Lithium secondary battery with suppressed metal elution

Abstract

The present invention relates to a lithium secondary battery, which is excellent in economy and safety since it contains an iron phosphate compound of Chemical Formula 2 having an olivine structure as a positive electrode active material, and also contains an electrolyte additive of Chemical Formula 1 having a specific molecular weight in an electrolyte, which can improve the increase in internal resistance of the battery as charging and discharging proceeds, and can effectively prevent iron ions from leaching out of the positive electrode active material, thereby providing the advantage of excellent battery performance and life.

Description

[Technical Field]

[0001] The present invention relates to a lithium secondary battery in which the elution of metals, particularly transition metals, into an electrolyte is suppressed.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0034565, filed March 21, 2022, and Korean Patent Application No. 10-2023-0013766, filed February 1, 2023, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference. [Background technology]

[0003] BACKGROUND ART In recent years, secondary batteries have been widely used not only in small devices such as portable electronic devices but also in medium to large devices such as battery packs for hybrid cars and electric cars or power storage devices.

[0004] Carbon materials are primarily used as the negative electrode active material for these lithium secondary batteries, with lithium metal, sulfur compounds, silicon compounds, tin compounds, etc. also being considered. Lithium-containing cobalt oxide (LiCoO2) is primarily used as the positive electrode active material, with lithium-containing manganese oxides such as LiMnO2 with a layered crystal structure and LiMn2O4 with a spinel crystal structure, and lithium-containing nickel oxide (LiNiO2) also being considered.

[0005] LiCoO2 is currently widely used due to its excellent cycle characteristics and other physical properties, but its low safety and high cost due to limited cobalt resources as a raw material limit its mass use as a power source in fields such as electric vehicles. LiNiO2 is difficult to apply to actual mass production processes at reasonable cost due to the characteristics of its manufacturing method, and lithium manganese oxides such as LiMnO2 and LiMn2O4 have the disadvantage of poor cycle characteristics.

[0006] Therefore, recently, a method of using lithium transition metal phosphates as a positive electrode active material has been studied. Lithium transition metal phosphates are generally classified into two groups: Li, which has a Nasicon crystal structure, and Li, which has a Nasicon crystal structure. x They are divided into M2(PO4)3 and LiMPO4 with an olivine crystal structure, and are being researched as materials with superior high-temperature stability compared to the existing LiCoO2. Currently, Li3V2(PO4)3 is known as a compound with a Nasicon crystal structure, and LiFePO4 and Li(Mn,Fe)PO4 are the most widely researched compounds with an olivine crystal structure. Of the above olivine crystal structures, LiFePO4 in particular has a voltage of 3.5V and a density of 3.6g / cm compared to lithium. 3 It has a high volume density and a theoretical capacity of 170 mAh / g. It is made from iron (Fe), which has better high-temperature stability than cobalt (Co) and is inexpensive, so it has great potential for use as a positive electrode active material in lithium secondary batteries in the future.

[0007] However, due to its low electrical conductivity, LiMPO4, when used as a positive electrode active material, suffers from a significant increase in internal resistance as the battery is charged and discharged. Furthermore, during the charge and discharge process, iron (Fe) ions in LiMPO4 can dissolve into the electrolyte, which can induce side reactions in the electrolyte, resulting in a decrease in performance such as the charge and discharge capacity retention rate of the battery.

[0008] To solve this problem, conventional technologies have been developed to form a coating layer on the surface of the positive electrode active material to prevent the leaching of iron (Fe) ions, or to capture the leached iron (Fe) ions.

[0009] However, forming a coating layer on the surface of a positive electrode active material requires additional processes, such as doping the positive electrode active material, to prevent the coating layer from peeling off during charging and discharging of the battery, which makes the manufacturing process complicated and limits its economic viability. In addition, the technology for capturing leached iron ions has a problem in that, unlike other transition metal ions, iron (Fe) ions have a large effective nuclear charge, resulting in low collection efficiency.

