Electrode assembly and secondary battery including the same

Abstract

The present invention relates to an electrode assembly and a secondary battery including the same. Specifically, the electrode assembly includes a core portion of an electrode assembly in which a negative electrode, a separator, and a positive electrode are sequentially stacked and wound, the negative electrode including a negative electrode active material layer portion including a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector, and a negative electrode non-coating portion including a negative electrode current collector on which the negative electrode active material layer is not provided, and the longitudinal length of the negative electrode active material layer portion extending from a longitudinal end of the positive electrode is adjusted to a specific range, and the electrode assembly and a secondary battery including the same.

Description

[Technical Field]

[0001] The present invention relates to an electrode assembly and a secondary battery including the same, and more particularly to an electrode assembly in which the amount of a core negative electrode is adjusted and a cylindrical secondary battery including the same. This application claims the benefit of Korean Patent Application No. 10-2023-0088068, filed with the Korean Intellectual Property Office on July 7, 2023, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] In the case of cylindrical batteries, a jelly roll-like electrode assembly is manufactured by winding long electrodes of a specified width into a roll. This electrode assembly is inserted into a battery case to manufacture a cylindrical battery, and the electrodes repeatedly contract and expand during charging and discharging. In particular, if an in-tab is located in the core of the electrode assembly or if a silicon-based active material is added to the negative electrode, which increases the degree of contraction and expansion of the electrode assembly, the pressure acting on the core of the electrode assembly increases significantly.

[0003] Recently, as low resistance / high capacity designs have become more common, electrode assemblies have increasingly included multiple tabs or silicon-based active materials. This increases the possibility of deformation of the electrode assembly located in the core due to contraction / expansion of the electrode assembly. In particular, if a separator located between the negative and positive electrodes is damaged, the negative and positive electrodes may come into direct contact with each other, resulting in an internal short circuit, which may cause heat generation and fire.

[0004] To solve the problems of separator breakage and internal short circuiting caused by deformation of the electrode assembly, it is necessary to develop technology that can protect the negative electrode and separator in the corresponding area and prevent internal short circuiting. Summary of the Invention [Problem to be solved by the invention]

[0005] SUMMARY OF THE INVENTION An object of the present invention is to provide an electrode assembly with a modified design and a secondary battery including the same.

[0006] However, the problems to be solved by the present invention are not limited to those described above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0007] In one embodiment of the present invention, there is provided an electrode assembly having a core portion in which a negative electrode, a separator, and a positive electrode are sequentially stacked and wound, wherein the negative electrode includes a negative electrode active material layer portion including a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; and a negative electrode uncoated portion including a negative electrode current collector on which the negative electrode active material layer is not provided, wherein the longitudinal length of the negative electrode active material layer portion extending from a longitudinal end of the positive electrode is 0.5 turns or less from the longitudinal end of the positive electrode.

[0008] Another embodiment of the present invention provides a secondary battery including the electrode assembly; and a battery case for accommodating the electrode assembly. [Effects of the Invention]

[0009] In an electrode assembly according to an embodiment of the present invention, the amount of core negative electrode added can be adjusted to facilitate sliding of the negative electrode during charging and discharging of the battery, and the relative sliding of the positive electrode end relative to the negative electrode can be reduced.

[0010] In addition, the secondary battery according to an embodiment of the present invention can prevent damage to the negative electrode and the separator due to relative sliding of the positive electrode end portion, and can prevent internal short circuits between the positive electrode and the negative electrode, thereby improving battery stability and life characteristics.

[0011] The effects of the present invention are not limited to those described above, and effects not mentioned will be clearly understood by those skilled in the art from the present specification and the accompanying drawings. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a diagram schematically illustrating an electrode assembly according to an embodiment of the present invention. [Figure 2] 1 is a diagram schematically illustrating an electrode assembly according to an embodiment of the present invention. [Figure 3] 1 is a diagram schematically illustrating an electrode assembly according to an embodiment of the present invention. [Figure 4] 1 shows CT images showing the evaluation results of cycle stability of secondary batteries according to Example 1, Comparative Example 1, and Comparative Example 2. [Figure 5] 10A and 10B are diagrams illustrating a method for evaluating whether or not a separator membrane in a core portion of an electrode assembly is damaged, according to an embodiment of the present invention. [Figure 6] 1 is a graph showing the results of evaluation of whether or not the separator in the core portion of the secondary batteries according to Example 1 and Comparative Example 2 is damaged. [Figure 7] 1A and 1B are views showing an electrode assembly and a secondary battery including the same according to an embodiment of the present invention; [Figure 8] 1A and 1B are views showing an electrode assembly and a secondary battery including the same according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0013] Throughout this specification, when a part is said to "comprise" a certain element, this means that it may further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0014] Throughout this specification, when an element is said to be "on" another element, this includes not only when the element is in contact with the other element, but also when there is another element between the two elements.

[0015] Throughout this specification, the term "winding axis of the electrode assembly" may refer to an imaginary line located at the center of the core hollow H of the wound electrode assembly 1. The core hollow H may be formed at the position of a winding core C that was removed after being used to wind the components included in the electrode assembly.

[0016] As a result, the components included in the wound electrode assembly may each have a defined winding axis direction of the electrode assembly, i.e., a direction parallel to the winding axis of the electrode assembly (Z-axis direction).

