Lithium secondary battery and method for manufacturing the same

Abstract

The present invention relates to a lithium secondary battery and a method for manufacturing the same, which contains a silicon material as a negative electrode active material, has a high energy density, and has a polymer coating on a positive electrode active layer derived from an electrolyte additive contained in an electrolyte composition, which can reduce irreversible capacity loss caused by the silicon material in the negative electrode during initial charging and can suppress gas generation during charging and discharging, thereby improving battery safety. Therefore, the lithium secondary battery can be useful as a power source for medium- to large-sized devices such as electric vehicles.

Description

[Technical Field]

[0001] This invention relates to a lithium secondary battery with excellent battery capacity and a method for manufacturing the same.

[0002] This application claims priority under Korean Patent Application No. 10-2022-0075481 dated June 21, 2022, and all content disclosed in the said Korean Patent Application is incorporated herein as part of this specification. [Background technology]

[0003] As technological development and demand for mobile devices increase, the demand for rechargeable batteries as an energy source is rapidly growing. Among these rechargeable batteries, lithium-ion batteries, which have high energy density and operating potential, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] In recent years, lithium-ion batteries have been used as power sources for medium- and large-sized devices such as electric vehicles, leading to a growing demand for higher capacity, higher energy density, and lower costs in lithium-ion batteries.

[0005] While carbon-based materials are primarily used as negative electrode materials in lithium-ion batteries, negative electrodes made of carbon-based materials are theoretically limited to a maximum capacity of 372 mAh / g (844 mAh / cc), thus limiting capacity increases. Furthermore, lithium metal, which has been considered as a negative electrode material, has a very high energy density and can achieve high capacity, but it suffers from safety issues due to dendrite growth during repeated charge and discharge cycles, as well as a short cycle life.

[0006] Therefore, the use of anode active materials with high energy density as materials that exhibit high capacity and can replace lithium metal has become inevitable, and many studies and proposals have been made regarding silicon, tin, or alloys thereof. In particular, silicon-based materials are promising as high-capacity anode materials because they reversibly intercalate and release lithium through compound formation reactions with lithium, and have a theoretical maximum capacity of approximately 4200 mAh / g (9366 mAh / cc, specific gravity 2.23), which is much larger than that of carbon-based materials.

[0007] However, these negative electrode materials have the problem of low initial efficiency and large irreversible capacity loss during initial charging and discharging. To improve this problem, research has been actively conducted on irreversible positive electrode additives that can replenish the lithium ions lost due to irreversible reactions occurring at the negative electrode during initial charging, thereby improving capacity during subsequent charging and discharging. As a result, irreversible additives such as Li2NiO2 and Li6CoO4 have been developed.

[0008] However, these existing irreversible additives are generally manufactured by reacting precursors such as cobalt oxide or nickel oxide with an excess amount of lithium oxide. Because these irreversible additives are structurally unstable, they generate gas when charging occurs, and this generated gas can induce volume expansion of the electrode assembly, which can act as one of the main factors leading to a decrease in battery performance.

[0009] Furthermore, irreversible additives can deform into a thermally unstable structure when stored at high temperatures above 60°C after initial charging and discharging. This can lead to the release of additional gases and accelerate the battery's self-discharge, which is a limitation.

[0010] Therefore, there is a need to develop technology that can increase the energy density of a battery by using a silicon-based material as the negative electrode material, improve the irreversible capacity loss during the initial charging and discharging of the battery, and at the same time overcome safety problems such as the generation of internal gases. [Prior art documents]

Patent Document

[0011]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] Therefore, an object of the present invention is to provide a lithium secondary battery and a method for manufacturing the same, which contain a silicon material as a negative electrode material, reduce the loss of irreversible capacity during initial charging, and improve the safety problems of the battery caused by the generation of internal gas and the like.

Means for Solving the Problems

[0013] To solve the above - described problems, in one embodiment, the present invention provides a lithium secondary battery including an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and an electrolyte composition including a non - aqueous organic solvent, a lithium salt, and an electrolyte additive. The positive electrode sequentially includes a positive electrode active layer containing a positive electrode active material and a polymer film on a positive electrode current collector. The negative electrode includes a negative electrode active layer containing a carbon material and a silicon material as negative electrode active materials on a negative electrode current collector, and the ratio (DC / CC) of the initial discharge capacity (DC) to the initial charge capacity (CC) is 0.7 to 1.2.

[0014] Here, the positive electrode can satisfy the following formula 1 during XPS analysis.

[0015] [Formula 1] 0.5 ≤ P C / P N ≤ 5

[0016] P C represents the intensity of the peak existing at 284.0 ± 0.5 eV, and P N represents the intensity of the peak existing at 402.5 ± 0.5 eV.

[0017] Further, the positive electrode includes a positive electrode active layer containing a positive electrode active material, and the positive electrode active material may contain an iron phosphate compound represented by the following Chemical Formula 1.

[0018] [Chemical Formula 1] LiFe a M 1 1-a XO4

[0019] In the above Chemical Formula 1, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, X is one or more selected from the group consisting of P, Si, S, As, and Sb, and a is 0 ≦ a ≦ 0.5.

