Electrode assembly and lithium secondary battery including the same

Abstract

The present invention relates to an electrode assembly including: an electrode stack including a positive electrode including a positive active material layer, a negative electrode including a negative active material layer, and a separator disposed between the positive electrode and the negative electrode; and at least one fixing member fixing the electrode stack by winding the electrode stack in a total width direction, wherein the positive electrode includes a positive electrode sliding portion in which a thickness of the positive active material layer is reduced, the fixing member includes a first fixing member overlapping the positive electrode sliding portion and a second fixing member not overlapping the positive electrode sliding portion, and a thickness of the first fixing member is greater than a thickness of the second fixing member.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2023-0137204, filed on October 13, 2023, and Korean Patent Application No. 10-2024-0138669, filed on October 11, 2024, the disclosures of which are incorporated herein by reference.

[0002] The present invention relates to an electrode assembly and a lithium secondary battery including the electrode assembly, and more particularly, to an electrode assembly that can transmit uniform pressure to an end portion when inserted into a module and a lithium secondary battery including the electrode assembly. Background Technology

[0003] With advancements in technology for electric vehicles, energy storage systems (ESS), and portable electronic devices, the demand for lithium-ion batteries as energy sources has increased rapidly.

[0004] Lithium secondary batteries are classified into bag type, can type, etc. according to the material of the box used to house the electrode components, and the electrode components can be classified into wound type, stacked type, stacked laminated type or stacked folded type according to their manufacturing method and shape.

[0005] Among them, the pouch-type secondary battery is prepared by pressing a flexible pouch film laminate to form a cup-shaped part, accommodating the electrode assembly in the cup-shaped part, and sealing the sealing part after injecting electrolyte. The can-type secondary battery is prepared by accommodating the electrode assembly in a can made of metal material, injecting electrolyte, and then assembling a top cover onto the upper part of the can to seal the can.

[0006] An electrode assembly is a structure comprising a positive electrode, a negative electrode, and a separator disposed between the positive and negative electrodes. The positive and negative electrodes are each prepared by coating an electrode current collector with an electrode slurry to form an active material layer, followed by drying and rolling the coated current collector. In the case where the electrode is prepared by coating with an electrode slurry, a sliding portion is formed at the end portion of the active material layer, wherein the thickness of the active material layer gradually decreases. In the region where the electrode sliding portion is formed, lithium plating (Li plating) may easily occur because the distance between the positive and negative active material layers increases and the adhesion to the separator decreases.

[0007] Furthermore, the cell thickness at the end portion of the electrode assembly is relatively reduced due to the sliding portion. In the case of stacking many cells to form a module, the thickness difference between the central portion and the end portion of the cell increases, and therefore, due to the smaller pressure applied to the end portion of the cell in the module, expansion and lithium plating may be further increased.

[0008] Furthermore, as the electrolyte is consumed during repeated charging and discharging of the lithium secondary battery, the amount of electrolyte decreases. If the amount of electrolyte in the battery decreases, the electrolyte may not be able to reach the electrode end portion, thereby reducing electrolyte permeability. Consequently, the mobility of lithium ions may decrease, leading to lithium plating (Li plating).

[0009] Recently, with the increasing use of lithium-ion batteries as a power source for electric vehicles, lithium-ion batteries with a relatively long overall length relative to their total width (hereinafter referred to as "long cells" for convenience) are being developed, taking into account battery housing space and location. Lithium-ion batteries with this long cell structure are advantageous because they can achieve higher capacity and have excellent space efficiency compared to conventional lithium-ion batteries. However, a problem exists: the pressure at the end portion of the electrode assembly near the electrode tabs decreases with repeated charging and discharging, reducing the adhesion between the separator and the electrode, and thus further exacerbating lithium plating.

[0010] Lithium plating is the phenomenon where lithium removed from the positive electrode does not embed into the negative electrode but instead deposits on the surface of the negative electrode. If lithium plating occurs, the corresponding area turns gray. Lithium plating can not only degrade battery performance and significantly reduce long-term lifespan, but it can also limit the battery's fast-charging capacity and lead to combustion or explosion. Summary of the Invention

[0011] Technical issues

[0012] One aspect of the present invention provides an electrode assembly that is developed to compensate for the reduction in thickness at the end portion of the unit by providing a thicker fixing member in the region overlapping with the positive electrode sliding portion, thereby enabling pressure to be uniformly transmitted throughout the entire electrode assembly when inserted into a module, and to suppress expansion and lithium plating by improving the adhesion between the electrode and the separator.

[0013] Another aspect of the present invention provides a lithium secondary battery in which the performance and safety of the battery can be prevented from deteriorating due to lithium deposition by including the above-described electrode assembly.

[0014] Technical solution

[0015] According to an embodiment, the present invention provides an electrode assembly comprising: an electrode stack including a positive electrode containing a positive active material layer, a negative electrode containing a negative active material layer, and a separator disposed between the positive and negative electrodes; and at least one fixing member that fixes the electrode stack by winding the electrode stack along its total width direction.

