Method for measuring the resistance of the negative electrode of a secondary battery and a secondary battery capable of measuring the resistance of the negative electrode

Abstract

The negative electrode included in the secondary battery according to the present invention is a freestanding type containing a negative electrode active material and without a separate current collector, and the negative electrode includes a first negative electrode tab and a second negative electrode tab extending in opposite directions to each other. The resistance of the negative electrode alone can be measured independently without disassembling the secondary battery, and the resistance of the negative electrode can be accurately measured. Furthermore, the degree of deterioration of the negative electrode can be determined by measuring the resistance of the negative electrode in the secondary battery.

Description

Technical Field

[0001] The present invention relates to a method for measuring the resistance of a negative electrode for a secondary battery and a secondary battery capable of measuring the resistance of the negative electrode.

[0002] This application claims priority based on Korean Patent Application No. 10-2023-0112035 filed on August 25, 2023, and all the contents disclosed in the specification and drawings of the application are incorporated into this application.

Background Art

[0003] A secondary battery capable of repeatedly charging and discharging has been in the spotlight as a means to replace fossil energy. Secondary batteries have mainly been used in traditional handheld devices such as mobile phones, video cameras, and power tools. However, in recent years, their application fields have been gradually expanding to electric vehicles (EV, HEV, PHEV), large-capacity power storage devices (ESS), uninterruptible power supply systems (UPS), etc.

[0004] A secondary battery includes an electrode assembly including a positive electrode, a negative electrode, and a separator sandwiched therebetween, an active material coated on the positive electrode and the negative electrode, and an electrolyte that electrochemically reacts. A lithium-ion secondary battery in which lithium ions act as working ions to cause an electrochemical reaction at the positive electrode and the negative electrode during charging and discharging is a representative example. In existing lithium-ion secondary batteries, lamination has been applied during the assembly process to realize the adhesive force between the electrodes and the separator in the electrode assembly. Lamination is a process of bonding the separator and the electrode by heat. Lamination heats and adheres the separator and the electrode laminated vertically, and as a result, increases the adhesive force between the separator and the electrode. The rough surface shape of an existing electrode composed of an active material, a conductive material, and a binder makes it easy to form an adhesion between the electrode and the separator by performing lamination with the separator.

[0005] Recently, as part of efforts to improve the energy density of lithium-ion secondary batteries, considerable attention has been drawn to the need to develop next-generation secondary batteries that directly utilize lithium metal thin films as the negative electrode. Lithium metal has a high tendency to ionize, low density, and extremely low potential at the standard electrode, resulting in extremely high specific capacity. When problems such as internal short circuits due to the growth of lithium dendrites and the risk of explosion due to exposure to moisture are solved, lithium metal has the advantage of being able to obtain the highest energy density, making it worthwhile to continue research into it.

[0006] In the case of lithium-ion secondary batteries, performance deteriorates further as the usage period increases compared to the initial state. In particular, in lithium metal batteries that use a lithium metal thin film directly as the negative electrode, lithium metal easily forms large-area dendritic crystals (dendrites), which react with salts and additives in the electrolyte to form a solid electrolyte interface (SEI), continuously consuming the salts and additives in the electrolyte, and as a result, accelerating battery degradation.

[0007] For this reason, various research efforts are being undertaken to estimate the lifespan or degradation of lithium metal batteries. It is known that the resistance of lithium metal batteries is related to changes in the chemical composition and physical structure of the lithium metal. It is thought that by using this relationship and observing the resistance of the negative electrode of a lithium metal battery in operation, the degradation of the lithium metal battery can be estimated. However, in the structure of a typical lithium metal battery, there is currently no technology to independently measure the resistance of only the lithium metal negative electrode in operation, and therefore, efforts are needed to develop such technology. [Overview of the project] [Problems that the invention aims to solve]

[0008] Therefore, the problem that the present invention aims to solve is to provide a secondary battery and a method for measuring the same that can measure the resistance of the negative electrode while the secondary battery is in operation and determine the degree of degradation. [Means for solving the problem]

[0009] According to one aspect of the present invention, a secondary battery in the following form is provided.

[0010] A secondary battery according to the first embodiment is In a secondary battery comprising a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode, The aforementioned negative electrode is a free-standing type containing a negative electrode active material and not having a separate current collector. The negative electrode active material comprises at least one lithium-based material selected from lithium metal and lithium alloys. The negative electrode includes a first negative electrode tab and a second negative electrode tab, each extending in opposite directions.

[0011] A second aspect is, in the first aspect, The positive electrode has a positive electrode active material layer formed on at least one side of the positive electrode current collector, and the positive electrode active material layer may contain a sulfur-carbon composite and a binder polymer.