[0010] Therefore, while the positive electrode contains LiMPO4 having an olivine structure as the positive electrode active material, metal ions (M + There is a need for the development of a technology that can more effectively suppress and / or prevent the dissolution of ) into electrolytes. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Korean Patent Publication No. 10-2021-0111077 Summary of the Invention [Problem to be solved by the invention]

[0012] Therefore, an object of the present invention is to provide a positive electrode active material containing LiMPO4 having an olivine structure, while improving the increase in internal resistance that occurs with charging and discharging the battery, and to provide a positive electrode active material containing LiMPO4 having an olivine structure, while improving the increase in internal resistance that occurs with charging and discharging the battery, while reducing the metal ions (M + The present invention aims to provide a lithium secondary battery in which the elution of . [Means for solving the problem]

[0013] In order to solve the above-mentioned 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 disposed between the positive electrode and the negative electrode; and an electrolyte composition including a lithium salt, an electrolyte additive having a unit represented by Chemical Formula 1 below, and a non-aqueous solvent, wherein the positive electrode includes a positive electrode active layer containing a positive electrode active material, the positive electrode active material including a metal oxide containing iron (Fe) atoms, and the electrolyte additive has a weight average molecular weight of less than 40,000 g / mol.

[0014] [ka]

[0015] In the above chemical formula 1, R1, R2, and R3 are each hydrogen or an alkyl group having 1 to 6 carbon atoms, R4 and R5 are each an alkylene group having 1 to 6 carbon atoms, p, q, and r are each an integer of 0 to 5, and m and n are each an integer of 10 to 200.

[0016] Specifically, in the unit represented by the above chemical formula 1, R1, R2, and R3 are each hydrogen or a methyl group, R4 and R5 are each an ethylene group or a propylene group, and p, q, and r are each an integer of 0 to 2.

[0017] In addition, the ratio of m to n in the unit represented by Chemical Formula 1 may be 1:1.01-10.

[0018] The electrolyte additive may have a weight average molecular weight of 5,000 to 30,000 g / mole.

[0019] The electrolyte additive may have a bimodal distribution of molecular weight and a polydispersity index (PDI) of 1.2 to 5.0.

[0020] The electrolyte additive may be contained in an amount of less than 5% by weight based on the weight of the entire electrolyte composition.

[0021] Meanwhile, the positive electrode includes a positive electrode active material containing iron (Fe) atoms in a positive electrode active layer, and the positive electrode active material may be a metal phosphate represented by the following Chemical Formula 2.

[0022] [Chemical formula 2] Life x M 1 1-x XO4

[0023] In the above chemical formula 2, M 1is 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, and X is one or more elements selected from the group consisting of P, Si, S, As, and Sb, and x is in the range of 0≦x≦0.5.

[0024] The negative electrode may include a negative electrode active layer containing a negative electrode active material, and the negative electrode active material may include one or more carbon materials selected from the group consisting of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, carbon black, acetylene black, and ketjen black.

[0025] The negative electrode active material may be silicon (Si), silicon carbide (SiC), or silicon oxide (SiO q , where 0.8≦q≦2.5).

[0026] In this case, the silicon material may be contained in an amount of 1 to 20% by weight based on the total weight of the negative electrode active material.

[0027] Furthermore, in one embodiment, the present invention provides a lithium secondary battery module including the lithium secondary battery according to the present invention and a module case in which the lithium secondary battery is mounted. [Effects of the Invention]

[0028] The lithium secondary battery according to the present invention is economical and safe because it contains an iron phosphate compound of Chemical Formula 2 having an olivine structure as a positive electrode active material, and contains an electrolyte additive of Chemical Formula 1 having a specific molecular weight in the electrolyte, which can improve the increase in internal resistance of the battery as charging and discharging progresses, and can effectively prevent iron ions from leaching out of the positive electrode active material, thereby providing excellent battery performance and lifespan. DETAILED DESCRIPTION OF THE INVENTION

[0029] Because the present invention is susceptible to various modifications and can have various embodiments, specific embodiments are described in detail in the detailed description.

[0030] However, this is not intended to limit the invention to any particular embodiment, but should be understood to include all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

[0031] In the present invention, the terms "comprise" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the presence or possibility of addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0032] Furthermore, in the present invention, when a layer, film, region, plate, or other portion is described as being "on" another portion, this includes not only the case where it is "directly on" the other portion, but also the case where there is another portion therebetween. Conversely, when a layer, film, region, plate, or other portion is described as being "under" the other portion, this includes not only the case where it is "directly under" the other portion, but also the case where there is another portion therebetween. Furthermore, in this application, being "located on" can include not only the case where it is located at the top, but also the case where it is located at the bottom.

[0033] The present invention will now be described in more detail.

[0034] <Lithium 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 separator disposed between the positive electrode and the negative electrode; and an electrolyte composition including a lithium salt, an electrolyte additive having a unit represented by Chemical Formula 1 below, and a non-aqueous solvent, wherein the positive electrode comprises a positive electrode active layer containing a positive electrode active material, the positive electrode active material comprising a metal oxide containing iron (Fe) atoms, and the electrolyte additive has a weight average molecular weight of less than 40,000 g / mol.