[0016] In this case, the positive electrode includes a positive electrode sliding portion in which the thickness of the positive electrode active material layer is reduced.

[0017] In addition, the fixing member includes a first fixing member that overlaps with the positive electrode sliding portion and a second fixing member that does not overlap with the sliding portion, and the thickness of the first fixing member is greater than the thickness of the second fixing member.

[0018] Preferably, when the thickness of the first fixing member is T1, the thickness of the second fixing member is T2, and the thickness of the positive electrode is T... C At that time, the electrode assembly satisfies [Formula 1].

[0019] [Formula 1]

[0020] 0 <T1-T2≤0.5T C

[0021] The negative electrode may include a negative electrode sliding portion in which the thickness of the negative electrode active material layer is reduced, and the fixing member may overlap with at least a portion of the region corresponding to the negative electrode sliding portion.

[0022] The ratio (L / W) of the total length (L) of the electrode stack can be 3 or greater, and the ratio of the total length (L) of the electrode stack to the total width (W) can preferably be in the range of 3 to 7.

[0023] The electrode assembly according to the invention may include 2 to 10 fixing members, wherein, in this case, the fixing members may be arranged in a horizontally symmetrical position along the total length direction, and preferably, the fixing members are arranged to be separated from each other at equal intervals.

[0024] The fastening member may include a porous structure, and specifically may be a strip on one surface of a substrate having an adhesive layer formed thereon.

[0025] The fixing component can have a width of 10mm to 50mm.

[0026] According to another embodiment, the present invention provides a lithium secondary battery comprising: an electrode assembly according to the present invention; an electrolyte; and a battery case housing the electrode assembly and the electrolyte. In this case, the battery case may be a pouch-type battery case.

[0027] Beneficial effects

[0028] In the electrode assembly according to the invention, since the first fixing member is arranged to overlap with the area where the positive electrode sliding portion is formed, the close contact between the electrode sliding portion and the separator is firmly maintained, and thus the reduction in lithium mobility due to poor interfacial adhesion between the electrode sliding portion and the separator can be minimized. Therefore, the electrode assembly according to the invention can prevent lithium from being desorbed from the electrode sliding portion.

[0029] Furthermore, since the reduction in thickness at the end portion of the electrode assembly is compensated by making the first fixing member, which is located in the region overlapping with the positive electrode sliding portion, thicker than the second fixing member, pressure can be uniformly transmitted to the entire electrode assembly when inserted into the module, and thus, expansion and lithium plating due to the reduced pressure at the end portion of the cell can be suppressed.

[0030] Furthermore, when the fixing member is arranged in a long unit with a total length that is relatively long relative to the total width, as in the present invention, the reduction of electrolyte solution in the electrode sliding part can be minimized, thus preventing the degradation of battery performance.

[0031] Therefore, if the electrode assembly according to the invention is used, excellent performance and safety can be achieved until the end of the lithium secondary battery's life. Attached Figure Description

[0032] Figure 1 This is a top view of an electrode assembly according to an embodiment of the present invention.

[0033] Figure 2 This is a cross-sectional view of an electrode stack according to an embodiment of the present invention.

[0034] Figure 3 The photograph shows whether lithium plating occurs after 800 charge and discharge cycles of a module using the electrode assembly prepared in the example.

[0035] Figure 4 The photograph shows whether lithium plating occurs after 800 charge and discharge cycles of a module using the electrode assembly prepared in Comparative Example 1.

[0036] Figure 5 The photograph shows whether lithium plating occurs after 800 charge and discharge cycles of a module using the electrode assembly prepared in Comparative Example 2. Detailed Implementation

[0037] The invention will be described in detail below.

[0038] As a result of extensive research into developing lithium secondary batteries that achieve excellent performance and safety until the end of the battery's lifespan, the inventors have discovered that by using thick fixing members to fix the area corresponding to the positive electrode sliding portion during the formation of the electrode assembly, not only can the interfacial adhesion between the electrode sliding portion and the separator be improved, but also the pressure can be transmitted relatively evenly to the entire cell after the module is assembled. Therefore, the degradation of battery performance can be minimized until the end of the battery's lifespan, and excellent safety can be achieved by suppressing fire and / or explosion due to lithium plating, thus leading to the completion of this invention.

[0039] Specifically, the electrode assembly according to the present invention is characterized in that the electrode assembly includes: an electrode stack including a positive electrode, a negative electrode and a separator; and at least one fixing member, the fixing member fixing the outer surface of the electrode stack by wrapping around the outer surface of the electrode stack along the total width direction, wherein the positive electrode includes a positive electrode sliding portion with a reduced thickness of the positive electrode active material layer, the fixing member includes a first fixing member disposed to overlap with a region corresponding to the positive electrode sliding portion and a second fixing member disposed in a region not corresponding to the positive electrode sliding portion, and the thickness of the first fixing member is greater than the thickness of the second fixing member.

