Cylindrical secondary battery

Abstract

The present invention relates to a cylindrical secondary battery having a large form factor applicable to medium to large devices such as automobiles, which exhibits excellent fast charging characteristics, improves swelling problems caused by side reactions and gas generation, suppresses lithium precipitation, and has high capacity characteristics.

Description

[Technical Field]

[0001] The present invention relates to a cylindrical secondary battery having a large form factor applicable to medium- to large-sized devices such as automobiles, which exhibits excellent fast charging characteristics, improves swelling caused by side reactions and gas generation, suppresses lithium precipitation, and has high capacity characteristics.

[0002] This application claims priority based on Korean Patent Application No. 10-2021-0142208 filed on October 22, 2021, and Korean Patent Application No. 10-2021-0179523 filed on December 15, 2021, and the contents disclosed in the specifications and drawings of those applications are incorporated into this application in their entirety. [Background technology]

[0003] Secondary batteries, which are easily applicable to various products and have electrical properties such as high energy density, are widely used not only in portable devices but also in electric vehicles (EVs) and hybrid electric vehicles (HEVs), which are powered by electrical sources. These secondary batteries are attracting attention as a new energy source because they not only have the primary advantage of dramatically reducing the use of fossil fuels, but also because they are environmentally friendly and improve energy efficiency by producing no by-products associated with energy use.

[0004] Currently, widely used types of secondary batteries include lithium-ion batteries, lithium polymer batteries, nickel-cadmium batteries, nickel-metal hydride batteries, and nickel-zinc batteries. The operating voltage of such unit secondary battery cells, i.e., unit battery cells, is approximately 2.5V to 4.5V. Therefore, if a higher output voltage is required, a battery pack is constructed by connecting multiple battery cells in series. Alternatively, a battery pack can be constructed by connecting multiple battery cells in parallel depending on the required charge / discharge capacity of the battery pack. Therefore, the number and electrical connection form of battery cells included in the battery pack can be variously set depending on the required output voltage or charge / discharge capacity.

[0005] Meanwhile, known types of unit secondary batteries include cylindrical, prismatic, and pouch-type batteries. Among these, cylindrical secondary batteries include a separator, which is an insulator, interposed between a positive electrode and a negative electrode, which is then wound up to form a jelly-roll-shaped electrode assembly, which is then inserted into a battery can to complete the battery. Strip-shaped electrode tabs are connected to the positive electrode and negative electrode, respectively, and the electrode tabs electrically connect the electrode assembly to electrode terminals exposed to the outside. However, conventional cylindrical secondary batteries with this structure have problems such as high resistance, excessive heat generation, and poor current collection efficiency due to current concentration in the strip-shaped electrode tabs connected to the positive electrode and / or negative electrode.

[0006] However, resistance and heat generation were not a major issue for small cylindrical secondary batteries with form factors such as 18650 (cylindrical secondary battery with a diameter of 18 mm and a height of 65 mm) and 21700 (cylindrical secondary battery with a diameter of 21 mm and a height of 70 mm), which have been mainly used up until now.

[0007] However, due to recent demands for increased driving range and faster charging speeds for electric vehicles, the development and use of larger form factors, such as enlarged cylindrical secondary batteries, such as the 46800 (a cylindrical secondary battery with a diameter of 46 mm and a height of 80 mm), is being considered. Furthermore, in order to improve the fast charging characteristics of such enlarged cylindrical secondary batteries, so-called tab-less cylindrical secondary batteries are being developed, which use the current collectors of the uncoated portions of the positive and negative electrodes as electrode tabs instead of separate strip-shaped electrode tabs.

[0008] The enlarged cylindrical secondary battery not only exhibits relatively large capacity characteristics and energy density, but also has the advantage of improving production efficiency and reducing production costs for cylindrical secondary batteries for electric vehicles. Furthermore, the application of the tabless structure increases the electrical connection area and efficiency between the electrode tabs and the electrode terminals, thereby reducing current concentration on the electrode tabs, increasing current collection efficiency, and improving fast charging characteristics.

[0009] However, in large cylindrical secondary batteries employing the above-mentioned tabless structure, a large current is applied to each of the positive and / or negative electrodes in a short period of time during rapid charging, which can cause side reactions and the resulting generation of gas at each electrode. In particular, to achieve high capacity and rapid charging characteristics, silicon-based negative electrode active materials are typically used in the negative electrodes of these large cylindrical secondary batteries. However, such silicon-based active materials can undergo large volume changes and side reactions during charging and discharging, which can lead to serious problems such as side reactions and gas generation at the negative electrode, resulting in swelling and lithium precipitation.

[0010] Therefore, there is a need to develop technology that can improve the fast charging characteristics of large cylindrical secondary batteries that can be used in medium to large devices such as automobiles, while reducing side reactions, gas generation, swelling, and lithium deposition that occur at the negative electrode. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention aims to provide a cylindrical secondary battery that has a large form factor applicable to medium- to large-sized devices, exhibits excellent fast charging characteristics, improves swelling caused by side reactions and gas generation, suppresses lithium deposition, and has high capacity characteristics.

[0012] Another object of the present invention is to provide a battery pack including the cylindrical secondary battery. [Means for solving the problem]

[0013] In order to solve the above problems, according to one aspect of the present invention, there is provided a cylindrical secondary battery according to the following embodiment.

[0014] According to a first embodiment of the present invention, A cylindrical secondary battery comprising: a jelly-roll-shaped electrode assembly in which a positive electrode including a positive electrode active material layer on a current collector, a negative electrode including a negative electrode active material layer on a current collector, and a separator interposed between the positive electrode and the negative electrode are wound up; and a battery can containing the electrode assembly, The negative electrode active material layer is a lower layer region in surface contact with the current collector and containing a silicon-based compound and natural graphite as active materials; a mixed region in surface contact with the lower layer region and containing a silicon-based compound, natural graphite, and artificial graphite as active materials; an upper layer region in surface contact with the mixed region and containing a silicon-based compound and artificial graphite as active materials; A cylindrical secondary battery having a diameter of 35 mm or more and a height of 75 mm or more based on the maximum diameter and maximum height of the battery can is provided.