[0035] [ka]

[0036] In the above chemical formula 1, R1, R2, and R3 are each hydrogen or an alkyl group having 1 to 6 carbon atoms, R4 and R5 are each an alkylene group having 1 to 6 carbon atoms, p, q, and r are each an integer of 0 to 5, and m and n are each an integer of 10 to 200.

[0037] The lithium secondary battery according to the present invention includes an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and an electrolyte composition impregnated in the electrode assembly.

[0038] In this case, the positive electrode may include a positive electrode active layer containing a metal oxide having iron (Fe) atoms as a positive electrode active material on a positive electrode current collector, and an electrolyte additive having a specific chemical structure and molecular weight is included in the electrolyte composition to prevent and / or suppress leaching of iron (Fe) ions from the positive electrode active material during charging and discharging of the battery.

[0039] Specifically, the electrolyte additive used in the present invention may have a unit represented by the following Chemical Formula 1:

[0040] [ka]

[0041] In the above chemical formula 1, R1, R2, and R3 are each hydrogen or an alkyl group having 1 to 6 carbon atoms, R4 and R5 are each an alkylene group having 1 to 6 carbon atoms, p, q, and r are each an integer of 0 to 5, and m and n are each an integer of 10 to 200.

[0042] More specifically, in the unit represented by the above chemical formula 1, R1, R2, and R3 are each hydrogen or a methyl group, R4 and R5 are each an ethylene group or a propylene group, and p, q, and r are each an integer of 0 to 2.

[0043] As an example, the unit represented by Chemical Formula 1 may include one or more of the following <Structural Formula 1> to <Structural Formula 4>.

[0044] [ka]

[0045] [ka]

[0046] [ka]

[0047] [ka]

[0048] The unit represented by the above chemical formula 1 contains a repeating unit derived from an alkyl acrylate having 1 to 6 carbon atoms, and may have excellent solubility in organic solvents, specifically non-aqueous solvents.

[0049] Furthermore, the unit represented by Chemical Formula 1 includes a repeating unit containing a cyano group (-CN), which can induce a coordinate bond between metal ions, specifically iron (Fe) ions, leaching from the positive electrode active material and the cyano group, thereby easily capturing iron (Fe) ions and preventing an increase in the concentration of iron (Fe) ions in the electrolyte. Iron (Fe) ions generally have a large ionic size, unlike other transition metal ions, resulting in a low effective nuclear charge and low capture efficiency. However, the unit represented by Chemical Formula 1 includes multiple repeating units containing a cyano group (-CN), which can coordinate with iron (Fe) ions with a small effective nuclear charge, thereby more efficiently suppressing the leaching of iron (Fe) ions.

[0050] In the present invention, the ratio of m, the number of repeating units derived from alkyl acrylate having 1 to 6 carbon atoms, to n, the number of repeating units containing a cyano group, can be adjusted to satisfy a certain range in order to optimize the solubility of the unit represented by Chemical Formula 1 in non-aqueous solvents and the metal ion capture efficiency. Specifically, the ratio of m to n in the unit represented by Chemical Formula 1 can be 1:1.01 to 10, more specifically, 1:2 to 10, 1:2 to 8, 1:2 to 6, 1:3 to 7, 1:5 to 10, or 1:3 to 5. In Chemical Formula 1, if the ratio of n is less than 1.01, not only will the metal ion capture efficiency be significantly reduced, but the battery resistance may also increase, resulting in a reduced charge / discharge capacity. Furthermore, if the ratio of n exceeds 10, the ionic conductivity may decrease, reducing the battery safety at high temperatures.

[0051] The electrolyte additive may have a weight-average molecular weight of less than 40,000 g / mole, specifically 1,000 to 40,000 g / mole, 2,000 to 35,000 g / mole, 5,000 to 30,000 g / mole, 5,000 to 25,000 g / mole, 5,000 to 15,000 g / mole, 8,000 to 19,000 g / mole, or 10,000 to 20,000 g / mole. If the weight-average molecular weight of the electrolyte additive is 40,000 g / mole or more, the initial resistance and resistance increase rate of the battery, as well as the impregnation of the electrolyte, may increase significantly, resulting in a decrease in capacity. In addition, in this case, the electrolyte additive itself may aggregate, significantly reducing the efficiency of capturing eluted metal ions, or even if aggregation is not induced, the electrolyte additive may form a precipitate with the captured metal ions, blocking the pores of the separator and degrading the electrical characteristics of the battery. Furthermore, if the weight-average molecular weight of the electrolyte additive is less than 1,000 g / mole, the metal ion capturing ability of the electrolyte additive may not be fully realized, and the concentration of metal ions eluted in the electrolyte composition may increase significantly.