[0020] Further, the electrolyte additive may be a polymer compound containing nitrogen (N) and carbon (C). [[ID=​​​​​​​​​​​​​​​​​​​​​​​​However, it may contain one or more of the following (0.8 ≤ q ≤ 2.5), and its content may be 1 to 20% by weight relative to the total weight of the negative electrode active material.

[0027] Furthermore, in one embodiment, the present invention provides a method for manufacturing a lithium secondary battery, comprising the steps of: assembling a secondary battery by injecting an electrolyte composition into a battery case into which an electrode assembly including a positive electrode, a negative electrode, and a separation membrane disposed between the positive electrode and the negative electrode is inserted; and charging the assembled secondary battery to a state of charge (SOC) of 10% or more, thereby forming a polymer film on the positive electrode active layer containing a positive electrode active material, wherein the electrolyte composition comprises a non-aqueous organic solvent, a lithium salt, and an electrolyte additive; the negative electrode comprises a negative electrode active layer on a negative electrode current collector containing a carbon material and a silicon material as negative electrode active material; and the assembled secondary battery has an initial discharge capacity (DC) to initial charge capacity (CC) ratio (DC / CC) of 0.7 to 1.2.

[0028] Here, the above charging can be performed at a C-rate of 0.01C to 3.0C at 25 to 70°C. [Effects of the Invention]

[0029] The lithium secondary battery according to the present invention contains a silicon material as the negative electrode active material, resulting in a high energy density. A polymer coating derived from an electrolyte additive contained in the electrolyte composition is provided on the positive electrode active layer. This not only reduces the irreversible capacity loss that occurs during initial charging due to the silicon material in the negative electrode, but also suppresses the generation of gases during charging and discharging, thereby improving the safety of the battery. Therefore, it can be usefully used as a power source for medium- and large-sized devices such as electric vehicles. [Modes for carrying out the invention]

[0030] Since the present invention can be modified in various ways and may have a variety of embodiments, specific embodiments will be described in detail in the detailed description.

[0031] However, this should not be understood as limiting the present invention to any particular embodiment, but rather as including all modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0032] In the present invention, terms such as "includes" and "have" are intended to specify the existence of features, numbers, stages, operations, components, parts, or combinations thereof as described in the specification, and should be understood not to preemptively exclude the existence or possibility of adding one or more other features, numbers, stages, operations, components, parts, or combinations thereof.

[0033] Furthermore, in this invention, when a part such as a layer, film, region, or plate is described as being "on top" of another part, this includes not only the case where it is "directly on top" of the other part, but also the case where another part is located in between. Conversely, when a part such as a layer, film, region, or plate is described as being "below" another part, this includes not only the case where it is "directly below" the other part, but also the case where another part is located in between. Also, in this application, being "on top" may include being located not only at the top but also at the bottom.

[0034] Furthermore, in this invention, "cross-sectional structure" means the structure of a plane that is cut perpendicularly to the surface of the active material layer.

[0035] The present invention will be described in more detail below.

[0036] <Lithium-ion secondary battery> In one embodiment, the present invention provides a lithium secondary battery comprising an electrode assembly including a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, and an electrolyte composition including a non-aqueous organic solvent, a lithium salt, and an electrolyte additive, wherein the positive electrode sequentially comprises a positive electrode active layer containing a positive electrode active material and a polymer coating on a positive electrode current collector, and the negative electrode comprises a negative electrode active layer containing a carbon material and a silicon material as negative electrode active material on a negative electrode current collector, and the ratio of initial discharge capacity (DC) to initial charge capacity (CC) (DC / CC) is 0.7 to 1.2.

[0037] The lithium secondary battery according to the present invention includes an electrode assembly comprising a positive electrode and a negative electrode, with a separation membrane interposed between the positive electrode and the negative electrode. Furthermore, the positive electrode and the negative electrode are connected by a lithium ion (Li) exchange that takes place between them. + It has a structure impregnated with an electrolyte for the movement of )

[0038] In this case, the negative electrode comprises a negative electrode active layer manufactured by coating, drying, and pressing a negative electrode active material onto a negative electrode current collector, and may optionally further contain conductive materials, binders, and other additives in the negative electrode active layer.

[0039] The above negative electrode active material may include a carbon material and a silicon material. The above carbon material means a carbon material mainly composed of carbon atoms, and such carbon materials may include graphite with a completely layered crystalline structure like natural graphite, soft carbon having a low-crystallinity layered crystalline structure (graphene structure; a structure in which planes of hexagonal honeycomb patterns of carbon are arranged in layers), hard carbon in which these structures are mixed with amorphous parts, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, acetylene black, Ketjen black, carbon nanotubes, fullerene, activated carbon, graphene, carbon nanotubes, etc. Preferably, it may include one or more selected from the group consisting of natural graphite, artificial graphite, graphene, and carbon nanotubes. More preferably, the above carbon material includes natural graphite and / or artificial graphite, and may include one or more of graphene and carbon nanotubes together with the above natural graphite and / or artificial graphite. In this case, the carbon material may contain 50 parts by weight or less of graphene and / or carbon nanotubes per 100 parts by weight of the total carbon material, and more specifically, it may contain 1 to 40 parts by weight or 5 to 20 parts by weight of graphene and / or carbon nanotubes per 100 parts by weight of the total carbon material.

