 2 More specifically, 450 mg / 25 cm 2 Up to 740 mg / 25 cm 2 It exhibits particularly good performance in positive electrodes containing lithium iron phosphate.

Claims

1. A lithium secondary battery, comprising: Positive electrode, negative electrode, membrane, 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 of the positive electrode is 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 ;and The non-aqueous electrolyte contains lithium salt, organic solvent, and additives. The organic solvent comprises cyclic carbonate solvents and linear carbonate solvents, wherein the cyclic carbonate solvent includes ethylene carbonate, and the linear carbonate solvent includes dimethyl carbonate and ethyl methyl carbonate. The organic solvent comprises 10 vol% to 35 vol% ethylene carbonate, 5 vol% to 55 vol% dimethyl carbonate, and 20 vol% to 70 vol% ethyl methyl carbonate, and the volume ratio of the cyclic carbonate solvent to the linear carbonate solvent is 10:90 to 35:

65. The additive contains vinylene carbonate, and The weight ratio of vinylene carbonate to dimethyl carbonate is greater than 0 and less than 0.

2.

2. The lithium secondary battery as described in claim 1, wherein, The loading of the positive electrode is 450 mg / 25 cm⁻¹. 2 Up to 700mg / 25 cm 2 .

3. The lithium secondary battery as described in claim 1, wherein, The loading of the positive electrode is 500 mg / 25 cm⁻¹. 2 Up to 600mg / 25 cm 2 .

4. The lithium secondary battery as described in claim 1, wherein, In the non-aqueous electrolyte, the content of vinylene carbonate is from 0.01% to 7% by weight.

5. The lithium secondary battery as described in claim 1, wherein, The organic solvent contains 7% to 45% by volume of dimethyl carbonate.

6. The lithium secondary battery as described in claim 1, wherein, The organic solvent contains 30% to 45% by volume of dimethyl carbonate.

7. The lithium secondary battery as described in claim 1, wherein, The organic solvent contains 35% to 45% by volume of dimethyl carbonate.

8. The lithium secondary battery as described in claim 1, wherein, The lithium salts include LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, and LiB. 10 Cl 10 At least one of LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2 or LiBETI (LiN(SO2CF2CF3)2).

9. The lithium secondary battery as described in claim 1, wherein, The concentration of lithium salt contained in the non-aqueous electrolyte is from 0.8 M to 3.0 M.

10. The lithium secondary battery as described in claim 1, wherein, The lithium iron phosphate particles contain a compound represented by formula A: [Formula A] Li 1+a Fe 1-s M s (PO 4-b )X b 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, where 0≤s≤0.5; -0.5≤a≤+0.5; 0≤b≤0.

1.

11. The lithium secondary battery as described in claim 1, wherein, The lithium iron phosphate particles contain LiFePO4.

12. The lithium secondary battery as described in claim 1, wherein, 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 The range is from 0.2 μm to 3.0 μm.

13. The lithium secondary battery as described in claim 1, wherein, The lithium iron phosphate particles have a carbon coating on their surface.

14. The lithium secondary battery as described in claim 1, wherein, The positive electrode active material does not contain lithium nickel oxides.

15. The lithium secondary battery as described in claim 1, wherein, The negative electrode contains carbon-based active materials.

16. The lithium secondary battery as described in claim 15, wherein, The carbon-based active material includes at least one of natural graphite or artificial graphite.

17. A method for preparing a lithium secondary battery, the method comprising: 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 of the positive electrode is 450 mg / 25 cm⁻¹. 2 Up to 740 mg / 25 cm 2 ; The electrode assembly is housed in the battery casing; Preparation of a non-aqueous electrolyte comprising lithium salt, organic solvent, and additives. The organic solvent comprises cyclic carbonate solvents and linear carbonate solvents, wherein the cyclic carbonate solvent includes ethylene carbonate, and the linear carbonate solvent includes dimethyl carbonate and ethyl methyl carbonate. The organic solvent comprises 10 vol% to 35 vol% ethylene carbonate, 5 vol% to 55 vol% dimethyl carbonate, and 20 vol% to 70 vol% ethyl methyl carbonate, and the volume ratio of the cyclic carbonate solvent to the linear carbonate solvent is 10:90 to 35:

65. The additive comprises vinylene carbonate, and the weight ratio of vinylene carbonate to dimethyl carbonate is 0.01 to 0.2; and The prepared non-aqueous electrolyte is injected or impregnated into the battery casing.

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