[0052] In addition, the molecular weight of the electrolyte additive may have a bimodal distribution. The bimodal molecular weight distribution may mean that the electrolyte additive contains two types of electrolyte additives that include a unit represented by Chemical Formula 1 and have different molecular weights. Here, the bimodal molecular weight distribution is measured by GPC and can be calculated using a standard polystyrene conversion method.

[0053] As one example, the electrolyte additive may include a first electrolyte additive having a unit represented by Formula 1 and a weight-average molecular weight of 12,000±500 g / mole, and a second electrolyte additive having a weight-average molecular weight of 15,000±500 g / mole. In this case, when the electrolyte additive is measured by GPC, a bimodal spectrum having one peak near the molecular weight of 12,000 and one peak near the molecular weight of 15,000 can be obtained. In this case, the second electrolyte additive may be included in an amount of 10 to 100 parts by weight per 100 parts by weight of the first electrolyte additive having a smaller weight-average molecular weight.

[0054] The present invention can effectively suppress metal ion elution from the positive electrode active material while minimizing an increase in resistance of the secondary battery by including an electrolyte additive having a bimodal molecular weight distribution.

[0055] The electrolyte additive may have a polydispersity index (PDI) of 1.2 to 5.0. The polydispersity index (PDI) is the weight average molecular weight (Mw) divided by the number average molecular weight (Mn) (Mw / Mn), and the electrolyte additive of the present invention may exhibit a polydispersity index of 1.2 to 4.5, 1.2 to 4.0, 1.2 to 3.5, 1.2 to 3.0, 1.2 to 2.5, 1.2 to 1.9, 1.5 to 2.5, 1.8 to 3.1, or 1.6 to 2.2.

[0056] As an example, the electrolyte additive may exhibit a polydispersity index (PDI) of 1.8 to 2.1.

[0057] In another example, the electrolyte additive includes a first electrolyte additive having a unit represented by Chemical Formula 1 and a weight-average molecular weight of 12,000±500 g / mole, and a second electrolyte additive having a weight-average molecular weight of 15,000±500 g / mole, and when the molecular weights exhibit a bibar distribution, the first electrolyte additive and the second electrolyte additive may each exhibit a polydispersity index of 1.6 to 2.0.

[0058] Furthermore, the electrolyte additive may be contained in an amount of less than 5 wt % relative to the total weight of the electrolyte composition, specifically 0.05 to 5 wt %, 0.05 to 4 wt %, 0.05 to 3 wt %, 0.1 to 2.5 wt %, 0.1 to 2.2 wt %, 0.2 to 1.6 wt %, 0.9 to 1.9 wt %, 1.6 to 2.3 wt %, or 0.1 to 0.8 wt %.

[0059] By adjusting the content of the electrolyte additive within the above range, the present invention can prevent an increase in the internal resistance of the battery and a decrease in ionic conductivity caused by an excessive amount of electrolyte additive, while reducing side reactions between the electrolyte composition and the positive electrode active layer, and can prevent a decrease in the metal ion capturing ability caused by an extremely small amount of electrolyte additive.

[0060] Meanwhile, the electrolyte composition includes the above-mentioned electrolyte additive, a lithium salt, and a non-aqueous solvent.

[0061] In this case, the lithium salt may be any one used in the art for non-aqueous electrolytes without any particular limitation. Specifically, the lithium salt may be LiCl, LiBr, LiI, LiClO4, LiBF4, LiB 10 Cl 10 , LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, and (FSO2)2NLi.

[0062] The concentrations of these lithium salts are not particularly limited, but a preferred lower limit of the concentration range is 0.5 mol / L or more, specifically 0.7 mol / L or more, more specifically 0.9 mol / L or more, and a preferred upper limit of the concentration range is 2.5 mol / L or less, specifically 2.0 mol / L or less, more specifically 1.5 mol / L or less. If the lithium salt concentration is below 0.5 mol / L, the ionic conductivity decreases, which may result in a decrease in the cycle characteristics and output characteristics of the nonaqueous electrolyte battery. If the lithium salt concentration exceeds 2.5 mol / L, the viscosity of the electrolyte for the nonaqueous electrolyte battery increases, which may also decrease the ionic conductivity, which may result in a decrease in the cycle characteristics and output characteristics of the nonaqueous electrolyte battery.