[0015] According to a second embodiment of the present invention, in the first embodiment, The positive electrode active material layer may contain, as an active material, a lithium nickel-based transition metal oxide in which the nickel content is 80 to 100 mol % based on the total amount of transition metals.

[0016] According to a third embodiment of the present invention, in the second embodiment, The lithium nickel-based transition metal oxide may be represented by the following Chemical Formula 1:

[0017] [C1] Li 1+a (Ni b Co c Mn d Al e M f )O2 In the above Chemical Formula 1, -0.1≦a≦0.2, 0.8≦b≦1.0, 0.01≦c≦0.15, 0.01≦d≦0.15, 0.01≦e≦0.1, and 0≦f≦0.05, and M is at least one selected from the group consisting of Mg, Ti, Zr, Nb, and W.

[0018] According to a fourth embodiment of the present invention, in any one of the first to third embodiments, The negative electrode active material layer may have a thickness of 40 to 200 μm.

[0019] According to a fifth embodiment of the present invention, in any one of the first to fourth embodiments, In the negative electrode active material layer, the mixed region may have a thickness of 20 to 80% of the thickness of the entire negative electrode active material layer, based on the cross-sectional thickness of the portion where the mixed region is formed at its thickest.

[0020] According to a sixth embodiment of the present invention, in any one of the first to fifth embodiments, The lower layer region may have a thickness of 10 to 40% of the thickness of the entire negative electrode active material layer.

[0021] According to a seventh embodiment of the present invention, in any one of the first to sixth embodiments, The upper layer region may have a thickness of 10 to 40% of the thickness of the entire negative electrode active material layer.

[0022] According to the eighth embodiment of the present invention, in any one of the first to seventh embodiments, The mixing region may contain the natural graphite and artificial graphite in a weight ratio (natural graphite: artificial graphite) of 2:8 to 8:2.

[0023] According to the ninth embodiment of the present invention, in any one of the first to eighth embodiments, The mixing region may have an active material distribution gradient in which the distribution ratio of the natural graphite decreases and the distribution ratio of the artificial graphite increases as it approaches the upper layer region.

[0024] According to the tenth embodiment of the present invention, in any one of the first to ninth embodiments, The natural graphite may have a particle form showing a sphericity exceeding 0.91 and an average particle diameter D50 of 5 to 30 μm.

[0025] According to the eleventh embodiment of the present invention, in any one of the first to tenth embodiments, The artificial graphite may include secondary particles formed by aggregation of primary particles and a carbon coating layer formed on the surface of the secondary particles.

[0026] According to the twelfth embodiment of the present invention, in any one of the first to eleventh embodiments, The carbon coating layer may be contained in an amount of 0.5 to 10% by weight based on the total weight of the artificial graphite.

[0027] The artificial graphite may have an average particle diameter D50 of 4 to 32 μm.

[0028] According to the fourteenth embodiment of the present invention, in any one of the first to thirteenth embodiments, The silicon-based compound may include Si, SiOx (0 < x ≤ 2), Si-Y alloy (Y is an element selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements excluding Si, transition metals, rare earth elements, and combinations thereof), or two or more of these.

[0029] According to a fifteenth embodiment of the present invention, in any one of the first to fourteenth embodiments, The silicon-based compound may be contained in an amount of 10 to 50 wt % based on the total amount of the active material contained in the negative electrode active material layer.

[0030] According to a 16th embodiment of the present invention, in any one of the 1st to 15th embodiments, In the negative electrode active material layer, the lower layer region, the mixed region, and the upper layer region each include the active material, the binder polymer, and the conductive material, and the lower layer region may include a binder polymer content (wt %) greater than that of the upper layer region relative to the total content (wt %) of each region.

[0031] According to a 17th embodiment of the present invention, in any one of the 1st to 16th embodiments, The lower layer region may contain 1 to 1.2 wt % of the binder polymer relative to the total content (wt %), and the upper layer region may contain 0.5 to 0.9 wt % of the binder polymer relative to the total content (wt %).

[0032] According to an 18th embodiment of the present invention, in any one of the 1st to 17th embodiments, The lower layer region and the upper layer region may contain the same or different polymers, and the binder polymer of the lower layer region may contain styrene butadiene rubber (SBR) or a mixture of styrene butadiene rubber (SBR) and an acrylic copolymer.

[0033] According to a 19th embodiment of the present invention, in any one of the 1st to 18th embodiments, In the case of a mixture of styrene butadiene rubber (SBR) and an acrylic copolymer, the styrene butadiene rubber may be contained in a greater amount than the acrylic copolymer.

[0034] According to a twentieth embodiment of the present invention, in any one of the first to nineteenth embodiments, The binder polymer of the upper layer region may include core-shell particles having a core portion made of styrene-butadiene rubber and a shell portion surrounding the outside of the core portion and made of an acrylic copolymer, or a mixture of the core-shell particles and styrene-butadiene rubber.

[0035] According to a 21st embodiment of the present invention, in the 20th embodiment, The binder polymer of the upper layer region may include a mixture of the core-shell particles and styrene-butadiene rubber, and the core-shell particles may be contained in a greater amount than the styrene-butadiene rubber.

[0036] According to a 22nd embodiment of the present invention, in the 20th or 21st embodiment, The core-shell particles may have an average particle size D50 of 30 to 100 nm, and the styrene-butadiene rubber may have an average particle size of 200 to 350 nm.

[0037] According to a 23rd embodiment of the present invention, in any one of the 1st to 22nd embodiments, the negative electrode active material layer has a QBR (Quantified Binder Ratio) of 2.0 or less, The QBR can be defined by the following mathematical formula:

[0038] QBR=Bs / Bf

[0039] In the above mathematical formula, Bs represents the average value of the Os atomic ratio in a surface region of the negative electrode active material layer extending from the outermost surface of the negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer; Bf represents the average value of the Os atomic ratio in a bottom region of the negative electrode active material layer extending from the interface of the negative electrode active material layer facing the current collector to within 15% of the total thickness of the negative electrode active material layer; The Os atomic ratio is analyzed by energy dispersive spectroscopy (EDS) after staining a cross section of the negative electrode active material with OsO4.