[0040] In cases where the thickness of the first fixing member disposed in the region overlapping with the positive electrode sliding portion, as in this invention, is greater than the thickness of the second fixing member disposed in another region, the reduction in thickness of the end portion of the cell can be compensated for by the thickness of the first fixing member. Therefore, expansion and lithium deposition that occur during module assembly due to the reduced thickness of the end portion of the cell can be minimized.

[0041] Furthermore, when the first fixing member is configured to overlap with the positive electrode sliding portion region as in this invention, the distance between the electrode sliding portion and the separator is maintained by the first fixing member, and even if repeated charging and discharging cause volume changes in the positive electrode active material layer and / or the negative electrode active material layer, the first fixing member prevents an increase in the distance between the electrode sliding portion and the separator. Therefore, since the interfacial adhesion between the separator and the electrode at the end portion of the electrode assembly and the electrolyte solution permeability can be maintained even with repeated charging and discharging, lithium plating due to a decrease in lithium mobility can be suppressed.

[0042] Figure 1 A top view of an electrode assembly according to an embodiment of the present invention is illustrated, and Figure 2A cross-sectional view of an electrode stack according to an embodiment of the present invention is illustrated. The invention will be described in more detail below with reference to the accompanying drawings. However, the drawings are presented for illustrative purposes only, and the scope of the invention is not limited thereto; various modifications may be made without departing from the purpose and spirit of the invention.

[0043] like Figure 1 As illustrated, the electrode assembly 1 according to the present invention includes an electrode stack 100 and one or more fixing members 200a and 200b for fixing the electrode stack 100 by winding the electrode stack along the total width direction, wherein the fixing members 200a and 200b include a first fixing member 200a that overlaps with the positive electrode sliding portion 12b and a second fixing member 200b that does not overlap with the positive electrode sliding portion 12b.

[0044] Reference Figure 2 The electrode stack 100 includes a positive electrode 10 comprising positive electrode active material layers 12a and 12b, a negative electrode 20 comprising negative electrode active material layers 22a and 22b, and a separator 30 disposed between the positive electrode 10 and the negative electrode 20. The electrode stack 100 can be a stacked electrode stack formed by cutting the positive electrode, separator, and negative electrode into certain sizes and then stacking them.

[0045] The positive electrode 10 can be formed, for example, as a structure in which positive electrode active material layers 12a and 12b are formed on one or both surfaces of the positive electrode current collector 14, wherein the positive electrode active material layers include a central portion 12a with a relatively constant thickness and a sliding portion 12b with a reduced thickness. Furthermore, the positive electrode 10 includes a positive electrode contact 16 for electrical connection to an external power source.

[0046] The positive electrode 10 of the lithium secondary battery is prepared by applying a positive electrode slurry to one or both surfaces of the positive electrode current collector 14, removing the solvent from the positive electrode slurry by a drying process, and then rolling. The positive electrode slurry is prepared by dispersing the positive electrode active material, conductive agent, and binder in a solvent such as dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water. In the case where the positive electrode active material layer is formed by the wet coating method described above, the amount of positive electrode slurry coated at the end portion of the positive electrode active material layer is reduced to form a positive electrode sliding portion 12b.

[0047] As the positive electrode current collector 14, various positive electrode current collectors used in the art can be used. For example, as the positive electrode current collector, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used. The positive electrode current collector generally can have a thickness of 3 μm to 500 μm, and micro-concavities and protrusions can be formed on the surface of the current collector to improve the adhesion of the positive electrode active material. The positive electrode current collector can be used in various shapes, such as in the shapes of a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0048] The positive electrode active material layers 12a and 12b can include a positive electrode active material, a conductive agent, and a binder.

[0049] The positive electrode active material is a compound capable of reversibly inserting and extracting lithium. Among them, various positive electrode active materials used in the art can be used, and their types are not particularly limited. For example, lithium iron phosphate-based oxides (e.g., LiFe 1-x M x PO4, 0 ≤ x < 1), lithium manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium cobalt-based oxides (e.g., LiCoO2, etc.), lithium nickel-based oxides (e.g., LiNiO2, etc.), lithium nickel manganese-based oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2)), lithium nickel cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1)), lithium manganese cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2)), lithium nickel manganese cobalt-based oxides (e.g., Li(Ni p1 Co q1 Mn r1 )O2 (where 0 < p1 < 1, 0 < q1 < 1, 0 < r1 < 1, and p1 + q1 + r1 = 1) or Li(Ni p2 Co q2 Mn r2 )O4 (where 0 < p2 < 2, 0 < q2 < 2, 0 < r2 < 2, and p2 + q2 + r2 = 2)), or lithium nickel cobalt transition metal (M) oxides (e.g., Li(Ni p3 Co q3 Mn r3 M s3)O2 (where M is selected from the group consisting of aluminum (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and p3, q3, r3, and s3 are the atomic fractions of each independent element, where 0 < p3 < 1, 0 < q3 < 1, 0 < r3 < 1, 0 < s3 < 1, and p2 + q2 + r3 + s2 = 1)), or a combination thereof can be used as a positive electrode active material.