[0040] Furthermore, the above silicon material is a particle containing silicon (Si) as the main metallic component, and includes silicon (Si), silicon carbide (SiC), and silicon oxide (SiO₂). q However, it may contain one or more of the following (0.8 ≤ q ≤ 2.5). As one example, the silicon material may include a mixture of silicon (Si) particles and silicon carbide (SiC). As another example, the silicon material may include silicon (Si) particles, silicon monoxide (SiO) particles, silicon dioxide (SiO2) particles, or a mixture of these particles.

[0041] Furthermore, the silicon material may have a form in which crystalline particles and amorphous particles are mixed, and the proportion of amorphous particles may be 50 to 100 parts by weight per 100 parts by weight of the total silicon material, specifically 50 to 90 parts by weight, 60 to 80 parts by weight, or 85 to 100 parts by weight. By controlling the proportion of amorphous particles contained in the silicon material within the above range, the present invention can improve thermal stability and flexibility without degrading the electrical properties of the electrode.

[0042] Furthermore, the silicon material may be included in an amount of 1 to 20 parts by weight per 100 parts by weight of the negative electrode active layer, specifically in an amount of 5 to 20 parts by weight, 3 to 10 parts by weight, 8 to 15 parts by weight, 13 to 18 parts by weight, 2 to 7 parts by weight, or 4 to 17 parts by weight per 100 parts by weight of the negative electrode active layer.

[0043] The present invention makes it possible to increase the energy density of the battery while suppressing excessive volume expansion of the negative electrode due to silicon material during charging and discharging by adjusting the content of silicon material in the negative electrode active material to the range described above.

[0044] On the other hand, the positive electrode, like the negative electrode, includes a positive electrode active layer manufactured by coating, drying, and pressing a positive electrode slurry containing a positive electrode active material and a positive electrode additive onto a positive electrode current collector, and the positive electrode active layer may selectively further contain conductive materials, binders, and other additives as needed.

[0045] In this case, the positive electrode active material may include a compound containing iron (Fe) as an element, and in some cases, other transition metals (M 1 ) may have a doped form. For example, the above positive electrode active material may contain an iron phosphate compound represented by the following chemical formula 1.

[0046] [Chemical formula 1] LiFe a M 1 1-a XO4

[0047] In the above chemical formula 1, M 1X is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, X is one or more elements selected from the group consisting of P, Si, S, As, and Sb, and a is 0 ≤ a ≤ 0.5.

[0048] The iron phosphate compound represented by the above chemical formula 1 has an olivin structure and exhibits superior structural stability, making it a promising active material with excellent lifespan characteristics and superior safety in all aspects, including overcharging and over-discharging. In particular, the above iron phosphate compound has excellent high-temperature stability due to the strong bonding force of PO4, and because it contains iron, which is abundant and inexpensive as a resource, it is less expensive than the LiCoO2, LiNiO2, or LiMn2O4 mentioned above, and also has low toxicity, thus having less impact on the environment.

[0049] Furthermore, lithium metal oxides containing two or more elements such as nickel (Ni), cobalt (Co), manganese (Mn), and aluminum (Al) exhibit large voltage fluctuations during charging, which may prevent stable polymerization of electrolyte additives contained in the electrolyte composition, potentially leading to internal short circuits in the battery. However, iron phosphate compounds represented by chemical formula 1 do not exhibit large voltage fluctuations during charging and discharging, allowing for stable polymerization of electrolyte additives contained in the electrolyte composition on the positive electrode surface. This not only reduces irreversible capacity loss during initial charging of the battery but also suppresses the generation of gases during charging and discharging, thereby improving battery safety.

[0050] Furthermore, the content of the above-mentioned positive electrode active material may be 85 to 95 parts by weight per 100 parts by weight of the positive electrode active layer, specifically 88 to 95 parts by weight, 90 to 95 parts by weight, 86 to 90 parts by weight, or 92 to 95 parts by weight.

[0051] Furthermore, the positive electrode active layer may further contain a binder, conductive material, additives, etc., along with the positive electrode active material.

[0052] As one example, the positive electrode active layer may contain a conductive material to improve the performance of the positive electrode, such as its electrical conductivity. Such a conductive material may be one or more carbon-based substances selected from the group consisting of natural graphite, artificial graphite, graphene, carbon nanotubes, carbon black, acetylene black, Ketjenblack, and carbon fibers. For example, the conductive material may contain acetylene black.

[0053] Furthermore, the conductive material may be included in an amount of 0.1 to 5 parts by weight per 100 parts by weight of the positive electrode active layer, specifically in an amount of 0.5 to 4 parts by weight, 1 to 3.5 parts by weight of the conductive material, or 0.5 to 1.5 parts by weight.

[0054] Furthermore, the positive electrode active layer may further contain a binder to fix the positive electrode active material, conductive material, etc., that constitute the active layer, while also providing adhesion to the positive electrode current collector. Such a binder may include one or more resins selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. As one example, the binder may include polyvinylidene fluoride.

[0055] Furthermore, the above-mentioned binder may be included in an amount of 1 to 10 parts by weight per 100 parts by weight of the entire positive electrode active layer, specifically in an amount of 2 to 8 parts by weight, or 1 to 5 parts by weight of conductive material.