[0040] According to a 24th embodiment of the present invention, in any one of the 1st to 23rd embodiments, The positive electrode and the negative electrode have an uncoated portion where no active material layer is formed along an edge of one side (long side) of the current collector in a direction parallel to the winding direction, and at least a portion of the current collector in the uncoated portion may define an electrode tab.

[0041] According to a 25th embodiment of the present invention, in the 24th embodiment, At least a portion of the current collector defining the electrode tab may be processed in the form of a plurality of segments that can be bent independently, and the segments may be connected to the electrode terminals with a wide contact area, so that the cylindrical secondary battery may have a tabless secondary battery structure.

[0042] According to a 26th embodiment of the present invention, in any one of the 1st to 25th embodiments, The form factor ratio, defined as the diameter divided by the height, may be greater than 0.4.

[0043] According to a 27th embodiment of the present invention, in any one of the 1st to 26th embodiments, The cylindrical secondary battery may be a 46110 cell, a 48750 cell, a 48110 cell, a 48800 cell, or a 46800 cell.

[0044] According to a 28th embodiment of the present invention, there is provided a battery pack including the cylindrical secondary battery of any one of the 1st to 27th embodiments.

[0045] According to a 29th embodiment of the present invention, there is provided a vehicle including the battery pack of the 28th embodiment. [Effects of the Invention]

[0046] According to the present invention, there is provided a cylindrical secondary battery with a large form factor having a diameter of 35 mm or more and a height of 75 mm or more, in which a negative electrode active material layer is formed by being divided into three or more regions depending on the distribution of the active material.

[0047] Due to this distribution of active material in the negative electrode active material layer, the cylindrical secondary battery suppresses side reactions between the negative electrode and the electrolyte and gas generation, even when a relatively large current is applied in a short period of time, such as during fast charging, thereby significantly reducing swelling, electrolyte depletion, and lithium deposition. Furthermore, the distribution of a large amount of natural graphite in the lower layer region and the adjacent mixed region allows the active material layer to have excellent adhesion to the current collector and mechanical properties.

[0048] Therefore, the cylindrical secondary battery according to the present invention has a larger form factor than conventional batteries, while exhibiting excellent fast charging characteristics and reduced swelling and lithium precipitation due to reduced side reactions and gas generation, and is therefore suitable for use as a secondary battery for medium to large devices such as automobiles that require large capacity and fast charging characteristics.

[0049] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters described in the drawings. [Brief explanation of the drawings]

[0050] [Figure 1] 2 is a plan view schematically illustrating an example of a positive electrode or a negative electrode included in a cylindrical secondary battery according to an embodiment; FIG. [Figure 2] 2 is a cross-sectional view schematically illustrating an example of an electrode assembly included in a cylindrical secondary battery according to an embodiment; FIG. [Figure 3] 1 is a cutaway perspective view schematically illustrating an example of a cylindrical secondary battery according to an embodiment; [Figure 4] 1 is a diagram illustrating an example of a battery pack including a cylindrical secondary battery according to an embodiment; [Figure 5] FIG. 2 is a schematic diagram of a vehicle including the battery pack. [Figure 6a] 2 is an electron microscope photograph of a cross section of a negative electrode in the cylindrical secondary battery of Example 1. [Figure 6b] 2 is an electron microscope photograph of a cross section of a negative electrode in the cylindrical secondary battery of Example 1. [Figure 6c] 1 is an electron microscope photograph of a cross section of a negative electrode in a cylindrical secondary battery of Comparative Example 1. [Figure 7] 1 shows the results of X-ray CT imaging of the cylindrical secondary battery of Example 1. [Figure 8] 1 shows the results of X-ray CT imaging of the cylindrical secondary battery of Comparative Example 1. [Figure 9] 10 shows the results of X-ray CT imaging of the cylindrical secondary battery of Comparative Example 2. [Figure 10] 1 is a photograph showing the surface of a negative electrode active material layer and the negative electrode active material layer attached to a separator, obtained by disassembling the cylindrical secondary battery of Example 1. [Figure 11] 1 is a photograph showing the surface of a negative electrode active material layer and the negative electrode active material layer attached to a separator, obtained by disassembling the cylindrical secondary battery of Comparative Example 1. [Figure 12] 1 is a photograph showing the surface of a negative electrode active material layer and the negative electrode active material layer attached to a separator, obtained by disassembling the cylindrical secondary battery of Comparative Example 2. [Figure 13a] 10 is a graph showing the results of a rapid charge / discharge test conducted on the cylindrical secondary batteries of Example 1 and Comparative Example 1 in Experimental Example 3. [Figure 13b] 10 is a graph showing the results of a rapid charge / discharge test conducted on the cylindrical secondary batteries of Example 1 and Comparative Example 1 in Experimental Example 3. [Figure 13c] 10 is a graph showing the results of a rapid charge / discharge test conducted on the cylindrical secondary batteries of Example 1 and Comparative Example 1 in Experimental Example 3. [Figure 13d] 10 is a graph showing the results of a rapid charge / discharge test conducted on the cylindrical secondary batteries of Example 1 and Comparative Example 1 in Experimental Example 3. [Figure 14] FIG. 2 is a schematic diagram illustrating how to calculate the QBR value of a negative electrode active material layer. [Figure 15a] 1 is a graph showing a change in normalized intensity of an Os component stained in a binder polymer of a first negative electrode active material layer of a negative electrode in Example 1, extracted and analyzed by EDS mapping, along a distance from a surface of the first negative electrode active material layer toward a current collector. [Figure 15b] 1 is a graph showing a change in normalized intensity of an Os component stained in a binder polymer of a second negative electrode active material layer of a negative electrode in Example 1, the Os component being extracted and analyzed by EDS mapping over a distance from the surface of the second negative electrode active material layer toward a current collector. DETAILED DESCRIPTION OF THE INVENTION

[0051] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms and words used in the specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as having meanings and concepts corresponding to the technical ideas of the present invention, in accordance with the principle that the inventor himself can appropriately define the concepts of terms in order to best explain the invention.