[0063] Furthermore, when a large amount of lithium salt is dissolved in a non-aqueous organic solvent at once, the liquid temperature may rise due to the heat of dissolution of the lithium salt. If the temperature of the non-aqueous organic solvent rises significantly due to the heat of dissolution of the lithium salt, decomposition of the fluorine-containing lithium salt may be accelerated, resulting in the production of hydrogen fluoride (HF). Hydrogen fluoride (HF) is undesirable because it can cause deterioration of battery performance. Therefore, the temperature at which the lithium salt is dissolved in the non-aqueous organic solvent is not particularly limited, but may be adjusted to −20 to 80° C., specifically, 0 to 60° C.

[0064] In addition, the non-aqueous organic solvent used in the electrolyte composition may be any organic solvent used in non-aqueous electrolytes in the art without any particular limitations. Specifically, examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidinone, ethylene carbonate (EC), propylene carbonate (PC), 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, phosphate triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0065] The non-aqueous organic solvent used in the present invention may be one type alone or two or more types mixed in any combination and ratio 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 viewpoints of their electrochemical stability against oxidation-reduction and chemical stability against heat and reactions with solutes.

[0066] Furthermore, the electrolyte composition may further contain an electrolyte auxiliary additive as needed to prevent the non-aqueous electrolyte from decomposing under high-power conditions, which may lead to the collapse of the negative electrode, and to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effect of suppressing battery expansion at high temperatures.

[0067] Specifically, the electrolyte auxiliary additive may include one or more of a cyclic carbonate compound, a sultone compound, and a sulfate compound, preferably a combination of these compounds, which may form a more uniform SEI film on the negative electrode surface during the initial activation process of the battery, improve high-temperature stability, and suppress gas generation due to electrolyte decomposition.

[0068] In this case, the cyclic carbonate compound may include one or more of vinylene carbonate (VC), vinylethylene carbonate (VEC), and fluoroethylene carbonate (FEC); the sultone compound may include one or more of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone; and the sulfate compound may include one or more of ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

[0069] The electrolyte auxiliary additive may be included in an amount of 0.01 to 10 wt %, more specifically 0.05 to 5 wt %, or 1.5 to 3 wt %, based on the total weight of the electrolyte composition. By adjusting the content of the electrolyte auxiliary additive within this range, the present invention can prevent the additive from remaining precipitated at room temperature due to an excessive amount of the auxiliary additive, which would reduce the resistance characteristics of the battery, while also preventing the auxiliary additive from being added in an extremely small amount, which would prevent the effect of improving high-temperature life characteristics from being fully realized.

[0070] The positive electrode includes a positive electrode active layer containing a positive electrode active material having iron (Fe) atoms on a positive electrode current collector. Specifically, the positive electrode includes a positive electrode active layer manufactured by applying a slurry containing the positive electrode active material on the positive electrode current collector, drying it, and pressing it, and may optionally further include a conductive material, a binder, and other additives as needed.

[0071] The positive electrode active material may include an iron phosphate compound having an olivine crystal structure, which is a material capable of electrochemically reacting on a positive electrode current collector and has excellent stability. For example, the positive electrode active material may include one or more iron phosphate compounds represented by Formula 2, which are capable of reversibly intercalating and deintercalating lithium ions.

[0072] [Chemical formula 2] Life x M 1 1-x XO4

[0073] In the above chemical formula 2, 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, and X is one or more elements selected from the group consisting of P, Si, S, As, and Sb, and x is in the range of 0≦x≦0.5.

[0074] As an example, the iron phosphate compound represented by the above chemical formula 2 is LiFePO4, LiFe 0.5 Mn 0.5 It may contain PO4 etc.

[0075] The content of the positive electrode active material may be 85 to 95 parts by weight, 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, per 100 parts by weight of the positive electrode active layer.

[0076] The positive electrode active layer may further contain a binder, a conductive material, and other additives in addition to the positive electrode active material.

[0077] In this case, the conductive material may be used to improve the performance of the positive electrode, such as electrical conductivity, and may include one or more materials selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon nanotubes, graphene, and carbon fibers. For example, the conductive material may include acetylene black.