[0012] The third aspect is the first aspect or the second aspect, The positive electrode may include a positive electrode tab.

[0013] The fourth aspect is an embodiment of any one of the first to third aspects, The negative electrode may be composed solely of lithium-based materials, at least one of lithium metals and lithium alloys.

[0014] The fifth aspect is an embodiment of any one of the first to fourth aspects, The first negative electrode tab and the second negative electrode tab may extend outward from the respective ends of both ends in the width direction or both ends in the longitudinal direction of the negative electrode.

[0015] Aspect 6 is in any one of Aspect 1 to Aspect 5, the first negative electrode tab and the second negative electrode tab may be located in a region that is line-symmetric about the negative electrode.

[0016] Aspect 7 is in any one of Aspect 1 to Aspect 6, the first negative electrode tab and the second negative electrode tab may be located in a region that is point-symmetric about the negative electrode.

[0017] Aspect 8 is in any one of Aspect 1 to Aspect 7, the first negative electrode tab and the second negative electrode tab may be configured such that a resistance measuring device for measuring the resistance of the negative electrode can be connected.

[0018] Aspect 9 is in any one of Aspect 1 to Aspect 8, the secondary battery may be such that by connecting a resistance measuring device to the first negative electrode tab and the second negative electrode tab, the electrical degradation degree of the negative electrode and the secondary battery can be observed.

[0019] Aspect 10 is in any one of Aspect 1 to Aspect 9, the positive electrode is one in which a positive electrode active material layer is formed on at least one side of a positive electrode current collector, and the loading amount of the negative electrode may be 1 to 2.5 times the loading amount of the positive electrode active material.

[0020] The method for measuring the resistance of the negative electrode according to Aspect 11 is a method for measuring the negative electrode resistance of a secondary battery according to any one of Aspect 1 to Aspect 10, including the step of connecting a resistance measuring device to the first negative electrode tab and the second negative electrode tab and measuring the resistance of the negative electrode.

[0021] Aspect 12 is in Aspect 11, The measurement of the resistance can be carried out using electrochemical impedance spectroscopy.

[0022] A 13th aspect is in the 11th aspect or the 12th aspect, The measurement of the resistance can be carried out under the condition of a voltage amplitude of 0.01 mV to 0.5 mV.

[0023] A 14th aspect is in any one of the 11th aspect to the 13th aspect, The measurement of the resistance can be carried out under the condition of a frequency of 100 Hz to 1000 Hz.

[0024] A 15th aspect is in any one of the 11th aspect to the 14th aspect, The measurement of the resistance can be carried out every time the state of charge (SOC) of the secondary battery changes by 5% to 20%.

Advantages of the Invention

[0025] Conventionally, in a secondary battery, in order to measure the resistance of the negative electrode, the secondary battery had to be disassembled to separately obtain the negative electrode before the resistance of the negative electrode could be measured. However, since the secondary battery according to an embodiment of the present invention includes a first negative electrode tab and a second negative electrode tab that extend in opposite directions, the resistance of only the negative electrode can be measured alone without disassembling the secondary battery to remove the negative electrode.

[0026] Also, the secondary battery according to an embodiment of the present invention can measure the electrical resistance between the first negative electrode tab and the second negative electrode tab in both directions during driving to observe the degree of electrical deterioration of the negative electrode, and from this, the degree of deterioration of the secondary battery can be estimated. The method for measuring the resistance of the negative electrode for the secondary battery according to the present invention can measure the resistance of only the negative electrode alone without disassembling the secondary battery.

[0027] The drawings accompanying this specification illustrate preferred embodiments of the present invention and serve to further illustrate the technical idea of ​​the invention along with its content; therefore, the present invention is not to be construed as being limited solely to what is depicted in the drawings. On the other hand, the shapes, sizes, scales, or ratios of elements in the drawings accompanying this specification may be exaggerated to emphasize a clearer explanation. [Brief explanation of the drawing]

[0028] [Figure 1] This is a schematic representation of the negative electrode of a conventional secondary battery. [Figure 2] This is a schematic representation of the negative electrode of a secondary battery according to one embodiment of the present invention. [Figure 3] This is a schematic representation of the negative electrode of a secondary battery according to one embodiment of the present invention. [Figure 4] This is a schematic representation of the negative electrode of a secondary battery according to one embodiment of the present invention. [Figure 5] This is a schematic representation of the negative and positive electrodes of a secondary battery according to one embodiment of the present invention. [Figure 6] This graph shows the change in resistance during one charge-discharge cycle of a secondary battery according to Example 1 of the present invention. [Figure 7] This graph shows the capacity retention rate of a secondary battery according to Example 1 of the present invention. [Figure 8] This graph shows the change in resistance of a secondary battery according to Example 1 of the present invention. [Modes for carrying out the invention]

[0029] The present invention will now be described in detail based on the attached drawings. The terms and words used in this specification and the claims are not to be interpreted in their ordinary or dictionary sense, but rather in accordance with the principle that the inventor may appropriately define the concepts of terms in order to best describe the invention, and are to be interpreted in the sense and concepts corresponding to the technical idea of ​​the present invention.