[0052] According to one embodiment of the present invention, there is provided a cylindrical secondary battery including a jelly-roll-shaped electrode assembly in which a positive electrode including a positive electrode active material layer on a current collector, a negative electrode including a negative electrode active material layer on a current collector, and a separator interposed between the positive electrode and the negative electrode are wound up, and a battery can accommodating the electrode assembly, The negative electrode active material layer is a lower layer region in surface contact with the current collector and containing a silicon-based compound and natural graphite as active materials; a mixed region in surface contact with the lower layer region and containing a silicon-based compound, natural graphite, and artificial graphite as active materials; an upper layer region in surface contact with the mixed region and containing a silicon-based compound and artificial graphite as active materials; A cylindrical secondary battery having a diameter of 35 mm or more and a height of 75 mm or more based on the maximum diameter and maximum height of the battery can is provided.

[0053] A cylindrical secondary battery having a large form factor with a diameter of 35 mm or more and a height of 75 mm or more is provided, in which the negative electrode active material layer is divided into three or more regions according to the distribution of the active material.

[0054] The cylindrical secondary battery of this embodiment has a larger form factor than conventional cylindrical secondary batteries, and the negative electrode active material layer is divided into three or more regions according to the distribution of the active material. More specifically, a lower region in contact with the current collector is distributed with a silicon-based compound and natural graphite, an upper region in contact with the separator is distributed with a silicon-based compound and artificial graphite, and a mixed region containing the silicon-based compound, natural graphite, and artificial graphite is formed between the lower and upper regions.

[0055] Since the negative electrode active material layer essentially contains a silicon-based compound as an active material, the cylindrical secondary battery according to an embodiment may exhibit high capacity characteristics, energy density, and excellent fast charging characteristics.

[0056] Furthermore, compared to natural graphite, artificial graphite has more lithium ion migration paths, resulting in higher electrical efficiency and favorable fast charging. Furthermore, its isotropic, stable structure reduces side reactions and swelling, resulting in favorable life characteristics for secondary batteries. Conversely, natural graphite is inferior to artificial graphite in terms of fast charging characteristics and stability, and may undergo relatively more side reactions. However, its relatively large surface area and the presence of reactive sites on the surface make it favorable for improving adhesion to current collectors.

[0057] In one embodiment of a cylindrical secondary battery, artificial graphite is primarily distributed in the upper layer region of the active material layer where electrochemical reactions primarily occur during charging and discharging and in the adjacent mixed region, while natural graphite is primarily distributed in the lower layer region that contacts the current collector and in the adjacent mixed region, thereby maximizing the advantages of these two active materials.

[0058] That is, by distributing artificial graphite near the upper layer region where electrochemical reactions occur during charge and discharge, it is possible to further improve fast charging characteristics while suppressing side reactions, swelling, and lithium precipitation during charge and discharge. Furthermore, by distributing relatively stable artificial graphite together with a silicon-based compound in the upper layer region, it is possible to relatively buffer the silicon-based compound, which undergoes large volume changes and side reactions during charge and discharge. Furthermore, by distributing natural graphite near the lower layer region that contacts the current collector, it is possible to significantly improve the adhesion of the negative electrode active material layer to the current collector, thereby improving the overall durability of the secondary battery. Furthermore, by mixing natural graphite and artificial graphite in the mixed region, it is possible to utilize the advantages of each active material and increase the lithium ion storage capacity due to the large surface area, thereby providing a large-sized cylindrical secondary battery with higher capacity characteristics and energy density.

[0059] As a result, in the cylindrical secondary battery of the embodiment, due to the above-mentioned various characteristics, side reactions between the negative electrode and the electrolyte and gas generation are suppressed even when a relatively large current is applied in a short period of time, such as during fast charging, and as a result, swelling, electrolyte depletion, lithium deposition, etc. are significantly reduced. In addition, the active material layer can have excellent adhesion and mechanical properties to the current collector.

[0060] Therefore, the cylindrical secondary battery of one embodiment has a larger form factor than existing batteries, while exhibiting excellent fast charging characteristics and reduced swelling and lithium precipitation due to reduced side reactions and gas generation, making it highly suitable for use as a secondary battery for medium to large devices such as automobiles that require large capacity and fast charging characteristics.

[0061] Hereinafter, each component of a cylindrical secondary battery according to an embodiment will be described in more detail with reference to the accompanying drawings. Fig. 1 is a plan view schematically illustrating an example of a positive electrode or a negative electrode included in a cylindrical secondary battery according to an embodiment, and Fig. 2 is a cross-sectional view schematically illustrating an example of an electrode assembly included in a cylindrical secondary battery according to an embodiment.

[0062] As shown in FIGS. 1 and 2, the cylindrical secondary battery includes a positive electrode 13 and a negative electrode 14, each having a positive electrode active material layer or a negative electrode active material layer 18 formed on a current collector 17, and a jelly-roll-shaped electrode assembly 10 wound up with a separator 15 interposed between the positive electrode 13 and the negative electrode 14.

[0063] In this case, the positive electrode 13 and the negative electrode 14 have non-coated portions 16 where the active material layer 18 is not formed along the end of one side (long side) of each current collector 17 in a direction parallel to the winding direction, and at least a part of the current collector 17 in the non-coated portion 16 may define each electrode tab 11, 12 of the positive electrode or the negative electrode.

[0064] 1, at least a portion of the current collector 17 defining each of the electrode tabs 11 and 12 is processed into the form of a plurality of segments that can be bent independently. The plurality of segments may have different shapes and sizes for each region, but this specification is not particularly limited thereto.

[0065] As will be described in more detail below, the plurality of segment pieces formed in the non-coated portion 16 can be electrically connected to the respective electrode terminals and function as electrode tabs 11 and 12, and the cylindrical secondary battery of one embodiment can have the form of a so-called tabless secondary battery without additional electrode tabs. This significantly improves the electrical connection area with the electrode terminals and current collection efficiency compared to existing batteries with additional electrode tabs, making this a structure advantageous for large-capacity batteries that require rapid charging.