[0078] The conductive material may be present in an amount of 0.5 to 5 parts by weight relative to 100 parts by weight of the positive electrode active layer, specifically 0.5 to 4 parts by weight; 0.5 to 3 parts by weight; 0.5 to 1 part by weight; 0.5 to 2 parts by weight; 1 to 3 parts by weight; 2 to 4 parts by weight; 1.5 to 3.5 parts by weight; 0.5 to 1.5 parts by weight; or 1 to 2 parts by weight.

[0079] The binder may include one or more resins selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof. As one example, the binder may include polyvinylidene fluoride.

[0080] The binder may be contained in an amount of 1 to 10 parts by weight, specifically 2 to 8 parts by weight, relative to 100 parts by weight of the entire positive electrode active layer; or the conductive material may be contained in an amount of 1 to 5 parts by weight.

[0081] Furthermore, the average thickness of the positive electrode active layer is not particularly limited, but may be specifically 50 μm to 300 μm, more specifically 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.

[0082] The positive electrode current collector may be made of a material that has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, etc. may be used. In the case of aluminum or stainless steel, it may be surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector may also be formed with fine irregularities on its surface to enhance adhesion of the positive electrode active material, and may be in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. The average thickness of the current collector may be preferably 3 to 500 μm, taking into consideration the conductivity and total thickness of the positive electrode to be manufactured.

[0083] Furthermore, the negative electrode may be prepared by coating a negative electrode active material on a negative electrode current collector, followed by drying and pressing to form a negative electrode active layer. If necessary, the negative electrode may further include a conductive material, an organic binder polymer, an additive, and the like, similar to those of the positive electrode.

[0084] Here, the negative electrode active material may include at least one selected from the group consisting of lithium metal, nickel metal, copper metal, SUS metal, a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of these metals with lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0085] As an example, the negative electrode active material may include one or more carbon materials selected from the group consisting of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, carbon black, acetylene black, and ketjen black.

[0086] In addition, the negative electrode active material may further contain a silicon material together with the carbon material to further increase the charge / discharge capacity of the battery. The silicon material refers to a material containing silicon atoms as a main component, and such silicon material may contain silicon (Si), silicon carbide (SiC), silicon monoxide (SiO), or silicon dioxide (SiO2) alone or in combination. When silicon monoxide (SiO) and silicon dioxide (SiO2) are uniformly mixed or composited as the silicon (Si)-containing material and contained in the negative electrode active layer, they are referred to as silicon oxide (SiO q , where 0.8≦q≦2.5).

[0087] The silicon material may be contained in an amount of 1 to 20 wt % based on the total weight of the negative electrode active material, specifically 3 to 10 wt %, 8 to 15 wt %, 13 to 18 wt %, or 2 to 8 wt %. By adjusting the content of the silicon material within the above range, the present invention can maximize the energy density of the battery.

[0088] The negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery, and may be made of, for example, copper, stainless steel, nickel, titanium, calcined carbon, etc. In the case of copper or stainless steel, it may also be surface-treated with carbon, nickel, titanium, silver, etc. The average thickness of the negative electrode current collector is preferably 1 to 500 μm, taking into account the conductivity and total thickness of the negative electrode to be manufactured.

[0089] Furthermore, the shape of the lithium secondary battery according to the present invention is not particularly limited, and may be, for example, cylindrical, prismatic, pouch-type, coin-type, etc. According to one embodiment of the present invention, the lithium metal secondary battery may be a cylindrical lithium metal secondary battery, a prismatic lithium metal secondary battery, a pouch-type lithium metal secondary battery, or a coin-type lithium metal secondary battery, and particularly may be a pouch-type lithium metal secondary battery.

[0090] The lithium secondary battery according to the present invention has the above-described configuration, which makes it excellent in terms of economy and safety, and can more effectively capture metal ions derived from the positive electrode active layer and significantly reduce the concentration of metal ions eluted into the electrolyte composition. This makes it possible to improve the increase in battery resistance and side reactions, and the decrease in performance, which are caused by eluted metal ions, even under high-temperature conditions.

[0091] <Lithium secondary battery module> In one embodiment, the present invention provides a lithium secondary battery module including the lithium secondary battery according to the present invention described above and a module case in which the lithium secondary battery is mounted.

[0092] A lithium secondary battery module according to the present invention is a battery module including a plurality of unit cells and a module case that houses the plurality of unit cells, wherein the unit cells include the lithium secondary battery according to the present invention.

[0093] The lithium secondary battery module includes a plurality of the above-described lithium secondary batteries of the present invention as unit cells, and has the advantages of exhibiting low initial resistance and resistance increase rate, high voltage retention rate even under high temperature conditions, and an extremely low concentration of iron (Fe) ions dissolved in the electrolyte composition.