[0056] Furthermore, the average thickness of the positive electrode active layer is not particularly limited, but it can be 50 μm to 300 μm, and more specifically, it can be 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0057] Furthermore, the positive electrode can be made of a material that has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, or calcined carbon can be used, and in the case of aluminum or stainless steel, it is also possible to use materials that have been surface-treated with carbon, nickel, titanium, silver, etc.

[0058] Furthermore, the positive electrode active layer includes a polymer coating. This polymer coating is formed on the surface of the positive electrode by polymerization of electrolyte additives contained in the electrolyte composition during the initial charging of the battery. At this time, the electrolyte additives generate electrons (e) through polymerization during the initial charging. - ) releases electrons (e - ) releases lithium ions (Li + This can prevent the lithium ions (Li) from being consumed on the surface of the negative electrode in the form of a polymer film. That is, the polymer film prevents the silicon material, which is the negative electrode active material, from being consumed on the surface of the negative electrode by forming an insulating inorganic film, for example, a solid electrolyte interphase (SEI), during the process of being formed on the positive electrode active layer during the initial charging of the battery. + Since the amount of ) can be minimized, the irreversible capacity of the positive electrode can be further increased. In addition, the polymer coating can act as a scavenger to remove moisture and / or acidic by-products, thereby reducing side reactions of the electrolyte. Furthermore, compared to irreversible additives such as Li2NiO2 and Li6CoO4 that are applied to compensate for the loss of irreversible capacity in the conventional battery, the amount of gas generated during battery charging and discharging is significantly less, resulting in the advantage of superior battery stability.

[0059] As one example, the lithium secondary battery according to the present invention can satisfy an initial discharge capacity (DC) to initial charge capacity (CC) ratio (DC / CC) of 0.7 to 1.2, specifically 0.8 to 1.1, 0.9 to 1.2, or 0.9 to 1.1. This means that even though the lithium secondary battery contains silicon material as the negative electrode active material, the irreversible capacity of the positive electrode increases, reducing the loss of irreversible capacity during the initial charging and discharging of the battery, thereby improving the energy density of the battery.

[0060] Such polymer coatings may originate from electrolyte additives contained in the electrolyte composition during the initial charging of the battery. Specifically, the electrolyte composition applied to the lithium secondary battery according to the present invention is a non-aqueous liquid electrolyte comprising a non-aqueous organic solvent, a lithium salt, and an electrolyte additive for the formation of a polymer coating.

[0061] In this case, the electrolyte additive may be capable of polymerizing on the surface of the positive electrode during the initial charging of the battery to form a polymer film. Specifically, the electrolyte additive may be a polymerizable compound containing elements such as nitrogen (N), oxygen (O), and sulfur (S), which have lone pairs of electrons, along with carbon (C).

[0062] As one example, the electrolyte additive described above may be a polymerizable compound containing nitrogen (N) and carbon (C), and more specifically, it may contain one or more nitrogen-containing polymerizable compounds selected from pyrrole, pyridine, and aniline groups. Here, the polymerizable compound containing pyrrole, pyridine, and aniline groups is a compound with pyrrole, pyridine, and / or aniline groups as its core, and may contain not only pyrrole, pyridine, and / or aniline, but also derivatives in which substituents have been introduced thereto. For example, the nitrogen-containing polymerizable compounds mentioned above may include pyrrole, 2,5-dimethylpyrrole, 2,4-dimethylpyrrole, 2-acetyl-N-methylpyrrole, 2-acetylpyrrole, N-methylpyrrole, N-methylaniline, N,N-dimethylaniline, phenylenediamine, p-toluidine, N,N-dimethyl-p-toluidine, 2-methylpyridine, 4-methylpyridine, and the like.

[0063] In this case, the positive electrode according to the present invention can satisfy the conditions of the following formula 1 during X-ray photoelectron spectroscopy (XPS) analysis.

[0064] [Formula 1] 0.5 ≤ P C / P N ≤5

[0065] P C This represents the intensity of the peak located at 284.0±0.5eV, and P N This represents the intensity of the peak located at 402.5±0.5eV.

[0066] Equation 1 above represents the proportion of peaks originating from the components constituting the polymer coating on the positive electrode active layer when performing X-ray photoelectron spectroscopy (XPS) analysis on an initially charged positive electrode. Specifically, the above P C This peak represents the carbon-carbon double bond (C=C) energy among the 1s bonds of carbon (C), and P NThis is the 1s bond of nitrogen (N) in quaternary ammonium (NH) + The peaks represent energy, and these ratios represent the bond energy between the nitrogen element of the polymer coating and the oxidation byproducts such as water (H2O) and / or HF generated internally during battery charging and discharging. These ratios can indicate the amount of water and / or oxidation byproducts captured by the polymer coating. By satisfying the amount of lithium ions captured represented by Formula 1 to 0.5 to 5, the present invention can suppress the generation of internal gas and battery degradation due to oxidation byproducts such as water (H2O) and / or HF generated internally during battery charging and discharging, thereby improving the safety and lifespan of the battery and further increasing the energy density of the battery. In this case, the positive electrode of the present invention satisfies the above Formula 1 to 0.5 to 4 (0.5 ≤ P C / P N ≤4), 0.5~3 (0.5≦P C / P N ≤3), 1~5(1≦P C / P N ≤5), 1.5~5 (1.5 ≤ P C / P N ≤5), 2~5(2≦P C / P N ≤5), 3~5(3≦P C / P N ≤5), or 1~2 (1 ≤ P C / P N It can satisfy the condition ≤ 2).