[0030] Therefore, the embodiments described herein and the configurations shown in the drawings represent only preferred embodiments of the present invention and do not represent the entire technical concept of the invention. It should be understood that there are various equivalent and modified embodiments that can be substituted for these at the time of filing this application.

[0031] Furthermore, throughout the specification, when a part of a specification is described as "include," "comprise," or "have" a certain component, this does not mean that other components are excluded, but rather that other components may be included, unless otherwise specified.

[0032] Throughout this specification, the phrase "A and / or B" means "A or B or both."

[0033] Throughout this specification, unless otherwise specified, temperatures refer to Celsius temperatures, and the unit is °C.

[0034] The first aspect of this invention relates to a secondary battery.

[0035] A secondary battery according to one aspect of the present invention includes a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode. The aforementioned negative electrode is a free-standing type containing a negative electrode active material and not having a separate current collector. The negative electrode active material comprises at least one lithium-based material selected from lithium metal and lithium alloys. The negative electrode includes a first negative electrode tab and a second negative electrode tab, each extending in opposite directions.

[0036] In one embodiment of the present invention, the positive electrode may have a positive electrode active material layer formed on at least one side of the positive electrode current collector. The positive electrode active material layer may contain a positive electrode active material and a binder polymer, and may further selectively contain a conductive material.

[0037] In one embodiment of the present invention, the positive electrode active material may include a sulfur-carbon composite. The sulfur-carbon composite may be a sulfur-based material supported on a porous carbon material.

[0038] The sulfur-based substance may contain at least one of sulfur (S8) and sulfur-based compounds. Here, the sulfur-based compound may refer collectively to any substance containing the element sulfur (S). The sulfur-based compound may include, for example, any sulfur-containing compound that can be formed by the reduction reaction of inorganic sulfur (S8) or the oxidation reaction of lithium sulfide (Li2S), more specifically lithium sulfide (Li2S), lithium polysulfide (Li2S) x (where x is an integer between 2 and 8), disulfide compounds, carbon-sulfur polymers ((C2S y ) n (y = 2.5 to 50, n ≥ 2), lithium sulfide (Li2S), or two or more of these may be included.

[0039] The porous carbon material may be crystalline or amorphous carbon material, and may be conductive carbon. The porous carbon material provides a framework that can uniformly and stably fix sulfur-based substances, compensating for the low electrical conductivity of sulfur-based substances and enabling smooth electrochemical reactions.

[0040] The porous carbon material can be produced by carbonizing a wide variety of carbon-based precursors, and may contain irregularities and / or pores inside, with an average pore diameter of 1 nm to 200 nm, and a porosity of 10 vol% to 90 vol% of the total volume of the carbon material. When the average diameter of the carbon material satisfies the above range, it is possible to maintain the mechanical strength of the carbon material.

[0041] The porous carbon material can be used without limitation as long as it is in the form of a sphere, rod, needle, plate, tube, or bulk, or any other form commonly used in lithium-sulfur batteries. Furthermore, the carbon material may have a high specific surface area. For example, the carbon material may be graphite, graphene, Super P, carbon black, Denka black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, carbon nanofiber, carbon nanotube (SWCNT, MWCNT), carbon nanowire, carbon nanoring, carbon fabric, and fullerene (C 60 It may include at least one selected from the group consisting of ).

[0042] The sulfur-carbon composite, in which the sulfur-based substance is supported on a porous carbon material, may contain 60 wt% to 90 wt% of the sulfur-based substance per 100 wt% of the total. By keeping the sulfur-based substance content within this range, the content of the sulfur-based substance and the porous carbon material become appropriate, making it possible to use only the appropriate amount of binder polymer to bind the sulfur-based substance to the porous carbon material. Therefore, since only the appropriate amount of binder polymer, which can cause an increase in resistance, can be used, the performance of the battery can be improved.

[0043] In the sulfur-carbon composite, the sulfur-based substance may be located externally in such a way that it fills at least a portion of the internal space (e.g., pores) of the porous carbon material, or that it coats at least a portion of the surface of the porous carbon material together with or independently thereof. In one specific embodiment, the sulfur-based substance may be located at less than 100%, 1% to 95%, or 60% to 90% of the surface of the porous carbon material. When the sulfur-based substance is present within these ranges, the composite exhibits excellent wettability to an electrolyte and excellent electrical conductivity.