[0066] However, in such a cylindrical secondary battery with a tabless structure and large size, since a large current may be applied to each electrode in a short period of time, it may be relatively important to suppress side reactions, swelling, and lithium precipitation while improving fast charging characteristics. To address this, in one embodiment, in a cylindrical secondary battery, the negative electrode active material layer may be divided into the above-mentioned lower layer region, mixed region, and upper layer region.

[0067] The thickness of each region of the negative electrode active material layer may be determined by analyzing a cross section of the active material layer using an electron microscope, etc. The thickness of each region may be calculated by measuring the thickness of each region based on the cross section of the active material layer where the mixed region is thickest in the entire active material layer in an electron microscope photograph of the active material layer.

[0068] In a specific embodiment, the total thickness of the negative electrode active material layer is not particularly limited, and may be, for example, 40 to 200 μm.

[0069] Among these, the mixed region in which the natural graphite and the artificial graphite are mixed may have a thickness of 20 to 80%, 30 to 70%, or 40 to 60% of the total thickness of the negative electrode active material layer, the lower layer region may have a thickness of 10 to 40%, 15 to 35%, or 20 to 30% of the total thickness of the negative electrode active material layer, and the upper layer region may have a thickness of 10 to 40%, 15 to 35%, or 20 to 30% of the total thickness of the negative electrode active material layer.

[0070] In a more specific example, the thickness of the upper and / or lower region may be 5 to 100 μm or 10 to 60 μm, respectively, and the thickness of the mixed region may be 20 to 180 μm or 25 to 150 μm.

[0071] By controlling the thickness of each of the distribution regions of natural graphite and artificial graphite, the fast charging characteristics of the cylindrical secondary battery of an embodiment are further improved, and side reactions, gas generation, swelling, and lithium precipitation in the negative electrode are further suppressed, and the cylindrical secondary battery can exhibit excellent capacity and life characteristics.

[0072] In the negative electrode active material layer, the mixed region may contain the natural graphite and artificial graphite in a weight ratio (natural graphite:artificial graphite) of 2:8 to 8:2, or 3:7 to 7:3, or 4:6 to 6:4. Furthermore, such a mixed region may have an active material distribution gradient in which the distribution ratio of the natural graphite decreases and the distribution ratio of the artificial graphite increases toward the upper layer region. Due to such a mixing ratio and distribution gradient in the mixed region, the effect of the mixture of natural graphite and artificial graphite in the mixed region may be maximized.

[0073] Meanwhile, the negative electrode active material layer includes, as active materials, artificial graphite, natural graphite, and a silicon-based compound in each region.

[0074] Among them, artificial graphite is made by mixing coke with a binder and baking and heating it at high temperatures of over 2,500°C, and the crystallinity is intentionally increased during the manufacturing process, resulting in a consistent and stable internal structure.Compared to natural graphite, it does not contain as many lithium ions, but has the advantage of having many paths for lithium ions to move, which is advantageous for fast charging, and has a relatively long charge / discharge life.

[0075] However, artificial graphite is typically used in the form of secondary particles. Therefore, secondary-particle artificial graphite can be obtained by granulating coke, a primary particle material, to produce secondary particles, followed by heat treatment to graphitize them. Conventional manufacturing methods, which do not control the size of primary particles, result in a large amount of fine particles that are not granulated or that separate from the secondary particles after granulation. This can lead to a decrease in the negative electrode adhesion (the resistance of negative electrode active material particles to detachment from the negative electrode) and poor high-temperature storage performance of the battery. Furthermore, the presence of fine particles in the secondary particles can lead to uneven pores in the negative electrode, increasing the pore resistance of the negative electrode and degrading the battery's lifespan and fast charging performance. To address these issues, a process of forming a carbon coating layer on the secondary particles has been used.

[0076] Accordingly, in one embodiment of the present invention, the artificial graphite may include at least one of artificial graphite without a carbon coating layer on its surface and artificial graphite with a carbon coating layer on its surface.

[0077] The artificial graphite is typically produced by carbonizing raw materials such as coal tar, coal-tar pitch, and petroleum-based heavy oil at 2,500°C or higher. After graphitization, the artificial graphite is used as a negative electrode active material through particle size adjustment such as pulverization and secondary particle formation.

[0078] In the case of artificial graphite, the crystals are randomly distributed within the particles, and the sphericity is lower than that of natural graphite, with a somewhat pointed shape.

[0079] Conventional spherical natural graphite has a long Li-ion migration distance based on the active material particle and relatively few intercalation sites, which can result in inferior output characteristics compared to secondary particle artificial graphite. However, secondary particle artificial graphite is produced by granulating small primary particles to create secondary particles with a particle size that is easy to manufacture electrodes with, so it maintains the primary particle's characteristics of short Li migration distance and many intercalation sites, which can result in excellent fast charging characteristics.

[0080] The artificial graphite used in one embodiment of the present invention may be commercially available mesophase carbon microbeads (MCMB), mesophase pitch-based carbon fiber (MPCF), artificial graphite graphitized in a block form, or artificial graphite graphitized in a powder form, and the sphericity of the artificial graphite may be 0.91 or less, or 0.6 to 0.91, or 0.7 to 0.9.

[0081] The secondary particles of artificial graphite without a carbon coating layer may be formed by granulating primary particles, i.e., the secondary particles may be structures formed by agglomeration of the primary particles during the granulation process.

[0082] The secondary particle artificial graphite having a carbon coating layer on its surface may include at least one of amorphous carbon and crystalline carbon in the carbon coating layer.

[0083] The crystalline carbon may further improve the conductivity of the negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0084] The amorphous carbon may be a carbon-based material formed by using at least one carbide selected from the group consisting of tar, pitch, and other organic materials, or a hydrocarbon as a source in a chemical vapor deposition process.

[0085] The carbonized organic material may be a carbonized organic material selected from carbonized sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose, and combinations thereof.

[0086] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, hexane, etc. The aromatic hydrocarbon of the substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, phenanthrene, etc.