[0067] As another example, the electrolyte additive may be a polymerizable compound containing sulfur (S) and carbon (C), and more specifically, a sulfur-containing polymerizable compound such as a thiophene group. Here, the polymerizable compound containing a thiophene group is a compound with a thiophene group as its core, and may contain not only thiophene but also derivatives in which substituents have been introduced to thiophene. For example, the sulfur-containing polymerizable compound may include halothiophene, 3,4-ethylenedioxythiophene, and the like.

[0068] Furthermore, the electrolyte additive may be applied in an amount that does not degrade the performance of the electrolyte composition. Specifically, the electrolyte additive may be included in an amount of 0.01 to 5% by weight relative to the total weight of the electrolyte composition, and more specifically, in an amount of 0.05 to 3% by weight, 0.05 to 2% by weight, 0.05 to 1.5% by weight, 1.1 to 1.9% by weight, 0.1 to 0.9% by weight, or 0.8 to 1.4% by weight relative to the total weight of the electrolyte composition.

[0069] The present invention prevents a polymer film from forming on the surface of the positive electrode active layer to a sufficient thickness due to a very small amount of electrolyte additive, while also preventing the electrolyte from gelling due to an excessive amount, by adjusting the content of the electrolyte additive within the above range.

[0070] In this sense, the thickness of the polymer coating can satisfy a specific range. Specifically, the polymer coating may have a thickness of 5 nm to 500 μm, and more specifically, it may have a thickness of 5 nm to 300 μm, 100 nm to 200 μm, 500 nm to 150 μm, 900 nm to 50 μm, 1 μm to 100 μm, 5 nm to 900 nm, or 50 nm to 500 nm.

[0071] On the other hand, the electrolyte composition containing the above-mentioned electrolyte additive may include a non-aqueous organic solvent and a lithium salt commonly used in this industry.

[0072] Specifically, the lithium salts mentioned above are LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB 10 Cl 10 It may contain one or more selected from the group consisting of LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, and (FSO2)2NLi.

[0073] There are no particular restrictions on the concentration of these lithium salts, but the lower limit of the suitable concentration range is 0.5 mol / L or higher, specifically 0.7 mol / L or higher, more specifically 0.9 mol / L or higher, or 1.2 mol / L or higher, and the upper limit of the suitable concentration range is 2.5 mol / L or lower, specifically 2.0 mol / L or lower, more specifically 1.8 mol / L or lower. If the lithium salt concentration falls below 0.5 mol / L, the ionic conductivity will decrease, which may reduce the cycle characteristics and output characteristics of the non-aqueous electrolyte battery. Insufficient lithium ions (Li) are present to form a solid electrolyte film (SEI) on the surface of the negative electrode active layer during the initial charging of the battery. + There is a limitation in that a non-uniform solid electrolyte film is formed because the electrolyte is not supplied. Furthermore, if the lithium salt concentration exceeds 2.5 mol / L, the viscosity of the electrolyte for non-aqueous electrolyte batteries may increase, which may reduce the ionic conductivity and potentially degrade the cycle characteristics and output characteristics of the non-aqueous electrolyte battery.

[0074] Furthermore, dissolving a large amount of lithium salt in a non-aqueous organic solvent at once may cause the liquid temperature to rise due to the heat of dissolution of the lithium salt. When the temperature of the non-aqueous organic solvent rises significantly due to the heat of dissolution of the lithium salt, in the case of lithium salts containing fluorine, decomposition may be accelerated, potentially generating hydrogen fluoride (HF). Hydrogen fluoride (HF) is undesirable because it degrades battery performance. Therefore, while the temperature at which the lithium salt is dissolved in the non-aqueous organic solvent is not particularly limited, it can be adjusted to -20 to 80°C, specifically to 0 to 60°C.

[0075] Furthermore, the non-aqueous organic solvent used in the above-mentioned electrolyte composition is not particularly limited and can be applied as long as it is used in non-aqueous electrolytes in this industry. Specifically, non-aqueous organic solvents such as N-methyl-2-pyrrolidinone, ethylene carbonate (EC), propylene carbonate, butylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), gamma-butyrolactone, 1,2-dimethoxyethane (DME), tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate may be used.

[0076] Furthermore, the non-aqueous organic solvents used in the present invention may be used individually, or two or more may be mixed in any combination and proportion depending on the application. Among these, propylene carbonate, ethylene carbonate, fluoroethylene carbonate, diethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate are particularly preferred from the viewpoint of their electrochemical stability against oxidation and reduction and chemical stability with respect to heat and reactions with solutes.

[0077] Furthermore, the electrolyte composition described above may further contain other additives in addition to the basic components mentioned above. Additives commonly used in the non-aqueous electrolyte of the present invention may be added in any proportion, as long as the gist of the present invention is not impaired. Specifically, examples include compounds having overcharge prevention effects, negative electrode film formation effects, and positive electrode protection effects, such as cyclohexylbenzene, biphenyl, t-butylbenzene, vinylene carbonate, vinylethylene carbonate, difluoroanisole, fluoroethylene carbonate, propanesultone, succinonitrile, and dimethylvinylene carbonate. It is also possible to solidify the electrolyte for non-aqueous electrolyte batteries using gelling agents or crosslinking polymers, as is the case when used in non-aqueous electrolyte batteries known as lithium polymer batteries.