[0017] Furthermore, based on a cross section (XY plane) perpendicular to the winding axis of the electrode assembly, the components included in the wound electrode assembly may each be defined in a direction facing the winding axis of the electrode assembly, i.e., a direction toward the center of the hollow H of the core portion of the wound electrode assembly 1.

[0018] The present invention will be described in detail below with reference to the drawings. However, the drawings are for illustrative purposes only and the scope of the present invention is not limited by the drawings.

[0019] In one embodiment of the present invention, there is provided an electrode assembly having a core portion in which a negative electrode, a separator, and a positive electrode are sequentially stacked and wound, wherein the negative electrode includes a negative electrode active material layer portion including a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; and a negative electrode uncoated portion including a negative electrode current collector on which the negative electrode active material layer is not provided, wherein the longitudinal length of the negative electrode active material layer portion extending from a longitudinal end of the positive electrode is 0.5 turns or less from the longitudinal end of the positive electrode.

[0020] In an electrode assembly according to an embodiment of the present invention, the amount of core negative electrode inserted can be adjusted to facilitate sliding of the negative electrode during battery charge and discharge, and the relative sliding of the positive electrode end relative to the negative electrode can be reduced. Furthermore, the relative sliding of the positive electrode end relative to the negative electrode during battery charge and discharge can be reduced, preventing damage to the negative electrode and separator due to deformation of the electrode assembly caused by electrode contraction / expansion, and preventing internal shorts between the positive and negative electrodes, thereby improving battery stability and lifespan characteristics. Specifically, the effect of reducing the relative sliding of the positive electrode end by adjusting the amount of core negative electrode inserted within the above range can be even more pronounced when a negative electrode in-tab is included.

[0021] Here, the "core portion" may refer to a hollow portion located on the winding shaft of the electrode assembly and a region including a part of the wound negative electrode / separator / positive electrode laminate structure, and may refer to a region within two turns of the positive electrode from one end of the positive electrode in the longitudinal direction located at the innermost corner of the electrode assembly.

[0022] 1 to 3 are diagrams schematically illustrating an electrode assembly according to an embodiment of the present invention.

[0023] 1 to 3, an electrode assembly according to an embodiment of the present invention may be an electrode assembly in which a negative electrode 100, 100′, a separator 20, 20′, and a positive electrode 300 are stacked and wound up, and in the core portion of the electrode assembly, the negative electrode 100 may include a negative electrode current collector 101 and negative electrode active material layers 102, 103 formed on at least one surface of the negative electrode current collector 101. Specifically, the negative electrode 100 may include a negative electrode active material layer portion 10 including the negative electrode current collector 101 and negative electrode active material layers 102, 103 formed on at least one surface of the negative electrode current collector 101; and a negative electrode non-coating portion 12 including the negative electrode current collector 101 on which the negative electrode active material layer is not formed.

[0024] According to one embodiment of the present invention, the positive electrode 300 includes a first surface that is in the winding axis direction of the electrode assembly and a second surface that is the opposite surface to the first surface, and the negative electrode 100 in contact with the first surface of the positive electrode may include the negative electrode active material layer portion 10 and the negative electrode uncoated portion 12 extending from a longitudinal end portion 310 of the positive electrode.

[0025] According to one embodiment of the present invention, the negative electrode active material layer portion 10 may refer to a region extending from a longitudinal end portion 310 of the positive electrode to a point where the negative electrode active material layer portion extends from the longitudinal end portion 310 of the positive electrode to a point where the negative electrode active material layer portion contacts the negative electrode uncoated portion, or may refer to a region having a predetermined longitudinal length L1.

[0026] According to one embodiment of the present invention, the negative electrode uncoated portion 12 may refer to a region extending from the negative electrode active material layer portion 10 and extending from a point where the negative electrode uncoated portion 12 contacts a longitudinal end of the negative electrode active material layer portion to a longitudinal end of the negative electrode, or may refer to a region having a predetermined longitudinal length L2.

[0027] According to one embodiment of the present invention, the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode may be 0.5 turns or less from the longitudinal end of the positive electrode. Specifically, the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode may be 0.45 turns or less or 0.4 turns or less from the longitudinal end of the positive electrode.

[0028] Here, one turn refers to the length required to wind the positive or negative electrode included in the electrode assembly 360° from a reference point, and the length may be determined depending on the outer diameter of the winding core C used to wind the electrode assembly, the thickness of the electrode, and the number of windings of the electrode located inside. For example, one turn of the negative electrode 100 in contact with the first surface of the positive electrode 300 may refer to the length required to wind the negative electrode 100 360° from the longitudinal end 310 of the positive electrode toward the winding start end of the negative electrode 100.

[0029] According to one embodiment of the present invention, the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode may be less than 70% of the circumference of the inner circumferential surface of the electrode assembly (100%). Specifically, the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode may be 68% or less, 66% or less, 64% or less, or 62% or less of the circumference of the inner circumferential surface of the electrode assembly (100%).

[0030] Here, the "around the inner circumferential surface" refers to the circumference of an imaginary circle whose center is the winding axis of the electrode assembly and whose radius is the longest value among the distances to the innermost stratum corneum that contacts the hollow H of the core part of the electrode assembly, and the length may be determined according to the outer diameter of the winding core C used to wind the electrode assembly. For example, the circumference of the inner circumferential surface may have a value of about 10 mm.

