Lithium-ion secondary battery having improved cycle life characteristic

Abstract

The present invention relates to a lithium-ion secondary battery comprising at least one unit laminate formed by interposing a separator between a positive electrode and a negative electrode, wherein the lithium-ion secondary battery comprises a coating layer formed on the edge of at least one of a positive current collector or a negative current collector located at the outermost edge. According to the present invention, the flow of electrons can be guided to the edge of the cell, and accordingly, by inducing a uniform flow of current, localized degradation can be prevented, thereby significantly improving the lifespan of the cell.

Description

Technology Field

[0001] The present invention relates to a lithium-ion secondary battery, and more specifically, to a lithium-ion secondary battery that prevents degradation of specific parts by controlling the current flow in a battery cell, thereby improving lifespan characteristics. Background Technology

[0002] With the development of portable electronic devices such as mobile phones and laptop computers, the demand for secondary batteries as an energy source is rapidly increasing. Recently, the use of secondary batteries as power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs) has become a reality. Accordingly, extensive research is being conducted on secondary batteries capable of meeting various requirements, and in particular, there is a rising trend in demand for lithium secondary batteries that possess high energy density, high discharge voltage, and high output.

[0003] Lithium-ion batteries used in electric vehicles and the like must possess high energy density and the ability to deliver high output in a short period of time. Furthermore, since they must be able to be used for more than 10 years under harsh conditions involving repeated charging and discharging with high current in a short period, significantly superior output characteristics and long-term lifespan characteristics compared to existing small lithium-ion batteries are inevitably required.

[0004] Lithium-ion batteries are broadly classified by their external shape into cylindrical, prismatic, and pouch types, and also by the form of the electrolyte into lithium-ion, lithium-ion polymer, and lithium-polymer batteries. Due to the recent trend toward the miniaturization of mobile devices, there is increasing demand for thin prismatic and pouch-type batteries; in particular, there is high interest in pouch-type batteries, which are easy to shape, have low manufacturing costs, and are lightweight.

[0005] Generally, lithium secondary battery cells have a rectangular shape, with a positive electrode and a negative electrode placed at both ends. In such cells, electron movement tends to concentrate towards the center of the cell, causing localized performance degradation in the center, which in turn leads to a problem of overall cell performance degradation. The problem to be solved

[0006] The present invention was devised to solve the above-mentioned problems and provides a lithium-ion secondary battery that exhibits a uniform current flow throughout by inducing the flow of electrons to the edges of the cell. means of solving the problem

[0007] According to one aspect of the present invention, a lithium-ion secondary battery comprising at least one unit laminate formed by interposing a separator between a positive electrode and a negative electrode, wherein the lithium-ion secondary battery comprises a coating layer formed on the edge of at least one of a positive current collector or a negative current collector located at the outermost edge.

[0008] The width of the coating layer formed on the anode current collector may be 25% or less of the width of the anode current collector.

[0009] The thickness of the coating layer formed on the anode current collector may be 25% or less of the thickness of the anode current collector.

[0010] The coating layer formed on the anode current collector may be made of a material that is the same as or different from the material of the anode current collector.

[0011] The coating layer formed on the anode current collector may be one or more selected from aluminum, gold, silver CNTs, and graphene.

[0012] The width of the coating layer formed on the above-mentioned negative current collector may be 25% or less of the width of the negative current collector.

[0013] The thickness of the coating layer formed on the above-mentioned cathode current collector may be 25% or less of the thickness of the cathode current collector.

[0014] The coating layer formed on the above-mentioned cathode current collector may be made of a material that is the same as or different from the material of the cathode current collector.

[0015] The coating layer formed on the above-mentioned cathode current collector may be one or more selected from copper, gold, silver CNTs, and graphene.

[0016] The above coating layer can be formed by any one selected from chemical vapor deposition, spray coating, roll-to-roll coating, and the like. Effects of the invention

[0017] According to the present invention, the flow of electrons can be guided to the edge of the cell, and accordingly, by inducing a uniform flow of current, local degradation can be prevented, thereby significantly improving the lifespan of the cell. Brief explanation of the drawing

[0018] Figure 1 schematically shows a lithium-ion secondary battery including a rectangular cell. Figure 2 schematically illustrates the flow of electrons in a lithium-ion secondary battery containing a rectangular cell. FIG. 3 schematically shows a lithium-ion secondary battery having a coating layer formed thereon according to one embodiment of the present invention. Specific details for implementing the invention

[0019] Preferred embodiments of the present invention will be described below with reference to various examples. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0020] The present invention relates to a lithium-ion battery, and more specifically, to a lithium-ion battery that prevents degradation of specific parts by controlling the current flow in a battery cell, thereby increasing the lifespan.