[0087] The carbon coating layer may be included in an amount of 0.5 wt % to 10.0 wt %, more specifically, 1 wt % to 8 wt %, or 2 wt % to 6 wt %, based on the total weight of the artificial graphite including the carbon coating layer. When this range is satisfied, the capacity per weight of the negative electrode active material particles is ensured, while the fast charging performance of the artificial graphite is improved.

[0088] The artificial graphite not having a carbon coating layer on its surface may have a D50 of 5 μm to 35 μm, preferably 7 μm to 33 μm, and more preferably 10 μm to 30 μm.

[0089] The artificial graphite having a carbon coating layer on its surface may have a D50 of 4 μm to 32 μm, preferably 6 μm to 30 μm, and more preferably 8 μm to 28 μm, or 8 μm to 21 μm.

[0090] In this specification, the average particle size D50 is the particle size at 50% of the cumulative particle number distribution by particle size. The D50 can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The particle size distribution is calculated by measuring the difference in diffraction patterns depending on particle size when the particles pass through a laser beam. The D50 can be measured by calculating the particle size at 50% of the cumulative particle number distribution by particle size measured by the analyzer.

[0091] Meanwhile, the natural graphite may generally be in the form of plate-like aggregates, i.e., flake-like natural graphite, before being processed. The flake-like natural graphite is produced from natural graphite raw materials (e.g., graphite ore), and specifically, may be produced through processes such as crushing the natural graphite raw materials, removing impurities by base treatment and / or acid treatment, washing, drying, and sieving.

[0092] The natural graphite may be spherical. The spherical natural graphite may be produced by spheronizing flake natural graphite. The spheronization may be performed, for example, using a vortex flow pulverizer. When the natural graphite is spherical, packing between active material particles may be more smoothly achieved, thereby significantly reducing the problem of thickness expansion of the negative active material during charge and discharge.

[0093] The natural graphite used in one embodiment of the present invention may have a sphericity of more than 0.91 and not more than 0.97, or 0.93 to 0.97, or 0.94 to 0.96.

[0094] The natural graphite may have an average particle size D50 of 5 to 30 μm, or 10 to 25 μm.

[0095] Natural graphite has soft properties, making it easy to deform during rolling, which makes it easy to increase the packing density. It also has the advantage of ensuring a sufficient contact area between active materials, which can be beneficial for ensuring adhesion. However, this can make it difficult to maintain porosity in the electrode structure, which can result in blocking of pores on the electrode surface or the creation of closed pores inside, reducing electrolyte impregnation. Artificial graphite, on the other hand, has hard properties that make it difficult to roll, and the contact area between active materials is smaller than that of natural graphite, resulting in relatively low adhesion. On the other hand, artificial graphite has the advantage of easily maintaining porosity, which can enable the realization of an electrode structure that is favorable for electrolyte impregnation.

[0096] In the above-described embodiment, in order to utilize the advantages of both natural and artificial graphite, the distribution gradient of the natural and artificial graphite in the active material layer is controlled.

[0097] Among these, the inclusion of artificial graphite in the upper layer region or the adjacent mixed region of the negative electrode active material layer facilitates impregnation with an electrolyte and ensures adhesion through binder migration during electrode fabrication. Furthermore, the inclusion of natural graphite in the lower layer region or the adjacent mixed region ensures adhesion through the active material properties of the natural graphite itself, even if binder migration to the upper layer region occurs.

[0098] Typically, fast charging can be characterized by a step charge, where the current density is initially high and then gradually decreased. The anode included in the secondary battery of the above-described embodiment can improve fast charging performance by taking into account the current characteristics during fast charging. The overall electrode structure has secondary artificial graphite, which has excellent fast charging performance, located on the surface of the electrode where Li ions migrate, and is also advantageously impregnated with electrolyte. When a large number of Li ions initially migrate, charging occurs quickly from the surface, reducing the density of Li ions migrating toward the current collector, which reduces the charging load on the natural graphite. Subsequently, as the current density decreases, charging becomes smoother overall.

[0099] Meanwhile, to form a mixed region containing natural graphite and artificial graphite in the negative electrode active material, the negative electrode active material layer may be formed by simultaneously or sequentially coating a lower region slurry containing a first negative electrode active material (natural graphite and a silicon-based compound) and an upper region slurry containing a second negative electrode active material (artificial graphite and a silicon-based compound) on a current collector, followed by simultaneous drying. The time interval between each slurry coating step may be 1 hour or less, 50 minutes or less, 40 minutes or less, 30 minutes or less, 20 minutes or less, 10 minutes or less, 5 minutes or less, or 30 seconds or less. By controlling the time interval, the thickness ratio of the mixed region can be optimized, thereby further improving various characteristics of the secondary battery according to an embodiment.

[0100] On the one hand, as the silicon-based compound contained as an active material together with the above-mentioned natural graphite and artificial graphite, it may contain at least one or more of Si, SiOx (0 < x ≤ 2), and Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, group 13 elements, group 14 elements excluding Si, transition metals, rare earth elements, and combinations thereof). Note that it is also possible to mix and use SiO2 with at least one of these.

[0101] The element Y may be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, or Po. Specifically, the silicon-based compound may apply SiOx (0 < x ≤ 2).

[0102] The silicon-based compound may be contained in an amount of 10 to 50% by weight, or 10 to 30% by weight, based on the total amount of the active material contained in the negative electrode active material layer. More specifically, the weight ratio of the natural graphite to the silicon-based compound in the negative electrode active material in the lower layer region may be 1:1 to 10:1, or 1:1 to 10:3. Also, the weight ratio of the artificial graphite to the silicon-based compound in the negative electrode active material in the upper layer region may be 1:1 to 10:1, or 1:1 to 10:3. Furthermore, in the mixed region, the weight ratio of the total of the natural graphite and artificial graphite to the silicon-based compound may be 1:1 to 10:1, or 1:1 to 10:3.

[0103] When the weight ratios of the natural graphite and / or artificial graphite to the silicon-based compound each satisfy the above range, high capacity and high energy density of the secondary battery can be ensured. If the content of the natural graphite or artificial graphite exceeds the above weight ratio range and is contained in excess, the energy density of the battery will decrease. If the silicon-based compound exceeds the above weight ratio range and is contained in excess, the durability of the battery will decrease, and side reactions and the like may increase.