[0031] According to one embodiment of the present invention, the negative electrode and separator may extend beyond the longitudinal end of the positive electrode in the core of the electrode assembly. Specifically, referring to FIG. 3, the negative electrode 100 and separator 20, 20' may extend beyond the longitudinal end 310 of the positive electrode. That is, the negative electrode and separator may be wound up together with the positive electrode after being wound up. For example, the negative electrode and separator may be wound together around a winding core for 0.5 turns or more, and then wound up together with the positive electrode. That is, in the core of the electrode assembly, the longitudinal ends of the negative electrode and separator may be located more inward than the longitudinal end of the positive electrode.

[0032] According to one embodiment of the present invention, the longitudinal length of the negative electrode extending from the longitudinal end of the positive electrode may be 1.5 turns or more and 2.5 turns or less from the longitudinal end of the positive electrode. Specifically, referring to Figures 1 and 3, the longitudinal length L1 + L2 of the negative electrode extending from the longitudinal end 310 of the positive electrode may be 1.6 turns or more or 1.7 turns or more from the longitudinal end 310 of the positive electrode, and may be 2.4 turns or less or 2.3 turns or less.

[0033] When the longitudinal length of the negative electrode extending from the longitudinal end of the positive electrode satisfies the above range, the length of the negative electrode uncoated portion may be suitable for further including a negative electrode tab 50, specifically a negative electrode in-tab, provided on one surface of the negative electrode current collector, as described below. For example, when a negative electrode in-tab is provided, the length L2 of the negative electrode uncoated portion may be approximately 19 mm. Furthermore, the relative sliding reduction effect of the positive electrode end by adjusting the longitudinal length of the negative electrode within the above range may be more excellent when a negative electrode in-tab is included.

[0034] According to one embodiment of the present invention, the longitudinal length of the negative electrode active material layer portion extending from a longitudinal end of the positive electrode may be 35% or less, based on 100% of the longitudinal length of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion. Specifically, referring to Figures 1 and 3, the longitudinal length L1 of the negative electrode active material layer portion extending from a longitudinal end 310 of the positive electrode may be 10% or more, 11% or more, or 12% or more, or may be 34% or less, 33% or less, or 32% or less, based on 100% of the longitudinal length L2 of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion.

[0035] When the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode satisfies the above-mentioned range, even when a negative electrode in-tab is included, the amount of core negative electrode inserted can be adjusted to facilitate sliding of the negative electrode during battery charge and discharge, and relative sliding of the positive electrode end relative to the negative electrode can be reduced. Specifically, the effect of reducing relative sliding of the positive electrode end by adjusting the longitudinal length of the negative electrode active material layer portion within the above-mentioned range can be more excellent when a negative electrode in-tab is included.

[0036] According to one embodiment of the present invention, the longitudinal length of the negative electrode extending from the longitudinal end of the positive electrode may be 0.5 turns or more and 1 turn or less from the longitudinal end of the positive electrode. Specifically, referring to Figures 2 and 3, the longitudinal length L1 + L2 of the negative electrode extending from the longitudinal end 310 of the positive electrode may be 0.6 turns or more or 0.7 turns or more, or 0.9 turns or less or 0.8 turns or less from the longitudinal end 310 of the positive electrode.

[0037] When the longitudinal length of the negative electrode extending from the longitudinal end of the positive electrode satisfies the above-mentioned range, the length of the negative electrode uncoated portion may be suitable for the case where a negative electrode in-tab is not provided. For example, when a negative electrode in-tab is not provided, the length L2 of the negative electrode uncoated portion may be approximately 3 mm. Furthermore, the relative sliding reduction effect of the positive electrode end by adjusting the longitudinal length of the negative electrode within the above-mentioned range may be more excellent when a negative electrode in-tab is not provided.

[0038] According to one embodiment of the present invention, the longitudinal length of the negative electrode active material layer portion extending from a longitudinal end of the positive electrode may be 150% to 230% of the longitudinal length of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion (100%). Specifically, referring to Figures 2 and 3, the longitudinal length L1 of the negative electrode active material layer portion extending from a longitudinal end 310 of the positive electrode may be 155% to 160% or 165% or more, or 225% to 220% or less, or 215% to 100% of the longitudinal length L2 of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion.

[0039] When the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode satisfies the above-mentioned range, even if a negative electrode in-tab is not included, the amount of core negative electrode inserted can be adjusted to facilitate sliding of the negative electrode during battery charge and discharge, and relative sliding of the positive electrode end relative to the negative electrode can be reduced. Specifically, the effect of reducing relative sliding of the positive electrode end by adjusting the longitudinal length of the negative electrode active material layer portion within the above-mentioned range can be more excellent when a negative electrode in-tab is not included.

[0040] According to one embodiment of the present invention, the longitudinal length of the negative electrode may be greater than the longitudinal length of the positive electrode, and the longitudinal lengths of the separators located on one side and the opposite side of the negative electrode may be greater than the longitudinal length of the positive electrode.