[0021] In the present invention, the lithium-ion battery may be formed by including at least one unit laminate formed by interposing a separator between a negative electrode and a positive electrode. The positive electrode, negative electrode, and separator are not particularly limited; for example, the negative electrode may be formed by coating a negative electrode active material onto a negative electrode current collector, and for example, a copper foil may be used as the negative electrode current collector. As the negative electrode active material, for example, one or more carbon-based materials selected from crystalline artificial graphite, crystalline natural graphite, amorphous hard carbon, low-crystalline soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon, Si-based materials, LixFe2O3 (0≤x≤1), Li x WO2(0≤x≤1), Sn x Me 1-x Me y O z (Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group 1, 2, and 3 elements of the periodic table, halogens; 0 <x≤1; 1≤y≤3; 1≤z≤8) 등의 금속 복합 산화물, 리튬 금속, 리튬 합금, 규소계 합금, 주석계 합금, SiO, SiO2, SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5등의 금속 산화물, 폴리아세틸렌 등의 도전성 고분자, Li-Co-Ni 계 재료, 티타늄 산화물, 리튬 티타늄 산화물 등을 사용할 수 있다.

[0022] The positive electrode can be formed by coating a positive active material onto a positive current collector, and, for example, an aluminum foil can be used as the positive current collector. As the positive active material, for example, a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals, or a compound with the chemical formula Li 1+x Mn 2-xLithium manganese oxides such as O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc., lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiFe3O4, V2O5, Cu2V2O7, etc., chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented by O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x = 0.01 ~ 0.3); chemical formula LiMn 2-x M x Lithium manganese composite oxide, LiNi represented as O2 (where M = Co, Ni, Fe, Cr, Zn, or Ta, and x = 0.01 ~ 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu, or Zn). x Mn 2-x Lithium manganese complex oxide with a spinel structure represented by O4, LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions, disulfide compounds, Fe2(MoO4) 3, Lithium nickel manganese cobalt complex oxide (NMC), etc., may be used without restriction and are not limited to these alone.

[0023] The separator may be in the form of a polyolefin series such as polyethylene and polypropylene, a polyester series, a porous polymer membrane such as natural polymers such as pulp, cellulose, and cork, or a nonwoven fabric, and may have a single-layer or multi-layer structure. In addition, a porous multilayer membrane of a thermoplastic polyolefin polymer such as polyethylene or polypropylene (Tm 100 to 150 ℃) or a porous membrane of a high molecular weight thermoplastic heat-resistant polyolefin polymer (Tm 150 to 200 ℃) may be used.

[0024] FIG. 1 schematically shows a lithium-ion secondary battery including a rectangular cell, and FIG. 2 schematically shows the flow of electrons in the lithium-ion secondary battery including the rectangular cell.

[0025] Referring to FIGS. 1 and 2, electrons move from the negative electrode tab to the positive electrode tab along the length of the cell. These electrons react with lithium ions to undergo oxidation / reduction, thereby enabling the charging / discharging of the battery. At this time, since electrons move toward the side with lower resistance, regions where electron movement is restricted or insufficient occur, centered around each corner of the cell (region a in FIG. 1), as indicated in FIGS. 1 and 2. This is because, as electrons are supplied from the electrode tabs, the resistance increases as the length of the conductor increases, and electrons are supplied first to the region with lower resistance. Consequently, the flow of electrons becomes concentrated towards the center, which inevitably leads to localized degradation, resulting in a deterioration of the overall performance of the battery.

[0026] To solve these problems, one could consider increasing the size of the tab. However, increasing the size of the tab is not desirable for ensuring the sealing characteristics and durability of the cell. Accordingly, the inventors discovered that these problems can be solved by forming a coating layer capable of inducing the movement of electrons from the tab to the edge of the current collector through which electrons are transferred, and thus completed the present invention.

[0027] According to one aspect of the present invention, a lithium-ion secondary battery comprising at least one unit laminate formed by interposing a separator between a positive electrode and a negative electrode, wherein the lithium-ion secondary battery comprises a coating layer formed on the edge of at least one of a positive current collector or a negative current collector located at the outermost edge.