[0044] The method for producing the sulfur-carbon composite is not particularly limited, and methods commonly used in the industry can be employed. For example, a sulfur-carbon composite can be produced by mixing sulfur (S8) with a porous carbon material and heat-treating it.

[0045] The sulfur-carbon composite may be present in amounts of 50 wt% to 95 wt%, 60 wt% to 95 wt%, or 70 wt% to 90 wt% relative to 100 wt% of the total positive electrode active material layer. The presence of the sulfur-carbon composite within these ranges enables the positive electrode to undergo sufficient electrochemical reactions.

[0046] The aforementioned binder polymer holds the positive electrode active material in the positive electrode current collector and organically connects the positive electrode active materials to each other, further enhancing the binding force between them. Any binder polymer known in the industry can be used. For example, the binder polymer can be one, a mixture of two or more, or a copolymer selected from the group consisting of: a fluororesin-based binder containing polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); a rubber-based binder containing styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber, or styrene-isoprene rubber; a cellulose-based binder containing carboxyl methyl cellulose (CMC), starch, hydroxypropyl cellulose, or regenerated cellulose; a polyalcohol-based binder; a polyolefin-based binder containing polyethylene or polypropylene; a polyimide-based binder; a polyester-based binder; a polyacrylic-based binder; or a silane-based binder. The content of the binder polymer can be 0.5 wt% to 30 wt% relative to 100 wt% of the total positive electrode active material layer. When the content of the binder polymer satisfies this range, the physical properties of the positive electrode are improved, preventing the detachment of the active material and conductive material within the positive electrode. This allows for appropriate control of the ratio of active material to conductive material in the positive electrode, thereby ensuring the battery capacity.

[0047] The conductive material serves as a pathway to electrically connect the electrolyte and the positive electrode active material, allowing electrons to move from the positive electrode current collector to the positive electrode active material, and is a substance that improves electrical or ionic conductivity. Any conductive material can be used without limitation. For example, the conductive material can be carbon black such as Super-P, Denka Black, Acetylene Black, Ketjen Black, Channel Black, Furnace Black, Lamp Black, or Thermal Black; carbon derivatives such as carbon nanotubes, graphene, or fullerene; conductive fibers such as carbon fibers or metal fibers; metal powders such as carbon fluoride, aluminum, or nickel powder; or conductive polymers such as polyaniline, polythiophene, polyacetylene, or polypyrrole, either alone or in mixtures. The content of the conductive material can range from 0.01 wt% to 30 wt% based on a total of 100 wt% of the positive electrode active material layer. When the conductive material is included within the range described above, electrons can move more effectively from the positive electrode current collector to the positive electrode active material, and the ionic conductivity or electrical conductivity of the positive electrode can be appropriately maintained.

[0048] In one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it has high conductivity without causing a chemical change in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, silver, etc., and aluminum-cadmium alloys can be used.

[0049] In one embodiment of the present invention, the positive electrode may include one or more positive electrode tabs. The positive electrode tabs may contain the same material as the positive electrode current collector, or may consist solely of the same material as the positive electrode current collector.

[0050] The positive electrode tab may be formed on the positive electrode current collector. The positive electrode tab may be formed to extend outward from the positive electrode current collector.

[0051] In one embodiment of the present invention, the positive electrode tab may be formed by extending the positive electrode current collector itself. Alternatively, the positive electrode tab may be formed by electrically connecting a separate metal (or alloy) to the positive electrode current collector.

[0052] For example, a separate metal (or alloy) may be welded to the positive electrode current collector to form the positive electrode tab. The separate metal (or alloy) is not particularly limited as long as it has high conductivity without causing any chemical changes to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, silver, etc., and aluminum-cadmium alloys can be used.

[0053] In one embodiment of the present invention, the loading amount of the negative electrode may be 1 to 2.5 times the loading amount of the positive electrode.

[0054] The loading amount of the positive electrode active material can be 1 to 2.5 times that of the negative electrode active material. When the loading amounts of the positive and negative electrode active materials fall within this range, the secondary battery can have sufficient energy density and lifespan. Furthermore, when the negative electrode active material is contained within this range, the range of change in the negative electrode resistance is narrow, and the accuracy of resistance measurement can be increased.

[0055] The negative electrode according to the present invention is a freestanding type that includes a negative electrode active material and does not have a separate current collector. In one embodiment of the present invention, the negative electrode may consist only of the negative electrode active material.