[0041] When the negative electrode and separator extend beyond the longitudinal end of the positive electrode, lithium ions from the positive electrode can be more easily transferred to the negative electrode during the chemical reaction of the lithium ion battery. When the negative electrode is formed to be longer or wider, the area of ​​the negative electrode that receives lithium ions increases, preventing a decrease in charge / discharge efficiency and improving battery stability and lifespan characteristics.

[0042] According to one embodiment of the present invention, the weight of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode and further wound up may be 70% or more and less than 110% of the weight of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion (100%). Specifically, referring to Figures 1 and 3, the weight of the negative electrode active material layer portion 10 extending from the longitudinal end 310 of the positive electrode may be 70% or more and 100% or 80% or more and 100% of the weight of the negative electrode uncoated portion 12 extending from the end of the negative electrode active material layer portion (100%).

[0043] When the weight range of the negative electrode active material layer is satisfied, the amount of the core negative electrode added can be adjusted to facilitate sliding of the negative electrode during battery charge and discharge, and the relative sliding of the positive electrode end portion relative to the negative electrode can be reduced. Specifically, the effect of reducing the relative sliding of the positive electrode end portion by adjusting the weight of the core negative electrode active material layer within the above range can be more excellent when a negative electrode in-tab is included.

[0044] According to one embodiment of the present invention, the negative electrode active material layer portion 10 may include an end surface landed portion 11 including one surface of the negative electrode current collector on which the negative electrode active material layer is provided and the other surface on which the negative electrode active material layer is not provided and the negative electrode current collector is directly exposed. Specifically, the end surface landed portion 11 may include a negative electrode active material layer provided on one surface of the negative electrode current collector facing the winding shaft and a negative electrode active material layer not provided on the other surface, so that the negative electrode current collector is directly exposed. When the end surface landed portion is included, the amount of negative electrode active material applied to the region that does not face the positive electrode can be minimized, thereby ensuring economic efficiency. Furthermore, when a negative electrode active material layer is provided on the surface of the end surface landed portion facing the winding shaft of the negative electrode current collector, the formation of a step due to the thickness of the negative electrode active material can be minimized.

[0045] According to one embodiment of the present invention, the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end portion 310 of the positive electrode may be 5 mm or more and less than 7 mm. Specifically, referring to Figures 1 and 3, the longitudinal length L1 of the negative electrode active material layer portion extending from the longitudinal end portion 310 of the positive electrode may be 5.2 mm or more, 5.4 mm or more, or 5.6 mm or more, or may be 6.8 mm or less, 6.6 mm or less, or 6.4 mm or less.

[0046] When the longitudinal length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode satisfies the aforementioned range, the amount of core negative electrode added can be adjusted to facilitate sliding of the negative electrode during charging and discharging of the battery, and the relative sliding of the positive electrode end portion relative to the negative electrode can be reduced. Specifically, the effect of reducing the relative sliding of the positive electrode end portion by adjusting the longitudinal length of the negative electrode active material layer portion within the aforementioned range can be more excellent when a negative electrode in-tab is included.

[0047] According to one embodiment of the present invention, the negative electrode includes a negative electrode active material layer portion where a negative electrode active material layer is formed on the negative electrode current collector, and a negative electrode uncoated portion where no negative electrode active material layer is formed, and may include a tab on the negative electrode uncoated portion. Specifically, referring to FIGS. 1 and 3, the negative electrode 100 may include a negative electrode uncoated portion 12 and a negative electrode tab 50 provided on the negative electrode uncoated portion. Accordingly, the manufactured electrode assembly may include one or more negative electrode tabs. That is, the manufactured electrode assembly may include a negative electrode in-tab, and may further include a protective tape 40 to prevent short circuits due to the thickness of the negative electrode in-tab.

[0048] According to one embodiment of the present invention, the longitudinal length of the negative electrode end surface grounded portion may be 0 mm or more and 2 mm or less. Specifically, referring to FIG. 3, the longitudinal length L1' of the negative electrode end surface grounded portion may be 0.1 mm or more, 0.3 mm or more, 0.5 mm or more, or 0.7 mm or more, and may be 1.9 mm or less, 1.7 mm or less, 1.5 mm or less, or 1.3 mm or less, for example, 1 mm. When the longitudinal length of the negative electrode end surface grounded portion satisfies the above-mentioned range, the amount of core negative electrode material inserted can be adjusted to facilitate sliding of the negative electrode during battery charge and discharge, and relative sliding of the positive electrode end portion relative to the negative electrode can be reduced. Specifically, the effect of reducing relative sliding of the positive electrode end portion by adjusting the longitudinal length of the negative electrode end surface grounded portion within the above-mentioned range is more pronounced when a negative electrode in-tab is included.

[0049] According to one embodiment of the present invention, the negative electrode active material layer may include a negative electrode active material including at least one selected from the group consisting of a silicon-based material and a carbon-based material. The negative electrode active material layer may further include a negative electrode conductive material and a negative electrode binder, and the negative electrode active material, negative electrode conductive material, and negative electrode binder may be materials commonly used in the art without limitation.

[0050] According to one embodiment of the present invention, the negative electrode current collector may be any material that does not cause chemical changes in the battery and is conductive, and is not particularly limited. For example, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that effectively adsorbs carbon, such as copper or nickel, can be used as the negative electrode current collector. The thickness of the negative electrode current collector may be 6 μm or more and 80 μm or less, but is not limited thereto.