[0028] FIG. 3 schematically illustrates a lithium-ion secondary battery according to an embodiment of the present invention. In the present invention, a coating layer is formed along the edge of the positive current collector or negative current collector located at the outermost edge. Since electrons are transferred from the tab to the current collector, such a coating layer can serve as a path for electrons to move to each corner of the cell. More specifically, electrons have the characteristic of moving along a path with low resistance. Since the path of movement in the center of the electrode is the minimum distance electrons can travel, it becomes the path with the lowest resistance. However, such concentration of electrons toward the center of the electrode eventually causes the potential to decrease or increase, changing the direction toward higher activation energy; consequently, the electrons change their flow toward a direction with lower activation energy. This flow of electrons is continuously used from the center of the electrode, accelerating degradation in the center of the electrode. Therefore, by making the resistance in the direction of the electrode center (which is short and thin) and the direction of the electrode edge (which is long and thick) similar at the point where they initially possess the same activation energy, the concentration of electrons toward the center of the electrode can be prevented.

[0029] The coating layer coated on the anode current collector may be made of the same material as the anode current collector or may be made of a different material. For example, aluminum may be used as the anode current collector, and in this case, the material forming the coating layer may also be aluminum. In addition, the coating layer may be formed with a material that has excellent conductivity and is electrochemically stable, and for example, the coating layer may be formed using one or more selected from gold, silver, carbon nanotubes (CNT), and graphene.

[0030] The width of the coating layer formed on the anode current collector is preferably 25% or less of the width of the anode current collector, and preferably 10% to 25% of the width of the anode current collector. For example, if the current collector has a width of 100 mm, it can be formed with a width of 25 mm or less, or with a width of 10 μm to 25 mm. If the width of the coating layer exceeds 25% of the width of the anode current collector, the electron transfer path to the edge may be activated, resulting in the edge being heated first; on the other hand, if the width is excessively narrow, the effect of inducing electron transfer to the edge may be insufficient, so it is more preferable to form it with a width of 10% to 25% of the width of the anode current collector.

[0031] Meanwhile, the thickness of the coating layer formed on the anode current collector is preferably 25% or less of the anode current collector, and preferably 10 to 25% of the thickness of the anode current collector. If the thickness of the coating layer exceeds 25% of the thickness of the anode current collector, the electron transfer path to the edge may be activated, resulting in the edge being heated first; on the other hand, if the width is excessively narrow, the effect of inducing electron transfer to the edge may be insufficient, so it is more preferable to form it at 10 to 25% of the width of the anode current collector.

[0032] The coating layer coated on the current collector of the cathode may also be made of the same material as the current collector of the cathode or may be made of a different material. For example, copper may be used as the current collector of the cathode, and in this case, the material forming the coating layer may also be copper. Meanwhile, the coating layer may be formed with a material that has excellent conductivity and is electrochemically stable, and for example, one or more selected from gold, silver CNTs, and graphene may be used.

[0033] The width of the coating layer formed on the above-mentioned cathode current collector is preferably 25% or less of the width of the cathode current collector, and preferably 10% to 25% of the width of the cathode current collector. For example, if the current collector has a width of 100 mm, it can be formed with a width of 25 mm or less, or with a width of 10 μm to 25 mm. If the width of the coating layer exceeds 25% of the width of the cathode current collector, the electron movement path toward the edge may be activated, resulting in the edge being heated first; on the other hand, if the width is excessively narrow, the effect of inducing electron movement toward the edge may be insufficient, so it is more preferable to form it with a width of 10% to 25% of the width of the cathode current collector.

[0034] In addition, the thickness of the coating layer formed on the negative current collector is preferably 25% or less of the thickness of the negative current collector and 10 to 25% of the thickness of the positive current collector. If the thickness of the coating layer exceeds 25% of the thickness of the positive current collector, the electron movement path to the edge may be activated, resulting in the edge being heated first; on the other hand, if the width is excessively narrow, the effect of inducing electron movement to the edge may be insufficient, so it is more preferable to form it at 10 to 25% of the width of the negative current collector.

[0035] In the present invention, the method for forming the coating layer may be performed by any one selected from, for example, chemical vapor deposition (PVD), spray coating, and roll-to-roll coating. However, in order to increase the uniformity and purity of the coating layer, it is more preferable to form the coating layer by the chemical vapor deposition method.

[0036] As described above, according to the present invention, a coating layer is formed on the edge of the current collector to guide the flow of electrons to the edge of the cell, and accordingly, by inducing a uniform flow of current, local degradation can be prevented, thereby significantly improving the lifespan of the cell.

[0037] Examples

[0038] The present invention will be explained in more detail below with reference to examples. The following examples are intended to explain the present invention in more detail and do not limit the present invention.

[0039] A copper foil with a width of 300 mm and a thickness of 10 μm was used as the cathode current collector, and a copper foil with a width of 290 mm and a thickness of 10 μm was used as the anode current collector. Along the edges of the cathode and anode current collectors, a copper coating layer was formed on the cathode and an aluminum coating layer was formed on the anode with the thickness and width shown in Table 1 using chemical vapor deposition.