[0050] Based on the total weight of the positive electrode active material layer, the positive electrode active material can be included in an amount of 80 wt% to 99 wt%, preferably 85 wt% to 99 wt%, and more preferably 90 wt% to 99 wt%.

[0051] A conductive agent is used to provide conductivity to the electrode. Any conductive agent can be used without particular limitation as long as it has suitable electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of the conductive agent can 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 cracking carbon black, carbon fiber, and carbon nanotube; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the positive electrode active material layer, the conductive agent can usually be included in an amount of 1 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%.

[0052] The binder improves the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the binder can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the positive electrode active material layer, the binder can be included in an amount of 1 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%.

[0053] Next, the negative electrode 20 can be formed as a structure in which negative electrode active material layers 22a and 22b are formed on one or both surfaces of the negative electrode current collector 24, wherein the negative electrode active material layers include a central portion 22a with a relatively constant thickness and a sliding portion 22b with a reduced thickness. Furthermore, the negative electrode 20 includes a negative electrode contact 26 for electrical connection to an external power source.

[0054] The negative electrode 20 of the lithium secondary battery is prepared by applying a negative electrode slurry to one or both surfaces of the negative electrode current collector 24, removing the solvent from the negative electrode slurry through a drying process, and then rolling. The negative electrode slurry is prepared by dispersing the negative electrode active material, conductive agent, and binder in a solvent such as dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, or water. In cases where the negative electrode active material layer is formed by a wet coating method as described above, the amount of negative electrode slurry coated at the end portion of the negative electrode active material layer is reduced, forming a negative electrode sliding portion 22b. Although it varies depending on the negative electrode loading and coating conditions, the negative electrode sliding portion 22b is generally formed to be longer than the positive electrode sliding portion 12b.

[0055] As the negative electrode current collector, commonly used negative electrode current collectors in the art can be used, and materials such as copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. The negative electrode current collector typically has a thickness from 3 μm to 500 μm, and similar to the positive electrode current collector, micro-protrusions can be formed on the surface of the current collector to improve the adhesion of the negative electrode active material. The negative electrode current collector can be used in various shapes, such as membranes, sheets, foils, meshes, porous bodies, foams, nonwoven fabrics, etc.

[0056] The negative electrode active material layer may include a negative electrode active material, a conductive agent, and a binder.

[0057] As the negative electrode active material, compounds commonly used in the art that enable reversible insertion and extraction of lithium can be used, and their type is not particularly limited. Specific examples of negative electrode active materials can be: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; silicon-based materials, such as silicon (Si), Si-Me alloys (wherein Me is selected from at least one of the group consisting of aluminum (Al), tin (Sn), magnesium (Mg), copper (Cu), iron (Fe), lead (Pb), zinc (Zn), manganese (Mn), chromium (Cr), titanium (Ti), and nickel (Ni), and SiO2. y(where, 0 < y < 2) and Si-C composite materials; lithium metal thin films; and metal materials capable of alloying with lithium, such as Sn and Al, and any one of them or a mixture of two or more of them can be used.

[0058] Based on the total weight of the negative electrode active material layer, the negative electrode active material can be included in an amount of 80 wt% to 99 wt%, preferably 85 wt% to 99 wt%, and more preferably 90 wt% to 99 wt%.

[0059] The conductive agent is used to provide conductivity to the negative electrode. Among them, any conductive agent can be used without particular limitation as long as it has appropriate electronic conductivity and does not cause chemical changes in the battery. Specific examples of the conductive agent can 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 cracking carbon black, carbon fiber, and carbon nanotube; powders or fibers of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the negative electrode active material layer, the conductive agent can usually be included in an amount of 1 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%.

[0060] The binder improves the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Specific examples of the binder can be polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more of them can be used. Based on the total weight of the negative electrode active material layer, the binder can be included in an amount of 1 wt% to 30 wt%, preferably 1 wt% to 20 wt%, and more preferably 1 wt% to 10 wt%.

[0061] Next, separator 30 separates the negative and positive electrodes and provides a path for lithium ion movement. Any separator can be used without particular limitation, as long as it is typically used in lithium secondary batteries. Specifically, a porous polymer membrane or a laminated structure having two or more layers can be used as the separator. The porous polymer membrane is, for example, a porous polymer membrane prepared from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. Alternatively, typical porous nonwoven fabrics can be used, such as nonwoven fabrics formed from high-melting-point glass fibers or polyethylene terephthalate fibers. Furthermore, coated separators comprising ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength.

[0062] As in this invention, when the electrode sliding portion includes a reduced thickness of the active material layer at the ends of the positive and negative electrodes, the lithium mobility is lower than that of the central portion of the electrode because the separator and the electrode active material layer are not tightly attached to each other in the electrode sliding portion. Consequently, lithium plating occurs, in which lithium ions generated in the positive electrode during discharge are not intercalated into the negative electrode and are deposited on the surface of the negative electrode. Furthermore, with an increase in the total length of the electrode assembly, the pressure at the ends of the electrode assembly decreases, further reducing the adhesion of the separator at the ends of the electrode assembly. Moreover, with repeated charging and discharging consuming the electrolyte, the electrolyte may not be able to contact the electrode sliding portion, potentially accelerating battery performance degradation and lithium plating.