[0051] According to one embodiment of the present invention, the negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0052] According to one embodiment of the present invention, the negative electrode conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0053] According to one embodiment of the present invention, the electrode assembly may include multiple separators. For example, the electrode assembly may have a structure in which a separator / anode / separator / cathode are stacked in this order. The separator separates the anode and cathode and provides a path for lithium ions to move. Any separator typically used in secondary batteries may be used without limitation. It is particularly preferred that the separator exhibits low resistance to ion migration and excellent electrolyte humidification. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. The separation membrane may generally have a thickness of 10 μm to 20 μm. The separation membrane may be formed by coating a substrate layer of the above-mentioned separation membrane material with a slurry containing a ceramic component or a polymeric material to ensure heat resistance or mechanical strength, and may be used in a single-layer or multi-layer structure.

[0054] According to one embodiment of the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. Specifically, referring to FIG. 3, the positive electrode 300 may include a positive electrode current collector 301 and positive electrode active material layers 302 and 303 formed on one or both sides of the positive electrode current collector 301 and containing a positive electrode active material. That is, the positive electrode active material layer is formed on a positive electrode landed portion of the positive electrode current collector, and the surface not provided with the positive electrode active material layer may be referred to as a positive electrode un-landed portion.

[0055] According to one embodiment of the present invention, the positive electrode current collector may include a positive electrode ground portion on which a positive electrode active material is coated and a positive electrode uncoated portion on which no positive electrode active material is coated, and may include a tab on the positive electrode uncoated portion. Specifically, the positive electrode current collector may include a positive electrode uncoated portion and may include a positive electrode tab formed on the positive electrode uncoated portion.

[0056] According to one embodiment of the present invention, in the core portion of the electrode assembly, the positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector and having a longitudinal end portion at the same position as the positive electrode current collector. Specifically, referring to FIG. 3 , in the core portion of the electrode assembly, a positive electrode 300, a separator 20, 20′, and a negative electrode 100 are stacked and wound, and the positive electrode 300 may include a positive electrode current collector 301 and positive electrode active material layers 302, 303 disposed on at least one surface of the positive electrode current collector 301 and having a longitudinal end portion 310 at the same position as the positive electrode current collector 301. That is, one longitudinal end portion 310 of the positive electrode may have a free edge.

[0057] This reduces the area of ​​unnecessary uncoated portions of the positive electrode current collector, ensuring economic efficiency, and allows the slitting process to be performed after forming the active material layer on the electrode, thereby making it possible to more efficiently perform a roll-to-roll process including the slitting process and the winding process. Here, "the same position" means that the longitudinal ends are the same, and may also include cases where the ends are formed in substantially the same position due to process errors that may occur in the slitting process, etc.

[0058] According to one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and is conductive, but specifically, the positive electrode current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc. That is, the positive electrode current collector may be provided in the form of surface-treated stainless steel, aluminum foil, etc.

[0059] The positive electrode current collector may have a thickness of typically 3 to 50 μm, and may be provided with fine irregularities on its surface to enhance the adhesive strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0060] According to one embodiment of the present invention, the positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; or a compound having the chemical formula Li 1+x Mn 2-x O4 (0≦x≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2), vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-y M y Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦y≦0.3 is satisfied); 2-z M z Examples of the lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and 0.01≦z≦0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may be Li-metal.

[0061] According to one embodiment of the present invention, the positive electrode active material layer may further include a positive electrode conductive material and a positive electrode binder. The positive electrode conductive material is used to impart conductivity to the electrode and may be any material that does not cause chemical changes in the resulting battery and has electronic conductivity. Specifically, the positive electrode conductive material may be graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination.

[0062] The positive electrode binder improves adhesion between particles of the positive electrode active material and between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0063] According to one embodiment of the present invention, the angle between the initial position of the longitudinal end of the negative electrode relative to the winding axis of the electrode assembly and the position of the longitudinal end of the negative electrode after 200 or more additional charge and discharge cycles at 40°C or higher may be 5° or more. Specifically, the angle between the initial position of the longitudinal end of the negative electrode relative to the winding axis of the electrode assembly and the position of the longitudinal end of the negative electrode after 200 or more additional charge and discharge cycles at 40°C or higher may be 5° or more, 10° or more, 15° or more, or 20° or more, or may be 45° or less, 40° or less, 35° or less, 30° or less, or 25° or less, for example, 5° or more and 30° or less. More specifically, the initial position of the longitudinal end of the negative electrode may be measured after activation. Here, the term "after activation" may refer to after a predetermined cycle for manufacturing a secondary battery and completing a product has been performed. Specifically, the term "after activation" refers to the state before the start of active use, including multiple cycles for power supply purposes, i.e., before sale and subsequent storage, and may also include a state in which self-discharge occurs during storage.

[0064] When the above-mentioned angle range is satisfied, the amount of core negative electrode inserted can be adjusted to facilitate sliding of the negative electrode during battery charging and discharging, and the relative sliding of the positive electrode end relative to the negative electrode can be reduced.