[0056] In conventional secondary batteries, a negative electrode was used in which a negative electrode active material was coated on a negative electrode current collector. The negative electrode current collector was made of a material that does not cause chemical changes to the battery but has excellent conductivity, such as copper or stainless steel. However, when a voltage is applied to measure the resistance of such a negative electrode, the current flows through the negative electrode current collector, which has excellent conductivity. Therefore, the resistance measured at this time is essentially the resistance of the negative electrode current collector, which has excellent conductivity. The degree of degradation of the negative electrode is greatly influenced by the negative electrode active material into which lithium is inserted / removed. In the case of a negative electrode that includes a negative electrode current collector, as in conventional secondary batteries, even if the resistance of the negative electrode is measured, the resistance of the negative electrode current collector is measured, not the resistance of the negative electrode active material, so the degree of degradation of the negative electrode active material cannot be detected from the measured resistance. For this reason, the negative electrode included in the secondary battery according to the present invention is a freestanding type without a separate current collector, and the degree of degradation of the negative electrode active material can be determined by measuring the resistance of the negative electrode.

[0057] The negative electrode active material of the negative electrode according to the present invention includes at least one lithium-based material from among lithium metal and lithium alloys. The negative electrode may be composed solely of at least one lithium-based material from among lithium metal and lithium alloys.

[0058] The lithium alloy contains elements that can be alloyed with lithium, and examples of elements that can be alloyed with lithium include Si, Sn, C, Pt, Ir, Ni, Cu, Ti, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, or alloys thereof.

[0059] The negative electrode according to the present invention includes a first negative electrode tab and a second negative electrode tab.

[0060] Since the negative electrode of the present invention includes two or more negative electrode tabs, it is possible to measure the resistance of the negative electrode by connecting a resistance measuring device to different negative electrode tabs. The resistance is measured by applying current to one of the negative electrode tabs with the resistance measuring device and applying current from the other negative electrode tab to measure the resistance of the negative electrode.

[0061] However, if the distance between the negative electrode tabs is short, there is a risk that the current applied to one negative electrode tab may immediately flow to the other negative electrode tab, or that the current applied to one negative electrode tab may pass through only a portion of the negative electrode before flowing to the other negative electrode tab. Therefore, if the distance between the negative electrode tabs is short, there is a risk that the resistance of the negative electrode may not be measured accurately.

[0062] The first and second negative electrode tabs included in the negative electrode according to the present invention extend in opposite directions. Therefore, when current applied to one negative electrode tab flows out to the other negative electrode tab, it can flow through the maximum possible portion of the negative electrode, allowing for accurate measurement of the negative electrode's resistance and accurate determination of its degradation.

[0063] The fact that the first negative electrode tab and the second negative electrode tab extend in opposite directions means that the second negative electrode tab is provided on the opposite side of the negative electrode that does not come into contact with the side on which the first negative electrode tab is provided. For example, if the negative electrode has a rectangular parallelepiped structure, the first negative electrode tab may be provided on one side of the rectangular parallelepiped, and the second negative electrode tab may be provided on the opposite side that does not come into contact with the side on which the first negative electrode tab is provided, and that faces each other.

[0064] Figure 1 is a schematic representation of a negative electrode using conventional technology. The arrows in Figure 1 indicate the flow of current.

[0065] Referring to Figure 1, the first negative electrode tab 11 and the second negative electrode tab 12 extend in the same direction. In this case, the current applied to the first negative electrode tab 11 exits through only a portion of the negative electrode 10 when it exits through the second negative electrode tab 12. Therefore, it is not possible to accurately measure the overall resistance of the negative electrode 10, and there is a risk that the measured value will vary greatly each time the resistance is measured.

[0066] Figure 2 is a schematic representation of the negative electrode according to one embodiment of the present invention. The arrows in Figure 2 indicate the flow of current.

[0067] Referring to Figure 2, the first negative electrode tab 11 and the second negative electrode tab 12 extend in opposite directions from each other with the negative electrode 10 as the center. Therefore, the current applied to the first negative electrode tab 11 to measure the resistance of the negative electrode 10 can pass through the negative electrode 10 in the process of exiting to the second negative electrode tab 12. In this case, the overall resistance of the negative electrode 10 can be measured accurately, and the variation in the measured value each time the resistance is measured is reduced.

[0068] In one embodiment of the present invention, the first negative electrode tab and the second negative electrode tab may extend outward from the ends in each direction of both ends in the width direction or both ends in the longitudinal direction of the negative electrode. The width direction may mean the short side of the negative electrode, and the longitudinal direction may mean the long side of the negative electrode.

[0069] For example, the cross-section of the negative electrode (specifically, the horizontal cross-section) may have a rectangular shape with an aspect ratio of 1 or greater. Here, of two adjacent sides, the shorter side is defined as the width side, and the longer side is defined as the longitudinal side. The first negative electrode tab and the second negative electrode tab may extend outward from different width sides, or they may extend outward from different longitudinal sides.