[0063] Furthermore, when multiple cells are stacked and assembled into a module, the pressure is not properly transmitted to the end portions of the cells due to the accumulation and further increase of the thickness difference between the end portions and the central portion of the cells, and thus further exacerbates lithium deposition.

[0064] The present invention addresses the problem that, since the first fixing member 200a is configured to overlap with the positive electrode sliding portion 12b, and the thickness of the first fixing member 200a is manufactured to be greater than the thickness of the second fixing member 200b disposed in a region not overlapping with the positive electrode sliding portion 12b, the adhesion between the positive electrode sliding portion 12b and the separator 30 is improved to minimize the reduction in lithium mobility due to poor interfacial adhesion between the electrode sliding portion and the separator. Furthermore, the reduction in thickness of the end portion of the cell during module assembly is compensated so that pressure can be transmitted relatively uniformly to the entire electrode assembly in the module, and thus, expansion and lithium deposition can be minimized.

[0065] In this case, it is expected that the difference between the thickness T1 of the first fixing member and the thickness T2 of the second fixing member, i.e., T1-T2, will satisfy the following [Formula 1].

[0066] [Formula 1]

[0067] 0 <T1-T2≤0.5T C

[0068] In [Formula 1], T1 is the thickness of the first fixing member, T2 is the thickness of the second fixing member, and T C It is the thickness of the positive electrode.

[0069] When T1-T2 is 0 or less, there is no effect on the reduction in thickness of the end portion of the cell relative to the central portion of the cell, or the reduction in thickness may actually increase, thus exacerbating lithium plating, and when T1-T2 is greater than 0.5T... C In this case, because the thickness of the portion of the unit corresponding to the sliding part becomes greater than the thickness of the central portion of the unit, the external pressure on the module frame during cycling increases, which may lead to frame breakage due to uneven pressure. T1-T2 can preferably be between 0.01T. C Up to 0.5T C More preferably at 0.2T C Up to 0.3T C Within the range.

[0070] Specifically, the thickness T1 of the first fixing member can be the thickness T of the positive electrode. C The thickness T2 of the second fixing member is 0.2 to 0.5 times, preferably 0.25 to 0.45 times, and more preferably 0.3 to 0.4 times, and the thickness T2 of the second fixing member can be the thickness T of the positive electrode. C The thickness is 0.05 to 0.2 times, preferably 0.1 to 0.15 times, and more preferably 0.12 to 0.14 times. When the thickness of the first fixing member and the thickness of the second fixing member meet the above range, the reduction in thickness of the end portion of the unit is appropriately compensated, so that the pressure can be transmitted relatively uniformly to the entire electrode assembly in the module.

[0071] The thickness T of the positive electrode CThe thickness can vary depending on the capacity of the final battery, but it can be in the range of, for example, 100 μm to 200 μm, 120 μm to 180 μm, or 140 μm to 160 μm. In this case, the thickness of the positive electrode refers to the thickness after rolling. If the thickness of the positive electrode is too high, the energy density of the battery may decrease, and if the thickness of the positive electrode is too low, the coating processability of the positive electrode active material layer is reduced, the positive electrode active material may crack significantly during electrode rolling, and therefore, side reactions with the electrolyte solution increase, thereby increasing gas generation and potentially degrading lifetime characteristics.

[0072] The first fixing member 200a can be configured to overlap with at least a portion of the region corresponding to the negative electrode sliding portion 22b. When the first fixing member 200a is configured to overlap with at least a portion of the negative electrode sliding portion, the interfacial adhesion between the positive electrode and the separator, as well as the interfacial adhesion between the negative electrode and the separator, are both increased, thus the effect of suppressing lithium plating and performance degradation can be even better.

[0073] The ratio (L / W) of the total length L of the electrode stack can be 3 or greater, and preferably within the range of 3 to 7, more preferably 4 to 6. In this case, the total width and total length refer to the length in the width direction and the length direction of the component with the largest size among the positive electrode, negative electrode, and separator constituting the electrode stack, respectively. When the ratio of the total length to the total width of the electrode stack is 3 or greater, there is an advantage in achieving high capacity. However, when the total length is formed to be large relative to the total width as described above, the cell pressure at the end portion in the total length direction may decrease, which may lead to a problem where the electrolyte permeability at the end portion may decrease when the electrolyte is consumed due to charging and discharging. However, this problem can be solved when the first fixing member is arranged to overlap with the positive electrode sliding portion as in the present invention, because the pressure is maintained relatively uniformly up to the end of the electrode stack.

[0074] Specifically, the electrode stack according to the invention may have a total width of 50 mm to 200 mm, preferably 70 mm to 200 mm and more preferably 70 mm to 150 mm, and may have a total length of 200 mm to 1,000 mm, preferably 300 mm to 800 mm and more preferably 400 mm to 600 mm.