[0065] According to one embodiment of the present invention, when the battery is charged and discharged at 40°C or higher 200 times after activation, the angle between the positive electrode and the negative electrode may be 25° or less. Specifically, the positive electrode includes a first surface in the direction of the winding axis of the electrode assembly and a second surface opposite the first surface. A first extension line, which is drawn on the first surface of the positive electrode by extending a line connecting two points where the curvature direction changes within a distance of 5 mm from the longitudinal end of the positive electrode, and a second extension line, which is drawn on the surface of the negative electrode opposite the first surface of the positive electrode by extending a line connecting two points 5 mm from the longitudinal end of the positive electrode, may form an angle of 25° or less. When the above-mentioned angle range is satisfied, damage to the negative electrode and the separator due to relative sliding of the positive electrode end can be prevented, and internal short circuits between the positive electrode and the negative electrode can be prevented, thereby improving battery stability and life characteristics.

[0066] 7 and 8 are diagrams showing an electrode assembly according to an embodiment of the present invention and a secondary battery including the same.

[0067] One embodiment of the present invention provides a secondary battery including the electrode assembly and a battery case for housing the electrode assembly. Specifically, referring to Figures 7 and 8, the secondary battery 2 may include the electrode assembly 1 according to the above-described embodiment and a battery case 60 for housing the electrode assembly 1. More specifically, the electrode assembly 1 may or may not include a negative electrode tab 50.

[0068] A secondary battery according to an embodiment of the present invention can prevent damage to the negative electrode and separator due to relative sliding of the positive electrode end portion, and can prevent internal short circuits between the positive electrode and the negative electrode, thereby improving battery stability and life characteristics.

[0069] According to one embodiment of the present invention, the battery case may be cylindrical. Specifically, the battery case may be cylindrical, square, or pouch-shaped depending on the application. When the battery case is cylindrical, it is more suitable for accommodating an electrode assembly. When the battery case is cylindrical, the shape of a secondary battery including the electrode assembly and the battery case for accommodating the electrode assembly may also be cylindrical.

[0070] According to one embodiment of the present invention, the interior of the battery case may contain an electrolyte. Specifically, the electrolyte may be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used in manufacturing a lithium secondary battery, but is not limited to these. Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0071] According to one embodiment of the present invention, the non-aqueous organic solvent may be, for example, a non-protonic organic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, 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, ether, methyl propionate, or ethyl propionate.

[0072] According to one embodiment of the present invention, the metal salt may be a lithium salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be F - , Cl - , I - , NO3 - , N(CN)2- , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of:

[0073] According to one embodiment of the present invention, the electrolyte may further contain, in addition to the electrolyte constituents, one or more additives, such as a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, a cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, a nitrobenzene derivative, sulfur, a quinoneimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0074] Hereinafter, the present invention will be described in detail with reference to examples. However, the examples according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples in this specification are provided to more completely explain the present invention to those skilled in the art.

[0075] Example Example 1 Electrode assembly manufacturing An Al foil with a thickness of 15 μm and a width of 63.9 mm was prepared as a positive electrode current collector. A positive electrode active material slurry containing an NMCA (Ni-Mn-Co-Al) composite with a Ni content of 92% or more as a positive electrode active material and CNT as a conductive material was applied to the positive electrode current collector and dried to form a positive electrode active material layer, producing a positive electrode with a thickness of 154 μm.

[0076] Next, a Cu foil having a thickness of 8 μm and a width direction length of 65.1 mm was prepared as a negative electrode current collector, and a negative electrode active material slurry containing 50 parts by weight each of artificial graphite and natural graphite as negative electrode active materials was applied to the negative electrode current collector and dried to form a negative electrode active material layer, thereby producing a negative electrode having a thickness of 187 μm.

[0077] On the other hand, two polyethylene sheet-shaped separation membranes were prepared.

[0078] The negative and positive electrodes were then inserted in order to prepare an electrode assembly. The core of the electrode assembly had a negative active material layer portion (the negative electrode end surface coated portion had a longitudinal length of 1 mm) with a longitudinal length of 6 mm and a negative electrode uncoated portion with a longitudinal length of 19 mm, and was fabricated under the conditions shown in Table 1 below and with the structure shown in Figure 1.

[0079] Secondary battery manufacturing The electrode assembly was inserted into a cylindrical battery case, and an electrolyte solution prepared by mixing ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) in a volume ratio (vol%) of 20:5:75 and dissolving LiPF to a concentration of 0.7 M was injected into the case. The cylindrical battery case was then sealed with a cap assembly to manufacture a secondary battery.

[0080] Comparative Examples 1 and 2 An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that the longitudinal lengths of the negative electrode active material layer portion and the negative electrode uncoated portion, which extended from the longitudinal end of the positive electrode and were further wound, were adjusted according to the conditions shown in Table 1 below.

[0081] [Table 1]

[0082] Experimental example Experimental Example 1 - Cycle Stability Evaluation The secondary batteries prepared in Example 1, Comparative Example 1, and Comparative Example 2 were each charged to 30% SOC through three cycles at 4.2 V - 2.5 V, 0.2 C charge, and 0.2 C discharge to prepare activated secondary batteries. The activated secondary batteries were then charged for 200 cycles at 4.3 V - 2.85 V, 0.5 C / 1 C, and 55°C to prepare cycled secondary batteries. The cores of the activated secondary batteries and the cycled secondary batteries were subjected to computed tomography (CT) to check for core impingement and evaluate short-term cycle stability. The images are shown in Figure 4.