[0041] Cathode foil thickness Cathode coating layer width Cathode coating layer thickness anode foil thickness anode coating layer width Anode coating layer thickness Example 1 10 μm x x 20 μm 15mm 10 μm Example 2 10 μm 15mm 5 μm 20 μm x x Example 3 10 μm 15mm 5 μm 20 μm 15mm 10 μm Comparative Example 1 10 μm x x 20 μm x x Comparative Example 2 10 μm 5 μm 5 μm 20 μm 5 μm 10 μm Comparative Example 3 10 μm 40 mm 5 μm 20 μm 40 mm 10 μm Comparative Example 4 10 μm 15mm 1 μm 20 μm 15mm 1 μm

[0042] The initial capacity and resistance of secondary batteries manufactured using the positive and negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 5, as well as the cycle life and resistance according to the depth of discharge (DOD), were measured and are shown in Table 2.

[0044] fresh capacity (1C, discharge) fresh resistor(10 seconds, SOC 50%, discharge) DOD 0-100 Life after 1,000 cycles Resistance after DOD 0-100 / 1000 cycles Lifespan according to DOD 0-50 Example 1 98 Ah 2.0 mΩ 87% 145% 4000 cycle Example 2 100 Ah 2.0 mΩ 88% 140% 4500 cycle Example 3 98 Ah 2.0 mΩ 90% 130% 5000 cycle Comparative Example 1 100 Ah 2.0 mΩ 85% 160% 2500 cycle Comparative Example 2 100 Ah 2.0 mΩ 85% 160% 2500 cycle Comparative Example 3 97 Ah 2.0 mΩ 86% 150% 3000 cycle Comparative Example 4 100 Ah 2.0 mΩ 85% 160% 2500 cycle

[0045] Referring to Table 2, Comparative Example 1, in which no coating layer was formed along the edges of the cathode or anode, showed a significantly reduced lifespan of 2500 cycles according to DOD 0-50, and accordingly, it can be confirmed that rapid degradation occurred.

[0046] Comparative Example 2, in which the width of the coating layer was formed too thin, and Comparative Example 4, in which the thickness was formed too thin, had a negligible effect in inducing electron transfer to the edges, and thus showed experimental results similar to Comparative Example 1.

[0047] Meanwhile, Comparative Example 3, in which the width of the coating layer is formed too thickly, also exhibits lifespan and resistance characteristics similar to Comparative Example 1. Through this, it can be confirmed that when the width of the current collector exceeds 25%, deterioration at the edge of the electrode current collector is actually promoted, which does not contribute to improving the lifespan and resistance characteristics of the battery.

[0048] On the other hand, it can be confirmed that the lifespan characteristics of Examples 1 to 3 are improved compared to the comparative example by forming a coating layer to homogenize the electron transport path and prevent localized deterioration.

[0049] Although embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and it will be obvious to those skilled in the art that various modifications and variations are possible within the scope of the technical concept of the present invention as described in the claims. Explanation of the symbols

[0050] 10: Positive current collector or negative current collector 20: Positive tab or negative tab 30: Coating layer 40, 50, 60: Electronic movement path 100: Secondary battery

Claims

Claim 1 A lithium-ion secondary battery comprising at least one unit laminate formed by interposing a separator between a positive electrode formed by coating a positive active material on a positive current collector and a negative electrode formed by coating a negative active material on a negative current collector, wherein the lithium-ion secondary battery comprises a conductive coating layer formed along the edges of the positive current collector and the negative current collector located at the outermost edge, wherein the conductive coating layer formed on the positive current collector comprises one or more selected from aluminum, gold, silver, carbon nanotubes (CNT), and graphene, and the conductive coating layer formed on the negative current collector comprises one or more selected from copper, gold, silver, carbon nanotubes (CNT), and graphene, wherein the width of the coating layer is 10 to 25% of the width of the positive current collector or the negative current collector, and the thickness of the coating layer is 10 to 25% of the thickness of the positive current collector or the negative current collector. Claim 2 delete Claim 3 delete Claim 4 A lithium-ion secondary battery according to claim 1, wherein the coating layer formed on the positive current collector is made of a material identical to or different from the material of the positive current collector. Claim 5 delete Claim 6 delete Claim 7 delete Claim 8 A lithium-ion secondary battery according to claim 1, characterized in that the coating layer formed on the negative electrode current collector is made of a material that is the same as or different from the material of the negative electrode current collector. Claim 9 delete Claim 10 A lithium-ion secondary battery according to claim 1, wherein the coating layer is formed by any one selected from chemical vapor deposition, spray coating, and roll-to-roll coating.

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