[0075] The fixing members 200a and 200b may include a porous structure. When the fixing members include a porous structure, the electrolyte can be impregnated into the electrode stack through the fixing members, thus preventing a decrease in electrolyte permeability of the electrode stack due to the fixing members. Specifically, the fixing members 200a and 200b may be, but are not limited to, a terminating band on one surface of a polymer substrate layer having an adhesive layer formed thereon. The polymer material may be, for example, polyethylene terephthalate (PET), polyvinyl chloride (PVC), or polyethylene (PE), but is not limited to these.

[0076] It is desirable that the fixing members 200a and 200b have a width of approximately 10 mm to 50 mm or approximately 20 mm to 40 mm in the total width direction of the electrode stack. If the width of the fixing members 200a and 200b is too large, the contact area with the electrolyte will be reduced due to the increased outer surface area of ​​the electrode stack 100 covered by the fixing members 200a and 200b, and thus the electrolyte permeability may be reduced. Conversely, if the width of the fixing members 200a and 200b is too small, the effect of fixing the electrode stack may be reduced.

[0077] The electrode assembly according to the invention may include 2 to 10 fixing members, preferably 2 to 8 fixing members, and more preferably 3 to 7 fixing members. In this case, the fixing members may be arranged in a horizontally symmetrical position along the total length direction, and preferably, the fixing members may be arranged to be spaced apart from each other at equal intervals. When multiple fixing members are included and arranged as described above, the electrode stack having a long unit structure with a relatively long length can be securely fixed, and pressure variations depending on the position of the electrode stack can be prevented.

[0078] Next, the lithium secondary battery according to the present invention will be described.

[0079] The lithium secondary battery according to the present invention includes an electrode assembly according to the present invention, an electrolyte, and a battery case housing the electrode assembly and the electrolyte. Since the electrode assembly has already been described above, only the remaining components will be described below.

[0080] The lithium secondary battery according to the present invention can be prepared by accommodating an electrode assembly in a battery case, then injecting an electrolyte and sealing the battery case.

[0081] In this case, the battery box can be, for example, a pouch-type battery box.

[0082] The pouch-type battery box may include a barrier layer, a substrate layer disposed on one surface of the barrier layer, a sealant layer disposed on another surface of the barrier layer, and at least one cup-shaped portion recessed in one direction.

[0083] Specifically, the pouch-type battery box can be prepared by inserting a flexible pouch film laminate in which a substrate layer, a barrier layer and a sealant layer are sequentially laminated into a compression molding apparatus and stretching the pouch film laminate by applying pressure to a portion of the pouch film laminate with a punch to form a cup-shaped portion having a concave shape in one direction.

[0084] In this case, the substrate layer is set as the outermost layer of the pouch-type battery box to protect the electrode assembly from external impacts and to electrically insulate the electrode assembly.

[0085] The substrate layer may be formed of a polymeric material, and for example, may be formed of at least one polymeric material selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylenebenzobisoxazole), polyarylate and Teflon.

[0086] The substrate layer can have a single-layer structure or a multi-layer structure in which different polymer films are laminated. In the case of a multi-layer substrate layer, an adhesive layer can be provided between the polymer films.

[0087] For example, the substrate layer may have a laminated structure of polyethylene terephthalate (PET) film and nylon film. In this case, it is desirable that the nylon film is disposed on the barrier layer side, i.e., the inner side, and the polyethylene terephthalate film is disposed on the surface side of the bag.

[0088] Because polyethylene terephthalate (PET) possesses excellent durability and electrical insulation properties, its durability and insulation are excellent when applied to the surface side. However, regarding PET films, due to their weaker adhesion to the aluminum alloy film constituting the barrier layer and their different stretching behavior, delamination between the substrate layer and the barrier layer may occur during the forming process when the PET film is applied to the barrier layer side. Furthermore, the barrier layer may not be stretched uniformly, reducing formability. Conversely, because nylon films exhibit similar stretching behavior to the aluminum alloy film constituting the barrier layer, improved formability can be achieved when a nylon film is applied between the PET and the barrier layer.

[0089] The polyethylene terephthalate (PET) film can have a thickness of 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 7 μm to 15 μm, while the nylon film can have a thickness of 10 μm to 40 μm, preferably 10 μm to 35 μm, and more preferably 15 μm to 25 μm. When the thicknesses of the PET film and the nylon film meet the above ranges, the formability and stiffness after molding are excellent.

[0090] The substrate layer can have a total thickness of 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. In the case of a multilayer substrate layer, the thickness includes the adhesive layer. When the substrate layer meets the above ranges, durability, insulation properties, and formability are excellent. If the substrate layer is too thin, durability may decrease, and the substrate layer may be damaged during the molding process; if the substrate layer is too thick, formability may decrease, the total thickness of the bag may increase, and the battery housing space may decrease, thus reducing energy density.