[0104] Meanwhile, in one embodiment of the present invention, the negative electrode current collector used in the substrate on which the negative electrode active material layer is formed is not particularly limited as long as it is conductive and does not induce chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys.

[0105] The thickness of the current collector is not particularly limited, but may be a commonly applied thickness of 3 to 500 μm.

[0106] Furthermore, each region of the negative electrode active material layer may contain a binder polymer and a conductive material in addition to the active material described above. In this case, the lower layer region and the upper layer region may contain the binder polymer and the conductive material derived from the respective slurries used to form them, and the mixed region may contain a mixture of the binder polymer and the conductive material derived from the respective slurries.

[0107] In this case, the lower layer region may contain a larger content (wt%) of binder polymer than the upper layer region based on the total content (wt%) of each region. Specifically, the lower layer region may contain 1 to 1.2 wt% or 1 to 1.6 wt% of the binder polymer relative to its total content (wt%), and the upper layer region may contain 0.5 to 0.9 wt% or 0.4 to 0.85 wt% of the binder polymer relative to its total content (wt%).

[0108] In this case, when the ratio of the weight percent of the binder polymer in the lower layer region to the weight percent of the binder polymer in the upper layer region satisfies the above-mentioned relationship and range, excellent adhesive strength and fast charging performance are exhibited.

[0109] The binder polymers contained in the negative electrode active material layer may independently be various types of binder polymers such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butadiene rubber (SBR), fluororubber, acrylic copolymer, etc. In this case, the binder polymers contained in the lower layer region, the upper layer region, and the slurries for forming them may be the same or different.

[0110] Among the examples of the binder polymer, carboxymethyl cellulose (CMC), carboxyethyl cellulose, polyvinylpyrrolidone, etc. can act as a thickener to further enhance the dispersion stability of the slurry.

[0111] According to an embodiment of the present invention, the binder polymer mainly contained in the lower layer region of the negative electrode active material layer may be styrene butadiene rubber (SBR) alone or a mixture of styrene butadiene rubber and an acrylic copolymer.

[0112] In this case, the acrylic copolymer may be an acrylic ester copolymer, an acrylonitrile copolymer, or a combination thereof. Specifically, the acrylic ester copolymer may be a copolymer containing repeating units derived from a (meth)acrylic ester monomer and repeating units derived from a styrene monomer, a vinyl cyanide monomer, a (meth)acrylamide monomer, an unsaturated carboxylic acid monomer, or two or more of these monomers. Furthermore, the acrylonitrile copolymer may be a copolymer containing repeating units derived from an acrylonitrile monomer and repeating units derived from a (meth)acrylic ester monomer, an ethylenically unsaturated carboxylic acid ester monomer, an unsaturated carboxylic acid monomer, a co-diene monomer, a (meth)acrylamide monomer, a nitrile monomer, or two or more of these monomers.

[0113] Furthermore, when the binder polymer for the lower layer region is a mixture of styrene-butadiene rubber and an acrylic copolymer, the styrene-butadiene rubber content in the binder polymer may be greater than that of the acrylic copolymer. Specifically, the weight ratio of the styrene-butadiene rubber to the acrylic copolymer in the binder polymer for the lower layer region may be 51:49 to 99:1, or 70:30 to 99:1. When the styrene-butadiene rubber content in the binder polymer is greater than that of the acrylic copolymer and the weight ratio is within this range, excellent adhesive strength is exhibited.

[0114] In addition, the binder polymer mainly contained in the upper layer region of the negative electrode active material layer may be core-shell particles alone, each having a core portion made of styrene-butadiene rubber and a shell portion surrounding the core portion and made of an acrylic copolymer, or a mixture of the core-shell particles and styrene-butadiene rubber.

[0115] When the binder polymer for the upper layer region is a mixture, the content of the core-shell particles may be greater than that of the styrene-butadiene rubber. Specifically, the weight ratio of the core-shell particles to the styrene-butadiene rubber in the second binder polymer may be 51:49 to 99:1, or 70:30 to 99:1. When the content of the core-shell particles in the binder polymer is greater than that of the styrene-butadiene rubber and satisfies this weight ratio range, electrode processability is improved, for example, contamination of the electrode roll is prevented and electrode flexibility is increased.

[0116] The core-shell particles may have an average particle size of 30 to 100 nm, and the styrene-butadiene rubber may have an average particle size of 200 to 350 nm.

[0117] When the binder polymer for the upper layer region is a mixture, the average particle size of the styrene-butadiene rubber is larger than that of the core-shell particles. Specifically, when the average particle sizes of the core-shell particles and the styrene-butadiene rubber satisfy this range, contamination of the electrode roll is prevented, electrode flexibility is increased, and other electrode processability is improved.

[0118] The core-shell particles may contain 10 to 1 parts by weight, or 6 to 2 parts by weight, of a shell portion made of an acrylic copolymer that surrounds the outside of the core portion, relative to 100 parts by weight of a core portion made of styrene-butadiene rubber.

[0119] In this case, the acrylic copolymer constituting the shell may be an acrylic ester copolymer, an acrylonitrile copolymer, or a combination of these. Specifically, the acrylic ester copolymer may be a copolymer containing a repeating unit derived from a (meth)acrylic ester monomer and a repeating unit derived from a styrene monomer, a vinyl cyanide monomer, a (meth)acrylamide monomer, an unsaturated carboxylic acid monomer, or two or more of these monomers. Furthermore, the acrylonitrile copolymer may be a copolymer containing a repeating unit derived from an acrylonitrile monomer and a repeating unit derived from a (meth)acrylic ester monomer, an ethylenically unsaturated carboxylic acid ester monomer, an unsaturated carboxylic acid monomer, a conjugated diene monomer, a (meth)acrylamide monomer, a nitrile monomer, or two or more of these monomers.

[0120] Meanwhile, the conductive material contained in each of the lower and upper layer regions is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used. The conductive materials contained in each of the lower and upper layer regions may be the same or different.