[0094] Meanwhile, the present invention provides a battery pack including the battery module, and a device including the battery pack as a power source.

[0095] Specific examples of the device include, but are not limited to, power tools powered by battery-powered motors; electric vehicles including electric vehicles (EVs); hybrid electric vehicles (HEVs); and plug-in hybrid electric vehicles (PHEVs); electric bicycles (E-bikes); electric two-wheeled vehicles including electric scooters; electric golf carts; and power storage systems.

[0096] The present invention will be described in more detail below with reference to examples and experimental examples.

[0097] However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0098] <Example> a) Preparation of electrolyte composition LiPF6 (lithium salt) was dissolved at a concentration of 1.0M in a solvent prepared by mixing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) in a volume ratio of 3:4:3, and electrolyte additives were weighed and dissolved according to the types and contents shown in Table 1 below. Then, electrolyte auxiliary additives, vinylene carbonate (VC), 1,3-propane sultone (PS), and ethylene sulfate (Esa), were added at 2.5 wt %, 0.5 wt %, and 0.7 wt %, respectively, to prepare a nonaqueous electrolyte composition.

[0099] In Example 5, two types of electrolyte additives containing the unit represented by Chemical Formula 1 and having weight average molecular weights of 12,000 g / mole and 15,000 g / mole, respectively, were mixed and used.

[0100] In addition, gel permeation chromatography (GPC) was used to measure the weight-average molecular weight and PDI of the electrolyte additive, and the molecular weight distribution was analyzed from the resulting spectrum. For GPC, the Alliance 4 instrument was first stabilized. Once stabilized, a standard sample and a sample were injected into the instrument to obtain a chromatogram. The molecular weight was then calculated from the results obtained by the analytical method. (System: Alliance 4, Column: Agilent PL mixed B, Eluent: THF, Flow rate: 0.1 mL / min, Temperature: 40°C, Injection: 100 μL) The measurement results are shown in Table 1.

[0101] [Table 1]

[0102] b) Manufacture of lithium secondary batteries LiFePO4 was prepared as a positive electrode active material, and the prepared active material, carbon black as a conductive agent, and polyvinylidene fluoride as a binder were mixed in N-methylpyrrolidone (NMP) in a weight ratio of 94:3:3 to form a slurry. The slurry was cast on an aluminum sheet, dried in a vacuum oven at 120°C, and then rolled to produce a positive electrode.

[0103] Separately, natural graphite was prepared as a negative electrode active material, and 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. The slurry was cast on a copper sheet, dried in a vacuum oven at 130°C, and then rolled to produce a negative electrode.

[0104] The obtained positive and negative electrodes were interposed between 18 μm polypropylene separators, and inserted into a case. Then, the electrolyte solutions prepared in Examples 1 to 6 and Comparative Examples 1 to 3 were injected to prepare pouch-type lithium secondary batteries.

[0105] <Experimental Example> In order to evaluate the performance of the lithium secondary battery according to the present invention, the following experiment was carried out.

[0106] a) Initial resistance analysis The lithium secondary batteries prepared in the Examples and Comparative Examples were activated by charging at a current of 200 mA (0.1 C). The DC resistance of each activated lithium secondary battery was then measured, and the DC resistance deviation rate of each lithium secondary battery was calculated as the initial resistance based on the DC resistance value of the lithium secondary battery of Comparative Example 1, which contained the monomolecular electrolyte additive HTCN. The results are shown in Table 2 below.

[0107] B) Analysis of resistance increase rate and voltage maintenance rate after high-temperature cycling The lithium secondary batteries manufactured in each of the examples and comparative examples were activated by charging at a current of 200 mA (0.1 C). The activated lithium secondary batteries were then charged and discharged 300 times at 45°C with a charge / discharge current density of 0.33 C / 0.33 C, a charge cut-off voltage of 3.6 V, and a discharge cut-off voltage of 2.5 V. The DC resistance and charge / discharge capacity of the lithium secondary batteries after 300 charge / discharge cycles were measured, and the resistance increase rate and capacity retention rate were calculated from the measured results using the resistance and capacity at the initial charge / discharge as references. The results are shown in Table 2 below.

[0108] c) Analysis of metal ion elution amount after high-temperature cycle Since metals dissolved into the electrolyte are reduced on the surface of the negative electrode active material layer and induce side reactions, the content of metal ions remaining on the negative electrode surface was measured for lithium secondary batteries that had previously been analyzed for resistance increase rate and voltage retention rate after high-temperature cycling.