[0091] The barrier layer is used to ensure the mechanical strength of the pouch battery box, prevent gas or moisture from entering and leaving the secondary battery from the outside, and prevent electrolyte leakage.

[0092] The barrier layer can be formed of a metallic material, and for example, it can be formed of an aluminum alloy film. In this case, the aluminum alloy film may include aluminum and metallic elements other than aluminum, such as at least one selected from the group consisting of iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0093] The barrier layer can have a thickness of 40 μm to 100 μm, more preferably 50 μm to 80 μm, and even more preferably 60 μm to 80 μm. When the thickness of the barrier layer meets the above range, the formability is improved to increase the forming depth of the cup-shaped portion, or the occurrence of cracks and / or pinholes is reduced even when forming two cups, thereby improving resistance to external stress after forming.

[0094] The sealant layer is bonded by thermocompression to seal the pouch-type battery box, wherein the sealant layer is disposed on the inner layer of the battery box.

[0095] Since the sealant layer is the surface that comes into contact with the electrolyte and electrode assembly after the bag is formed, the sealant layer must have insulating properties and corrosion resistance, and since the sealant layer must completely seal the interior to prevent material from moving between the inside and outside, the sealant layer must have high sealing properties.

[0096] The sealant layer can be formed of a polymer material, and for example, it can be formed of at least one selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylenebenzobisoxazole), polyarylate, and Teflon, and particularly preferred is polypropylene (PP) which has excellent mechanical properties such as tensile strength, stiffness, surface hardness, abrasion resistance, and heat resistance, as well as chemical properties such as corrosion resistance.

[0097] More specifically, the sealant layer may include polypropylene, cast polypropylene (CPP), acid-modified polypropylene, polypropylene-butene-ethylene copolymer, or combinations thereof.

[0098] The sealant layer can have a single-layer structure or a multi-layer structure comprising two or more layers formed of different polymer materials.

[0099] The sealant layer can have a total thickness of 60 μm to 100 μm, preferably 60 μm to 90 μm, and more preferably 70 μm to 90 μm. If the sealant layer is too thin, the sealing durability and insulation properties may be reduced, and if the sealant layer is too thick, the volumetric energy density may be reduced due to reduced flexibility and increased total thickness of the bag film laminate.

[0100] Next, an electrolyte is used to move lithium ions generated by the electrochemical reaction of the electrodes during the charging and discharging of the secondary battery. Various electrolytes known in the art for lithium secondary batteries can be used, and their types are not particularly limited.

[0101] For example, electrolytes can include organic solvents and lithium salts.

[0102] Any organic solvent can be used without particular limitation, as long as it can serve as a medium through which the ions involved in the electrochemical reactions of the battery can move. Specifically, the following can be used as organic solvents: ester solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents, such as dibutyl ether or tetrahydrofuran; ketone solvents, such as cyclohexanone; aromatic hydrocarbon solvents, such as benzene and fluorobenzene; carbonate solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents, such as ethanol and isopropanol; nitriles, such as R-CN (where R is a linear, branched, or cyclic C2-C20 hydrocarbon group, and may include double-bonded aromatic rings or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Among these solvents, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant that can improve the charge / discharge performance of the battery and low viscosity linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) are even more preferred.

[0103] Lithium salts can be used without particular limitation, as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used as lithium salts. Lithium salts can be used in concentrations ranging from 0.1M to 5.0M, preferably from 0.1M to 3.0M. If the concentration of the lithium salt is within the above range, excellent electrolyte performance can be obtained because the electrolyte can have suitable conductivity and viscosity, and lithium ions can move efficiently.

[0104] In order to improve battery life characteristics, suppress battery capacity reduction, and improve battery discharge capacity, additives may be included in the electrolyte in addition to electrolyte components.

[0105] The invention will be described in more detail below with reference to specific examples.

[0106] Example

[0107] A positive electrode with dimensions of 70 mm x 500 mm, a negative electrode with dimensions of 75 mm x 505 mm, and a separator with dimensions of 80 mm x 510 mm are prepared, and the positive electrode, negative electrode, and separator are stacked in the order of positive electrode / separator / negative electrode to form an electrode stack.

[0108] In this case, the positive electrode is prepared by coating the two surfaces of an aluminum current collector with a thickness of 12 μm and a length of 500 mm with a positive electrode slurry, such that the total length of the positive electrode active material layer is 490 mm, and then drying and rolling the coated aluminum current collector, and forming a positive electrode sliding portion with a length of 8 mm at both ends of the positive electrode active material layer, and the total thickness of the positive electrode is 162 μm.

[0109] In addition, the negative electrode is prepared by coating the two surfaces of a copper current collector with a thickness of 8 μm and a length of 505 mm with a negative electrode slurry, so that the total length of the negative electrode active material layer is 495 mm, and then drying and rolling the coated copper current collector, and forming a negative electrode sliding portion with a length of 10 mm at both ends of the negative electrode active material layer, and the total thickness of the negative electrode is 215 μm.