[0078] The lithium secondary battery according to the present invention has the above-described configuration, which results in a high energy density and reduces the irreversible capacity loss that occurs during initial charging and discharging due to the silicon material of the negative electrode. Furthermore, it can suppress gas generation during charging and discharging, thereby improving the safety of the battery. Therefore, it can be usefully used as a power source for medium- and large-sized devices such as electric vehicles.

[0079] <Manufacturing method for lithium secondary batteries> Furthermore, in one embodiment, the present invention provides a method for manufacturing a lithium secondary battery according to the present invention as described above.

[0080] The method for manufacturing a lithium secondary battery according to the present invention can be carried out by injecting an electrolyte composition into a battery case into which an electrode assembly including a positive electrode, a negative electrode, and a separation membrane disposed between the positive electrode and the negative electrode is inserted to assemble a secondary battery, performing initial charging of the assembled secondary battery, and forming a polymer coating on the positive electrode active layer containing the positive electrode active material.

[0081] Specifically, the above-described method for manufacturing a lithium secondary battery includes the steps of assembling a secondary battery by injecting an electrolyte composition into a battery case into which an electrode assembly including a positive electrode, a negative electrode, and a separation membrane disposed between the positive electrode and the negative electrode is inserted; and charging the assembled secondary battery to a state of charge of 10% or more, thereby forming a polymer coating on the positive electrode active layer containing a positive electrode active material.

[0082] Here, the step of assembling the secondary battery described above includes the entire process of manufacturing the electrode assembly, inserting the manufactured electrode assembly into the battery case, and injecting the electrolyte composition, and the methods commonly used in the industry may be applied.

[0083] Furthermore, the electrolyte composition injected into the battery case is a non-aqueous liquid electrolyte as described above, and has a composition comprising a non-aqueous organic solvent, a lithium salt, and an electrolyte additive for forming a polymer coating.

[0084] Furthermore, the negative electrode comprises a negative electrode active layer manufactured by coating, drying, and pressing a negative electrode active material onto a negative electrode current collector as described above, and the negative electrode active layer contains a carbon material and a silicon material as the negative electrode active material.

[0085] Furthermore, the step of forming a polymer film on the positive electrode active layer involves initially charging the assembled secondary battery and forming a polymer film on the positive electrode active layer by electrochemical polymerization of the electrolyte additive. This allows for the uniform formation of a polymer film derived from the electrolyte additive on the positive electrode active layer.

[0086] The initial charging described above is a step in which the battery is charged, a solid electrolyte film (SEI) is formed on the surface of the negative electrode active layer, the battery is activated, and gases generated inside are removed. In order to form a polymer film on the surface of the positive electrode active layer, the present invention may be carried out so that the state of charge (SOC) of the lithium secondary battery is 10% or more. More specifically, the initial charging may be carried out at an SOC of 10% to 90%, 10% to 80%, 10% to 70%, 10% to 50%, 10% to 40%, 30% to 80%, 40% to 80%, 50% to 90%, or 45% to 65%. By performing initial charging to satisfy the above range of SOC, the present invention can prevent the polymer film from not being formed due to an extremely low battery potential, while preventing internal short circuits from occurring during the formation of the polymer film due to high SOC conditions.

[0087] Furthermore, the above initial charging can be carried out under certain temperature and C-rate conditions. Specifically, the above initial charging can be carried out at 25-70°C with a C-rate of 0.01-3.0C, more specifically at 30-65°C, 45-70°C, 25-50°C, or 30-50°C with a C-rate of 0.01-1.5C, 0.01-1.2C, 0.01-0.9C, 0.02-0.8C, or 0.05-0.9C.

[0088] As one example, the initial charging described above can be performed at 40-45°C with a charging termination voltage of 3.0-4.2V and a C-rate of 0.05-0.08C.

[0089] A polymer coating can be formed when a certain level of oxidation potential is applied, but the polymerization efficiency and the efficiency of electron release by lithium, the positive electrode active material, can differ depending on the conditions when the potential is applied, such as temperature and C-rate. Therefore, the present invention makes it possible to improve the battery capacity by applying a potential lower than or equivalent to the oxidation potential required to form a polymer coating during initial charging, while adjusting the charging temperature and C-rate to the range described above, thereby forming a polymer coating on the positive electrode active layer while minimizing the consumption of lithium, the positive electrode active material.

[0090] On the other hand, the initial charging described above can refer to the process required for the lithium secondary battery's State of Charge (SOC) to reach the above range, and may include aging, thermal treatment, etc., before reaching the above SOC range.

[0091] The present invention will be described in more detail below with reference to examples and experimental examples.

[0092] However, the following examples and experimental cases are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental cases.

[0093] <Examples and Comparative Examples: Manufacturing of Lithium-ion Secondary Batteries> An electrolyte composition was prepared by dissolving LiPF6 as a lithium salt at a concentration of 1.3 ± 0.1 M in a solvent prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70, and then adding polymerizable compounds as electrolyte additives, weighed based on the total weight of the electrolyte, as shown in Table 1 below.