[0070] Figures 3 and 4 illustrate how the first negative electrode tab 11 and the second negative electrode tab 12 are formed on different widthwise sides of the negative electrode 10.

[0071] The widthwise sides of the negative electrode 10 can be divided into two equal parts, defining one of the widthwise sides as region 10a and region 10b, and the other widthwise side as region 10c and region 10d. Then, with respect to the negative electrode 10 as the center, region 10a and region 10c are regions that are symmetrical with respect to each other along a line, and region 10b and region 10d are regions that are symmetrical with respect to each other along a line. Furthermore, region 10a and region 10d are regions that are point-symmetrical with respect to each other, and region 10b and region 10c are regions that are point-symmetrical with respect to each other.

[0072] The aforementioned line symmetry refers to the case where two regions of the widthwise edges of the negative pole can overlap when made symmetrical with respect to a straight line, while point symmetry refers to the case where two regions of the widthwise edges of the negative pole do not overlap when made symmetrical with respect to a straight line, but are at the same distance from each other with respect to a single point.

[0073] Referring to Figure 3, the first negative electrode tab 11 is located in region 10a, and the second negative electrode tab 12 is located in region 10c. In this case, the first negative electrode tab 11 and the second negative electrode tab 12 are located in regions that are symmetrical with respect to the negative electrode.

[0074] Referring to Figure 4, the first negative electrode tab 11 is located in region 10a, and the second negative electrode tab 12 is located in region 10d. In this case, the first negative electrode tab 11 and the second negative electrode tab 12 are located in regions that are point-symmetric with respect to the negative electrode.

[0075] Figures 3 and 4 illustrate the positioning of the first and second negative electrode tabs in a region that is point-symmetric or line-symmetric. The widthwise side is bisected to define regions 10a, 10b, 10c, and 10d. In the present invention, the positions of the first and second negative electrode tabs are not limited to regions 10a to 10d, but can be anywhere on the widthwise side or the longitudinal side. For example, the first negative electrode tab may be located in a region including the center of the widthwise side, and the second negative electrode tab may be located in a region including the center of the other widthwise side.

[0076] Thus, the first negative electrode tab and the second negative electrode tab in the negative electrode according to the present invention may be formed in a region that is symmetrical with respect to a line, or in a region that is symmetrical with respect to a point. When the first negative electrode tab and the second negative electrode tab are formed in a region that is symmetrical with respect to a line or a point, the current can pass through a large portion of the negative electrode when measuring the resistance, so the resistance of the negative electrode can be measured accurately.

[0077] In one embodiment of the present invention, the first negative electrode tab and / or the second negative electrode tab may be formed by extending the lithium-based material itself in the negative electrode of the lithium-based material. That is, the first negative electrode tab and the second negative electrode tab may be made of the same material as the negative electrode.

[0078] In one embodiment of the present invention, the first negative electrode tab and / or the second negative electrode tab may be formed by electrically connecting a separate metal (or alloy) to the negative electrode of a lithium-based material.

[0079] For example, a separate metal may be welded to the negative electrode to form the first negative electrode tab and / or the second negative electrode tab. The separate metal is not particularly limited as long as it has high conductivity without causing a chemical change to the battery. For example, copper, stainless steel, aluminum, nickel, titanium, palladium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, silver, etc., and aluminum-cadmium alloys can be used.

[0080] In one embodiment of the present invention, the first negative electrode tab and the second negative electrode tab may be configured to be connectable to a resistance measuring device for measuring the resistance of the negative electrode. The degree of deterioration of the negative electrode and the degree of deterioration of the secondary battery containing the negative electrode can be determined from the resistance value of the negative electrode measured by the resistance measuring device.

[0081] For example, when the secondary battery according to the present invention is housed in a pouch case, the first negative electrode tab and the second negative electrode tab may protrude outside the pouch case. Alternatively, separate electrode leads may be attached to the first negative electrode tab and / or the second negative electrode tab, and the attached electrode leads may protrude outside the pouch case. A resistance measuring device may be electrically connected to the protruding negative electrode tab or electrode lead to measure the resistance of the negative electrode.

[0082] A second aspect of the present invention relates to a method for measuring the resistance of the negative electrode of a secondary battery according to one aspect of the present invention.

[0083] A method for measuring the resistance of a negative electrode according to one aspect of the present invention is: A resistance measuring device is connected to the first negative electrode tab and the second negative electrode tab included in the negative electrode of a secondary battery according to one aspect of the present invention, and the resistance of the negative electrode is measured. The resistance measuring device may be connected directly or indirectly to the first negative electrode tab and the second negative electrode tab. For example, negative electrode leads may be attached to the first negative electrode tab and / or the second negative electrode tab, and the resistance measuring device may be connected to the negative electrode leads.