[0110] Next, two first fixing members and four second fixing members are wound around the outer surface of the electrode stack along the total width direction to fabricate the electrode assembly. In this case, the two ends of the first fixing members are arranged 5 mm away from the two end portions of the electrode stack, and the second fixing members are arranged at equal intervals between the first fixing members. A terminating strip (PET, DaehyunST) with a width of 30 mm and a thickness of 47 μm is used as the first fixing member, and a terminating strip (PET, DaehyunST) with a width of 30 mm and a thickness of 22 μm is used as the second fixing member.

[0111] Comparison Example 1

[0112] The electrode assembly was prepared in the same manner as in the example, except that the first fixing member was not used and six second fixing members were wound around the outer surface of the electrode stack along the total width direction to prepare the electrode assembly.

[0113] Comparison Example 2

[0114] The electrode assembly was prepared in the same manner as in the example, except that the two ends of the first fixing member were arranged 30 mm away from the two end portions of the electrode stack and three second fixing members were used.

[0115] Experimental Example 1

[0116] Lithium secondary battery cells are prepared by housing each electrode assembly in the example and comparative examples in a pouch battery case, injecting an electrolyte solution, and then sealing the pouch battery case. In the pouch battery case, nylon / polyethylene terephthalate / Al alloy film / polypropylene are sequentially laminated and a cup-shaped portion is formed.

[0117] Next, after defining a stack of four lithium-ion battery cells as a group and stacking a total of four groups on polyurethane pads, the polyurethane pads were stacked as the top layer to prepare a cell-pad assembly comprising 16 lithium-ion batteries and 5 polyurethane pads. The cell-pad assembly was then inserted into a module frame to prepare the module. After setting the charging of the lithium-ion battery module at 0.33C to 4.22V and the discharging of the lithium-ion battery module at 0.33C to 2.5V as one cycle and repeating this charge and discharge cycle 800 times, the electrode stacks were separated and the presence of lithium plating was visually confirmed.

[0118] The measurement results are presented in Table 1 below and Figures 3 to 5 In the middle, the condition where lithium plating is not visually observed is indicated as O, and the condition where lithium plating is observed is indicated as X.

[0119] [Table 1]

[0120] Example O Comparison Example 1 X Comparison Example 2 X

[0121] According to [Table 1], it can be confirmed that lithium plating did not occur in the lithium secondary battery using the electrode assembly of the example, but lithium plating occurred in the lithium secondary battery using the electrode assemblies of Comparative Example 1 and Comparative Example 2.

[0122] <Description of reference numerals> 1: Electrode assembly

[0123] 10: Positive electrode

[0124] 20: Negative electrode

[0125] 30: Divider

[0126] 100: Electrode stack

[0127] 200a: First fixing component; 200b: Second fixing component; 300: Electrolyte

Claims

1. An electrode assembly, comprising: An electrode stack, comprising a positive electrode containing a positive active material layer, a negative electrode containing a negative active material layer, and a separator disposed between the positive electrode and the negative electrode; and At least one fixing member, said fixing member fixing the electrode stack by wrapping the electrode stack around itself along its total width direction. The positive electrode includes a positive electrode sliding portion with a reduced thickness in the positive electrode active material layer. The fixing component includes a first fixing component that overlaps with the positive electrode sliding portion and a second fixing component that does not overlap with the sliding portion, and The thickness of the first fixing member is greater than the thickness of the second fixing member.

2. The electrode assembly according to claim 1, wherein, When the thickness of the first fixing member is T1, the thickness of the second fixing member is T2, and the thickness of the positive electrode is T... C At that time, the electrode assembly satisfies [Formula 1], [Formula 1] 0 < T1-T2≤ 0.5T C 。 3. The electrode assembly according to claim 1, wherein, The negative electrode includes a negative electrode sliding portion with a reduced thickness of the negative electrode active material layer.

4. The electrode assembly according to claim 1, wherein, The ratio (L / W) of the total length (L) to the total width (W) of the electrode stack is 3 or greater.

5. The electrode assembly according to claim 1, wherein, The ratio of the total length (L) to the total width (W) of the electrode stack is in the range of 3 to 7.

6. The electrode assembly according to claim 1, wherein, The electrode assembly includes 2 to 10 fixing components, and The fixing components are arranged in horizontally symmetrical positions along the total length direction.

7. The electrode assembly according to claim 6, wherein, The fixing members are arranged to be separated from each other at equal intervals.

8. The electrode assembly according to claim 1, wherein, The fixing component includes a porous structure.

9. The electrode assembly according to claim 1, wherein, The fixing member is a strip on one surface of a substrate with an adhesive layer formed thereon.

10. The electrode assembly according to claim 1, wherein, The fixing member has a width of 10 mm to 50 mm.

11. A lithium secondary battery, comprising: An electrode assembly according to any one of claims 1 to 10; an electrolyte; and a battery case, the battery case housing the electrode assembly and the electrolyte.

12. The lithium secondary battery according to claim 11, wherein, The battery box is a pouch-type battery box.

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