[0094] Separately, LiFePO4 with a particle size of 5 μm was prepared as the positive electrode active material. Carbon black, a carbon-based conductive material, and polyvinylidene fluoride, a binder, were mixed with N-methylpyrrolidone (NMP) in a weight ratio of 94:3:3 to form a slurry. This slurry was then cast onto an aluminum sheet, dried in a vacuum oven at 120°C, and then rolled to produce the positive electrode.

[0095] Also, graphite (natural graphite:synthetic graphite = 1:1 by weight) and silicon dioxide (SiO₂) q However, a negative electrode active material was prepared by mixing 1≦q≦2) in a weight ratio of 85:15. 97 parts by weight of the negative electrode active material and 3 parts by weight of styrene-butadiene rubber (SBR) were mixed with water to form a slurry, which was then cast onto a copper sheet, dried in a vacuum oven at 130°C, and then rolled to produce the negative electrode.

[0096] A separation membrane made of 18 μm polypropylene was interposed between the positive and negative electrodes obtained above, and after inserting them into a case, the previously manufactured electrolyte composition was injected as shown in Table 1 below to assemble the lithium secondary battery.

[0097] Initial charging was performed on each assembled lithium secondary battery. Specifically, the lithium secondary batteries were initially charged under the conditions shown in Table 1 below, to a charge termination voltage of 4.2V at 55±2℃, thereby producing activated lithium secondary batteries.

[0098] [Table 1]

[0099] <Comparative Example 6. Manufacturing of Lithium-ion Secondary Batteries> A lithium secondary battery was manufactured using the same method as in Example 1, except that the electrolyte composition did not contain an electrolyte additive.

[0100] <Comparative Example 7. Manufacturing of Lithium-ion Secondary Batteries> A lithium secondary battery was manufactured using the same method as in Example 1, except that the electrolyte composition did not contain electrolyte additives. This was done using a positive electrode slurry containing carbon black (a carbon-based conductive material), polyvinylidene fluoride (a binder), and lithium cobalt oxide (Li6CoO4) (an irreversible additive) in a weight ratio of 94:3:2:1.

[0101] <Example of experiment> To evaluate the physical properties and performance of the lithium secondary battery according to the present invention, the following experiments were conducted.

[0102] i) Analysis of polymer coatings Each lithium secondary battery manufactured in the examples and comparative examples was disassembled to separate the positive electrode, and X-ray photoelectron spectroscopy (XPS) was performed on the surface of the separated positive electrode to obtain the X-ray photoelectron spectroscopy spectrum. For this XPS analysis, a Thermo Fisher Scientific ESCALAB250 (acceleration voltage: 15kV, 150W, energy resolution: 1.0eV, analysis area: diameter 500μm, sputtering rate: 0.1nm / sec) was used.

[0103] From the obtained spectrum, a peak (P) located at 284±0.5eV was identified. C ) intensity and the peak (P) located at 402.5±1.0eV N The ratio of the intensity of (P C / P N The result was calculated and is shown in Table 2 below.

[0104] (b) Measurement of the initial charge / discharge capacity and irreversible capacity of the battery. Lithium secondary batteries were assembled as shown in the examples and comparative examples, and activated lithium secondary batteries were produced by initial charging of the assembled lithium secondary batteries as shown in Table 1. Subsequently, each activated lithium secondary battery was aged at 55±2℃ for 5 to 10 minutes, and the initial charge capacity (CC) was measured by fully charging it with a charge termination voltage of 4.2V under the same temperature conditions. Then, the initial discharge capacity (DC) was measured by discharging under conditions of discharge termination voltages of 0.5C and 2.5V. From the measured initial charge and discharge capacities, the ratio of initial discharge capacity (DC) to initial charge capacity (CC) (DC / CC) was calculated, and the results are shown in Table 2 below.

[0105] [Table 2]

[0106] As shown in Table 2 above, the lithium secondary battery according to the present invention, despite containing silicon as the negative electrode active material, exhibits low irreversible capacity loss during initial charging and discharging, excellent electrical performance, and high stability due to the low amount of gas generated during charging and discharging.

[0107] Specifically, the lithium secondary batteries manufactured in the examples showed that a polymer coating satisfying Equation 1 was formed on the positive electrode active layer during X-ray photoelectron spectroscopy (XPS) analysis, reducing the irreversible capacity during initial charge and discharge, and demonstrating a high ratio of initial discharge capacity (DC) to initial charge capacity (CC) (DC / CC).

[0108] In contrast, Comparative Example 1, a lithium secondary battery containing an excess amount of electrolyte additive in its electrolyte composition, showed that the electrolyte composition gelled around the positive electrode active layer, preventing the formation of a polymer film. This resulted in a significantly lower ratio of the battery's initial discharge capacity (DC) to initial charge capacity (CC) (DC / CC). Similarly, when a polymer film that did not satisfy Equation 1 was formed on the positive electrode active layer during X-ray photoelectron spectroscopy (XPS) analysis, the ratio of the battery's initial discharge capacity (DC) to initial charge capacity (CC) (DC / CC) was found to be remarkably low, below 0.7.

[0109] This indicates that when a polymer film satisfying Equation 1 is formed on the positive electrode active layer, the irreversible capacity of the positive electrode is increased during the initial charging of the battery, and the loss of irreversible capacity of the battery is reduced, meaning that the lithium secondary battery according to the present invention has excellent capacity and energy density.