[0083] Experimental Example 2 - Evaluation of negative electrode sliding The core parts of the secondary batteries manufactured in Example 1, Comparative Example 1, and Comparative Example 2 were subjected to computed tomography (CT) imaging. The angle change between the initial position of the longitudinal end of the negative electrode and the position of the longitudinal end of the negative electrode after cycling was measured relative to the winding axis of the electrode assembly. The angle change was measured again every 200 cycles, and the results are shown in FIG. 6 below.

[0084] Experimental Example 3 - Core Impingement Evaluation The secondary batteries manufactured in Example 1, Comparative Example 1, and Comparative Example 2 were prepared as follows. Each of the manufactured secondary batteries was charged to 30% SOC through three cycles of 4.2 V-2.5 V, 0.2 C charge, and 0.2 C discharge to prepare activated secondary batteries. The cores of the activated secondary batteries were subjected to computed tomography (CT) imaging to evaluate the occurrence of core impingement according to the method for evaluating the occurrence of core impingement described below. The occurrence of core impingement was evaluated every 200 cycles, and the results are shown in Figure 6 below.

[0085] Figure 5 shows a schematic diagram of a method for evaluating whether or not core impingement has occurred. Specifically, (a) of Figure 5 shows a schematic diagram of a method for evaluating whether or not core impingement has occurred when deformation has occurred in the negative electrode, and (b) of Figure 5 shows a schematic diagram of a method for evaluating whether or not core impingement has occurred when deformation has not occurred in the negative electrode.

[0086] 1) On the first surface of the positive electrode 300, a first extension line E1 is drawn by extending a straight line connecting the longitudinal end 310 of the positive electrode and a point 5 mm away from the end.

[0087] 2-1) When deformation occurs in the negative electrode In the core portion of the electrode assembly, on the surface of the negative electrode 100 facing the first surface of the positive electrode, a second extension line E2 is drawn by extending a straight line connecting two points where the curvature direction changes within a distance of 5 mm from the longitudinal end 310 of the positive electrode.

[0088] 2-2) When there is no deformation in the negative electrode In the core portion of the electrode assembly, on the surface of the negative electrode 100 facing the first surface of the positive electrode, a second extension line E2 is drawn by extending a straight line connecting two points that are 5 mm apart from the longitudinal end 310 of the positive electrode.

[0089] 3) When the angle from the first extension line E1 to the second extension line E2 counterclockwise, centered on the intersection of the first extension line E1 and the second extension line E2, exceeds 25°, it is determined that core impingement has occurred.

[0090] Meanwhile, the method for evaluating the occurrence of core impingement can be applied in the following manner: when a secondary battery (Unknown Cell) in an unknown state is obtained, the occurrence of core impingement is evaluated at the time of initial acquisition, and the occurrence of core impingement is re-evaluated every 200 cycles, and the result is compared and analyzed with the core impingement conditions of the secondary battery of the embodiment according to the present invention.

[0091] 4 to 6, it was confirmed that core impingement did not occur in the secondary battery manufactured in Example 1 during cycle stability evaluation, but core impingement occurred in both the secondary batteries manufactured in Comparative Examples 1 and 2. Specifically, it was confirmed that core impingement did not occur in the secondary batteries manufactured in Comparative Examples 1 and 2 after activation, but core impingement occurred after 200 cycles at 55°C, which is a high-temperature, long-term cycle.

[0092] 6, the secondary battery prepared in Example 1 exhibited a relatively high negative electrode core sliding of 5° or more, even though the change in angle between the initial position of the longitudinal end of the negative electrode relative to the winding axis of the electrode assembly and the position of the longitudinal end of the negative electrode after cycling gradually decreased after 200 cycles, whereas the secondary battery prepared in Comparative Example 2 exhibited a relatively low negative electrode core sliding of 5° or less even after 400 cycles due to the inhibition of negative electrode core sliding. Furthermore, the secondary battery prepared in Comparative Example 2 exhibited a significant increase in the angle between the positive and negative electrodes after 200 cycles, and after 400 cycles, the angle between the positive and negative electrodes was 25° or more, indicating the occurrence of core impingement.

[0093] As a result, it was confirmed that the secondary battery according to Example 1, in which the core negative electrode input amount was adjusted within a specific range, particularly when including a negative electrode in-tab, exhibited significantly reduced frequency and severity of damage to the negative electrode and separator due to the longitudinal end of the positive electrode, i.e., core impingement due to contraction / expansion of the electrode assembly, compared to the secondary batteries according to Comparative Examples 1 and 2. Specifically, it was confirmed that the secondary batteries according to Comparative Examples 1 and 2 had inferior battery stability and lifespan characteristics compared to the secondary battery according to Example 1, in which the longitudinal length of the negative electrode active material layer portion exceeded 0.5 turns from the longitudinal end of the positive electrode, i.e., 60% or more based on 100% of the circumference of the inner circumferential surface of the electrode assembly, or the weight of the negative electrode active material layer portion was 110% or more based on 100% of the weight of the negative electrode uncoated portion.