[0084] In one embodiment of the present invention, resistance can be measured using electrochemical impedance spectroscopy (EIS). EIS is a method of measuring resistance by applying an AC voltage to the object to be measured, and the results obtained when an AC voltage is applied to the sample as a frequency are analyzed using a Nyquist plot. EIS has the advantage of being able to perform a wider range of analyses compared to constant current or constant voltage measurement methods.

[0085] In one embodiment of the present invention, the amplitude range of the voltage when measuring the resistance may be 0.01mV to 0.5mV, or 0.01mV to 0.2mV, or 0.2mV to 0.5mV. When the amplitude of the voltage falls within this range, the accuracy of the measured resistance can be excellent while avoiding damage to the secondary battery.

[0086] In one embodiment of the present invention, the frequency of the voltage during the measurement of the resistance may be 100 Hz to 1000 Hz.

[0087] In one embodiment of the present invention, a secondary battery can be charged or discharged by connecting a charge / discharge device to one positive electrode tab and a first negative electrode tab. Simultaneously with the charging and discharging of the secondary battery, a resistance measuring device can be connected to the first negative electrode tab and the second negative electrode tab to measure the resistance of the negative electrode. That is, the resistance of the negative electrode can be measured simultaneously while the secondary battery is being charged or discharged, and the degree of deterioration of the negative electrode and the secondary battery can be determined from the measured resistance of the negative electrode.

[0088] In one embodiment of the present invention, the resistance can be measured each time the State of Charge (SOC) of the secondary battery changes by 5% to 20%, 5% to 10%, or 10% to 20%.

[0089] Figure 5 schematically illustrates a stacked structure with a separator in between, consisting of a negative electrode with two negative electrode tabs formed in opposite directions and a positive electrode with a single positive electrode tab. A secondary battery can be powered by connecting a charge / discharge device to the single positive electrode tab and the single negative electrode tab formed in the same direction. Furthermore, the resistance of the negative electrode can be measured by connecting a resistance measuring device to the two negative electrode tabs formed in opposite directions.

[0090] The present invention will be described in more detail below with reference to examples, but these examples are merely illustrative and the scope of the present invention is not necessarily limited to them.

[0091] [Example 1] Loading capacity: 5.16mAh / cm² 2 A lithium metal foil with a thickness of 50 μm was prepared, and a first negative electrode tab and a second negative electrode tab were formed by punching out the lithium metal foil. At this time, the first negative electrode tab and the second negative electrode tab were formed on the sides in the width direction of the lithium metal foil, and were formed in a region that is point-symmetric with respect to the lithium metal.

[0092] A sulfur-carbon composite was prepared in which sulfur (S8) was supported on carbon nanotubes as the positive electrode active material. The weight ratio of carbon nanotubes to sulfur was set to 3:7. The prepared sulfur-carbon composite was mixed with polyacrylic acid (PAA) as a binder polymer in a weight ratio of 96:4 and added to water to produce a positive electrode slurry composition. The positive electrode slurry composition was applied to an aluminum current collector and then dried to produce the positive electrode. The loading capacity of the produced positive electrode was 3.3 mAh / cm². 2 The aluminum current collector used had a positive electrode tab formed on one of its widthwise sides.

[0093] An electrode assembly was fabricated by positioning the positive and negative electrodes so that they faced each other, and sandwiching a polyethylene separator with a thickness of 16 μm and a porosity of 46 vol% between them. The electrode assembly was then placed in an aluminum pouch, and an electrolyte solution containing 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and 1 wt% lithium nitrate (LiNO3) dissolved in a solvent prepared by mixing 1,3-dioxolane (DOL) and dimethyl ether (DME) in a 1:1 volume ratio was injected to manufacture a secondary battery.

[0094] [Example of experiment] The capacity of the manufactured secondary battery was measured by connecting a charge / discharge device to the positive electrode tab and the first negative electrode tab and driving the secondary battery in the range of 1.8V to 2.5V in an environment of 25°C. Charging was performed in 0.2C CC mode, and discharging in 0.3C CC mode.

[0095] Then, resistance measuring devices were connected to the first and second negative electrode tabs, and the resistance was measured each time the State of Charge (SOC) fluctuated by 10% in the charge / discharge device. The resistance was measured using an EIS (Electronic Inductance) in the frequency range of 100 Hz to 1000 Hz, and the voltage amplitude was set to 0.2 mV.

[0096] Figure 6 is a graph showing the change in resistance during one charge-discharge cycle in the secondary battery of Example 1. The resistance was measured every 10% of SOC when the battery was charged from SOC 0% to SOC 100% and then discharged back to SOC 0%. The charging was performed at 25°C in 0.2C CC mode, and the discharging was performed at 25°C in 0.3C CC mode.