[0110] (h) Measurement of gas generation amount The lithium secondary batteries produced in Examples 2 and 4, and Comparative Examples 6 and 7 were fixed in ovens equipped with gas sensors, and the total amount of gas generated was measured after 50 charge-discharge cycles at 45°C and 0.3C. The results are shown in Table 3 below.

[0111] [Table 3]

[0112] As shown in Table 3 above, the lithium secondary battery according to the present invention generates significantly less gas inside the battery even when charging and discharging are performed.

[0113] Specifically, in the lithium secondary battery example in which a polymer coating was provided on the positive electrode active layer to improve the irreversible capacity loss that occurs during the initial charging and discharging of the secondary battery, it was confirmed that the amount of gas generated inside the battery was significantly low. On the other hand, the lithium secondary battery of the comparative example, which used an irreversible additive in the positive electrode, showed that the amount of gas generated by repeated charging and discharging was significantly high.

[0114] This means that the lithium secondary battery according to the present invention is highly safe even when charging and discharging are performed.

[0115] These results show that the lithium secondary battery according to the present invention has a high energy density, and the silicon material of the negative electrode not only reduces the irreversible capacity loss that occurs during initial charging and discharging, but also improves the safety of the battery by suppressing the generation of gas during charging and discharging.

[0116] While preferred embodiments of the present invention have been described above with reference to those skilled in the art or those with ordinary knowledge in the art, it will be understood that the present invention can be modified and altered in various ways without departing from the spirit and technical domain of the invention as described in the claims below.

[0117] Therefore, the technical scope of the present invention is not limited to what is described in the summary of the invention in the specification, but should be defined by the claims.

Claims

1. The electrode assembly includes a positive electrode, a negative electrode, and a separation membrane interposed between the positive and negative electrodes, and an electrolyte composition comprising a non-aqueous organic solvent, a lithium salt, and an electrolyte additive. The positive electrode has a structure in which a positive electrode active layer containing a positive electrode active material and a polymer coating are sequentially laminated on a positive electrode current collector. The aforementioned electrolyte additive is aniline, The polymer coating is a polymer of the electrolyte additive, The non-aqueous organic solvent is two or more selected from ethylene carbonate, fluoroethylene carbonate, and ethyl methyl carbonate. The negative electrode includes a negative electrode active layer on a negative electrode current collector containing a carbon material and a silicon material as negative electrode active material. A lithium secondary battery with an initial discharge capacity (DC) to initial charge capacity (CC) ratio (DC / CC) of 0.7 to 1.

2.

2. The positive electrode satisfies equation 1 below during XPS analysis. [Formula 1] 0.5≦P Ｃ / P Ｎ ≦5 P Ｃ This represents the intensity of the peak located at 284.0 ± 0.5 eV. P Ｎ The lithium secondary battery according to claim 1, wherein is the intensity of the peak present at 402.5 ± 0.5 eV.

3. The lithium secondary battery according to claim 1, wherein the electrolyte additive is contained in an amount of 0.01% to 5% by weight relative to the total weight of the electrolyte composition.

4. The lithium secondary battery according to claim 1, wherein the polymer coating has an average thickness of 5 nm to 500 μm.

5. The positive electrode active material contains an iron phosphate compound represented by the following chemical formula 1. [Chemical formula 1] LiFe ａ M １ １－ａ XO ４ In the aforementioned chemical formula 1, M １ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Co, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, X is one or more elements selected from the group consisting of P, Si, S, As, and Sb. A lithium secondary battery according to any one of claims 1 to 4, wherein a is 0 ≤ a ≤ 0.

5.

6. The lithium secondary battery according to claim 1, wherein the carbon material comprises one or more selected from the group consisting of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, carbon black, acetylene black, and Ketjenblack.

7. Silicon materials include silicon (Si), silicon carbide (SiC), and silicon oxide (SiO2). ｑ The lithium secondary battery according to claim 1, wherein it includes one or more of the following (0.8 ≤ q ≤ 2.5).

8. The lithium secondary battery according to claim 1, wherein the silicon material is contained in an amount of 1% to 20% by weight relative to the total weight of the negative electrode active material.

9. The steps include: assembling a secondary battery by injecting an electrolyte composition into a battery case into which an electrode assembly including a positive electrode, a negative electrode, and a separator membrane disposed between the positive and negative electrodes is inserted; and The assembled secondary battery is charged to a state of charge of 10% or more, and a polymer film is formed on the positive electrode active layer containing the positive electrode active material. The electrolyte composition comprises a non-aqueous organic solvent, a lithium salt, and an electrolyte additive. The aforementioned electrolyte additive is aniline, The polymer coating is a polymer of the electrolyte additive, The non-aqueous organic solvent is two or more selected from ethylene carbonate, fluoroethylene carbonate, and ethyl methyl carbonate. The negative electrode includes a negative electrode active layer on a negative electrode current collector containing a carbon material and a silicon material as negative electrode active material. A method for manufacturing a lithium secondary battery, wherein the assembled secondary battery has an initial discharge capacity (DC) to initial charge capacity (CC) ratio (DC / CC) of 0.7 to 1.

2.

10. A method for manufacturing a lithium secondary battery according to claim 9, wherein charging is performed at a temperature of 25°C to 70°C and a C-rate of 0.01C to 3.0C.

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