[0094] That is, the electrode assembly according to one embodiment of the present invention can adjust the amount of core negative electrode inserted to facilitate sliding of the negative electrode during battery charge and discharge, and can reduce relative sliding of the positive electrode end relative to the negative electrode. The secondary battery according to one embodiment of the present invention including this can prevent damage to the negative electrode and separator due to relative sliding of the positive electrode end, and can prevent internal short circuits between the positive electrode and negative electrode, thereby improving battery stability and life characteristics.

[0095] The above detailed description is illustrative and explanatory of the present invention. Furthermore, the foregoing merely illustrates and describes preferred embodiments of the present invention. As noted above, the present invention can be used in various other combinations, modifications, and environments, and can be changed or modified within the scope of the inventive concept disclosed herein, the scope of equivalents to the foregoing disclosure, and / or the skill or knowledge of the art. Therefore, the above detailed description of the invention is not intended to limit the present invention to the disclosed embodiments. Furthermore, the appended claims should be construed to include other embodiments. [Explanation of symbols]

[0096] C... core H: Hollow core 100...Negative electrode 101...Negative electrode current collector 102,103...Negative electrode active material layer 10...Negative electrode active material layer section 11... Negative end surface area 12 Negative electrode uncoated area 20,20'...Separation membrane 300...Positive electrode 301...Positive electrode current collector 302,303...Cathode active material layer 310 Longitudinal end of positive electrode 40 Protective tape 50 Negative electrode tab L1: Length of the negative electrode active material layer in the longitudinal direction L1': Length of the landed portion on the negative electrode end surface in the longitudinal direction L2: Length of the negative electrode uncoated area in the longitudinal direction E1: First extension E2: Second extension 60 Battery case 1...electrode assembly 2...Secondary battery

Claims

1. In a core part of an electrode assembly in which a negative electrode, a separator, and a positive electrode are stacked in order and wound, the negative electrode includes a negative electrode active material layer portion including a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector; and a negative electrode non-coating portion including a negative electrode current collector on which the negative electrode active material layer is not provided, The electrode assembly, wherein the length of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode is 0.5 turns or less from the longitudinal end of the positive electrode.

2. 2. The electrode assembly according to claim 1, wherein a longitudinal length of the negative electrode extending from a longitudinal end of the positive electrode is 1.5 turns or more and 2.5 turns or less from the longitudinal end of the positive electrode.

3. The length in the longitudinal direction of the negative electrode active material layer portion extending from the longitudinal end portion of the positive electrode is The electrode assembly of claim 2 , wherein the length of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion in the longitudinal direction is 35% or less, based on 100%.

4. 2. The electrode assembly according to claim 1, wherein a length of the negative electrode extending from a longitudinal end of the positive electrode is 0.5 turns or more and 1 turn or less from the longitudinal end of the positive electrode.

5. The length in the longitudinal direction of the negative electrode active material layer portion extending from the longitudinal end portion of the positive electrode is The electrode assembly of claim 4 , wherein the length of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion in the longitudinal direction is 150% to 230% based on 100%.

6. The weight of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode is The electrode assembly of claim 1 , wherein the weight of the negative electrode uncoated portion extending from the end of the negative electrode active material layer portion is 70% or more and less than 110% of 100% of the weight of the negative electrode uncoated portion.

7. The negative electrode active material layer portion is 2. The electrode assembly of claim 1, further comprising: an end surface land portion including one surface of the negative electrode current collector on which the negative electrode active material layer is provided; and another surface on which the negative electrode active material layer is not provided and on which the negative electrode current collector is directly exposed.

8. The electrode assembly according to claim 1 , wherein the length in the longitudinal direction of the negative electrode active material layer portion extending from the longitudinal end of the positive electrode is 5 mm or more and less than 7 mm.

9. With respect to the winding shaft of the electrode assembly, 2. The electrode assembly according to claim 1, wherein an angle between an initial position of the longitudinal end of the negative electrode and a position of the longitudinal end of the negative electrode after further charging and discharging at least 200 times at 40° C. or higher is 5° or greater.

10. 10. The electrode assembly of claim 9, wherein the initial position of the longitudinal end of the negative electrode is measured after activation.

11. the positive electrode includes a first surface in a winding axis direction of the electrode assembly and a second surface opposite to the first surface, a first extension line drawn on the first surface of the positive electrode by extending a straight line connecting two points at which the curvature direction changes within a distance of 5 mm from an end of the positive electrode in the longitudinal direction; and 2. The electrode assembly according to claim 1, wherein a second extension line drawn by extending a straight line connecting two points spaced 5 mm apart from longitudinal ends of the positive electrode on a surface of the negative electrode facing the first surface of the positive electrode forms an angle of 25° or less.

12. In the core portion of the electrode assembly, 2. The electrode assembly according to claim 1, wherein the positive electrode comprises: a positive electrode current collector; and a positive electrode active material layer provided on at least one surface of the positive electrode current collector and having a longitudinal end portion at the same position as the positive electrode current collector.

13. In the core portion of the electrode assembly, The electrode assembly of claim 1 , wherein the negative electrode uncoated portion further comprises a negative electrode tab provided on one surface of the negative electrode current collector.

14. An electrode assembly according to any one of claims 1 to 13; and a battery case for accommodating the electrode assembly; A secondary battery comprising:

15. The secondary battery according to claim 14 , wherein the battery case is cylindrical.

Images