[0097] It was observed that the resistance decreased as the SOC approached 100%, and gradually increased as the SOC approached 0%. This reflects the increase in resistance corresponding to the decrease in lithium metal foil thickness during discharge, and the decrease in resistance corresponding to the increase in lithium metal foil thickness during charging.

[0098] Figure 7 shows the discharge capacity retention rate measured while continuing the charge-discharge cycle for the secondary battery of Example 1, and Figure 8 shows the change in resistance measured while continuing the charge-discharge cycle for the secondary battery of Example 1. The charging was performed at 25°C in 0.2C CC mode, and the discharging was performed at 25°C in 0.3C CC mode.

[0099] At this time, the discharge capacity retention rate was calculated using the following formula.

[0100] Discharge capacity retention rate (%) = [(Discharge capacity in one cycle - Discharge capacity in n cycles) / Discharge capacity in one cycle] × 100

[0101] Referring to Figures 7 and 8, it can be seen that the highest point of resistance (the resistance at the SOC 0% point in each cycle) gradually increases as the cycle progresses. This is due to the increase in resistance as the lithium metal foil of the negative electrode deteriorates. In other words, by measuring the resistance of the lithium metal negative electrode using the negative electrode resistance measurement method according to the present invention, information regarding the deterioration state of the secondary battery can be obtained.

[0102] As described above, although the present invention has been explained with limited embodiments and drawings, the present invention is not limited in any way thereto, and it goes without saying that various modifications and variations can be made by persons with ordinary skill in the art to which the present invention pertains, within the equivalent scope of the technical idea of ​​the present invention and the appended claims. [Explanation of symbols]

[0103] 10 negative electrode 11. First negative electrode tab 12. Second negative electrode tab

Claims

1. A secondary battery comprising a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode, The aforementioned negative electrode is a free-standing type containing a negative electrode active material and not having a separate current collector. The negative electrode active material comprises at least one lithium-based material selected from lithium metal and lithium alloys. A secondary battery in which the negative electrode includes a first negative electrode tab and a second negative electrode tab, each extending in opposite directions.

2. The secondary battery according to claim 1, wherein the positive electrode has a positive electrode active material layer formed on at least one side of a positive electrode current collector, and the positive electrode active material layer comprises a sulfur-carbon composite and a binder polymer.

3. The secondary battery according to claim 1, wherein the positive electrode includes a positive electrode tab.

4. The secondary battery according to claim 1, wherein the negative electrode is composed of at least one lithium-based material selected from lithium metal and lithium alloy.

5. The secondary battery according to claim 1, wherein the first negative electrode tab and the second negative electrode tab extend outward from the respective ends of both ends in the width direction or both ends in the longitudinal direction of the negative electrode.

6. The secondary battery according to claim 1, wherein the first negative electrode tab and the second negative electrode tab are located in a region that is symmetrical with respect to the negative electrode.

7. The secondary battery according to claim 1, wherein the first negative electrode tab and the second negative electrode tab are located in a region that is point-symmetric with respect to the negative electrode.

8. The secondary battery according to claim 1, wherein the first negative electrode tab and the second negative electrode tab are configured to be connectable to a resistance measuring device for measuring the resistance of the negative electrode.

9. The secondary battery according to claim 1, wherein the degree of electrical degradation of the negative electrode and the secondary battery can be observed by connecting a resistance measuring device to the first negative electrode tab and the second negative electrode tab.

10. The secondary battery according to claim 1, wherein the positive electrode has a positive electrode active material layer formed on at least one side of the positive electrode current collector, and the loading amount of the negative electrode is 1 to 2.5 times the loading amount of the positive electrode.

11. A method for measuring the negative electrode resistance of a secondary battery according to any one of claims 1 to 10, A method for measuring the resistance of a negative electrode, comprising the step of connecting a resistance measuring device to the first negative electrode tab and the second negative electrode tab, and measuring the resistance of the negative electrode.

12. The method for measuring the resistance of a negative electrode according to claim 11, wherein the measurement of the resistance is performed using electrochemical impedance analysis.

13. The method for measuring the resistance of a negative electrode according to claim 11, wherein the measurement of the resistance is performed under conditions of a voltage amplitude of 0.01 mV to 0.5 mV.

14. The method for measuring the resistance of a negative electrode according to claim 11, wherein the measurement of the resistance is performed under conditions of a frequency of 100 Hz to 1000 Hz.

15. The method for measuring the resistance of a negative electrode according to claim 11, wherein the resistance measurement is performed each time the charge state (SOC: State Of Charge) of the secondary battery changes by 5% to 20%.

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