[0109] Specifically, the lithium secondary batteries of the Examples and Comparative Examples whose resistance increase rate and voltage retention rate were analyzed were disassembled to separate the negative electrodes, and the surface of the active material layer contained in the negative electrodes was scraped off to obtain active material layer powder. The active material layer powder was then subjected to inductively coupled plasma analysis (ICP) to measure the iron (Fe) ion content remaining on the negative electrode surface in ppm. The results are shown in Table 2 below.

[0110] [Table 2]

[0111] As shown in Table 2, the lithium secondary battery according to the present invention not only has a low internal resistance, but also has a low resistance increase rate, a high capacity retention rate, and a low metal elution rate even after high-temperature cycling.

[0112] These results demonstrate that the lithium secondary battery according to the present invention is economical and safe because it contains the iron phosphate compound of Formula 2 having an olivine structure as the positive electrode active material, and also has low battery resistance and effectively prevents metal ions from leaching out of the irreversible additive because it contains the electrolyte additive of Formula 1 having a specific molecular weight in the electrolyte, resulting in excellent battery performance and lifespan.

[0113] Although the present invention has been described above with reference to preferred embodiments, it will be understood by those skilled in the art or those with ordinary knowledge in the art that various modifications and changes can be made to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims below.

[0114] Therefore, the technical scope of the present invention should not be limited to the content described in the Summary of the Invention of the specification, but should be defined by the claims.

Claims

1. an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode; an electrolyte composition containing a lithium salt, an electrolyte additive having a unit represented by the following chemical formula 1, and a non-aqueous solvent, the positive electrode includes a positive electrode active layer containing a positive electrode active material, and the positive electrode active material includes a metal oxide containing iron (Fe) atoms; The electrolyte additive has a weight average molecular weight of less than 40,000 g / mol; 【Chemical 1】 In the above Chemical Formula 1, R １ , R ２ and R ３ are each hydrogen or an alkyl group having 1 to 6 carbon atoms, R ４ and R ５ are each an alkylene group having 1 to 6 carbon atoms, p, q, and r are each an integer of 0 to 5; m and n are each an integer of 10 to 200, The positive electrode active material containing iron (Fe) atoms is a metal phosphate represented by the following chemical formula 2: [Chemical formula 2] LiFe x M 1 1-x XO 4 In the above Chemical Formula 2, 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 at least one element selected from the group consisting of P, Si, S, As, and Sb; A lithium secondary battery, wherein x is 0≦x≦0.

5.

2. The unit represented by chemical formula 1 is R １ , R ２ and R ３ are each hydrogen or a methyl group, R ４ and R ５ are each an ethylene group or a propylene group, 2. The lithium secondary battery according to claim 1, wherein p, q, and r are each an integer of 0 to 2.

3. 2. The lithium secondary battery according to claim 1, wherein the ratio of m to n in the unit represented by Chemical Formula 1 is 1:1.01-10.

4. 2. The lithium secondary battery according to claim 1, wherein the electrolyte additive has a weight average molecular weight of 5,000 to 30,000 g / mole.

5. 2. The lithium secondary battery according to claim 1, wherein the electrolyte additive has a bimodal molecular weight distribution.

6. 2. The lithium secondary battery according to claim 1, wherein the electrolyte additive has a polydispersity index of 1.2 to 5.

0.

7. 2. The lithium secondary battery according to claim 1, wherein the electrolyte additive is contained in an amount of less than 5% by weight based on the weight of the entire electrolyte composition.

8. The lithium secondary battery according to claim 1, wherein the metal phosphate represented by chemical formula 2 is LiFePO 4 or LiFe 0.5 Mn 0.5 PO 4.

9. the negative electrode includes a negative electrode active layer including a negative electrode active material, 2. The lithium secondary battery according to claim 1, wherein the negative electrode active material comprises one or more carbon materials selected from the group consisting of natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, carbon black, acetylene black, and ketjen black.

10. The negative electrode active material is silicon (Si), silicon carbide (SiC), and silicon oxide (SiO ｑ 10. The lithium secondary battery according to claim 9, further comprising one or more silicon materials selected from the group consisting of q and q, where 0.8≦q≦2.

5.

11. The lithium secondary battery according to claim 10, wherein the silicon material is contained in an amount of 1 to 20 wt % based on the total weight of the negative electrode active material.

12. The lithium secondary battery according to claim 1; a module case in which the lithium secondary battery is mounted.

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