[0121] According to an embodiment of the present invention, the negative electrode active material layer has a Quantified Binder Ratio (QBR) of 2.0 or less, and the QBR may be defined by the following mathematical formula:

[0122] QBR=Bs / Bf

[0123] In the above mathematical formula, Bs represents the average value of the Os atomic ratio in a surface region of the negative electrode active material layer extending from the outermost surface of the negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer, and Bf represents the average value of the Os atomic ratio in a bottom region of the negative electrode active material layer extending from the interface of the negative electrode active material layer facing the current collector to within 15% of the total thickness of the negative electrode active material layer.

[0124] The Os atomic ratio can be determined by staining the binder polymer included in the negative electrode active material layer with OsO4 (osmium tetraoxide) and then analyzing a cross section of the negative electrode active material layer using energy dispersive X-ray spectroscopy (EDS). That is, the Os atomic ratio can be determined from the Os signal obtained from the EDS analysis results.

[0125] According to an embodiment of the present invention, the QBR may be calculated by the following method.

[0126] First, a negative electrode for which QBR is to be confirmed is selected, and the negative electrode is prepared in a size of 1 cm x 1 cm. The negative electrode is placed in a container containing OsO4 and sealed. After 3 hours, the negative electrode is removed and placed in a vacuum oven to dry for 48 hours. The binder polymer contained in the negative electrode active material layer can then be dyed using OsO4.

[0127] Then, a cross section of the dyed negative electrode is prepared using argon ion milling, and the components in the negative electrode active material layer of the prepared negative electrode cross section are subjected to EDS mapping using an energy dispersive X-ray (EDS) detector equipped with a scanning electron microscope (SEM).

[0128] A line profile is extracted in the thickness direction of the negative electrode active material layer from the EDS mapping results, and the average Os atomic ratio Bs of the Os-stained binder polymer in the surface region of the negative electrode active material layer and the average Os atomic ratio Bf of the Os-stained binder polymer in the bottom region of the negative electrode active material layer are extracted from the extracted line profile results, and the QBR value is calculated using the following equation.

[0129] QBR=Bs / Bf

[0130] In this case, the surface region of the negative electrode active material layer is a region extending from the outermost surface in the thickness direction of the negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer, and the bottom region of the negative electrode active material layer is a region extending from the interface of the negative electrode active material layer facing the current collector to within 15% of the total thickness of the negative electrode active material layer.

[0131] FIG. 14 is a schematic diagram for calculating the QBR value of the negative electrode active material layer.

[0132] In this case, the negative electrode active material layer has a surface region Es that extends from the outermost surface of the negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer, and a bottom region Ef of the negative electrode layer that extends from the interface of the negative electrode active material layer facing the current collector to within 15% of the total thickness of the negative electrode active material layer, based on the total thickness of the negative electrode active material layer.

[0133] 14, the X-axis represents the thickness of the negative electrode active material layer, i.e., the distance from the surface to the current collector, and the Y-axis represents the intensity of the Os atomic component. Line A represents the intensity of the Os atomic component of the Os-stained binder polymer extracted by EDS mapping of the negative electrode active material layer in the cross section of the negative electrode. Line B is a trend line showing the tendency of Line A, which is a line smoothed using the LOWESS smoothing method, i.e., a locally weighted scatterplot smoother method.

[0134] The QBR value is a value that indicates the uniformity of the binder polymer distribution in the thickness direction of the negative electrode active material layer, calculated by the ratio of the Os-dyed binder polymer content in the surface region to the Os-dyed binder polymer content in the bottom region of the negative electrode active material layer. In this case, the binder polymer content can be estimated from the Os atomic content of the Os-dyed binder polymer.

[0135] According to an embodiment of the present invention, the QBR value may be 0.95 or more, 0.97 or more, 1.0 or more, 1.2 or more, 1.5 or more, 1.6 or more, 1.62 or less, 1.7 or less, 1.9 or less, 1.95 or less, or 2.0 or less.

[0136] When the QBR value satisfies this range, migration of the binder polymer to the negative electrode surface is suppressed, the binder is uniformly distributed in the thickness direction of the negative electrode active material layer, the adhesion between the current collector and the electrode layer is improved, and the conductivity on the surface of the negative electrode active material layer and the resulting charge / discharge rate can also be improved.

[0137] On the other hand, according to one aspect of the present invention, the method for producing the negative electrode includes the steps of: a slurry for a lower layer region including the first negative electrode active material, a binder polymer for the lower layer region, a conductive material, and a dispersion medium; preparing a slurry for an upper layer region, the slurry including the second negative electrode active material, a binder polymer for the upper layer region, a conductive material, and a dispersion medium; coating one surface of a negative electrode current collector with the slurry for the lower layer region, and then coating the slurry for the upper layer region thereon; and drying the coated slurries simultaneously to form active material layers. As described above, in order to properly form a mixed region, the coating step should be performed such that the slurries are coated consecutively or within a predetermined time interval.

[0138] The negative electrode active materials (first negative electrode active material, second negative electrode active material), binder polymer, thickener, conductive material, etc. contained in each slurry are as described above. In addition, the dispersion medium may be N-methylpyrrolidone, acetone, water, etc., independently.

[0139] In this case, the lower layer region is formed from the coated lower layer region slurry, the upper layer region is formed from the upper layer region slurry, and the mixed region is formed by mixing these at the above-mentioned certain thickness ratio.

[0140] Each slurry may be coated using a device such as a double slot die. According to an embodiment of the present invention, when the slurry is coated onto the negative electrode current collector, the coating speed may be 10 m / min or more, 20 m / min or more, or 30 m / min or more. When the slurry coating speed satisfies this range, drying occurs before the binder polymer migrates, and therefore the binder polymer may be uniformly distributed in the thickness direction of the negative electrode active material layer.

[0141] In addition, the step of simultaneously drying the coated slurries to form the active material layers may include simultaneously drying the coated slurries to remove the dispersion medium from the slurries, rolling the slurries, and then vacuum drying the slurries to form the active material layers.

[0142] In this case, rolling may be performed by a method commonly used in the art, such as roll pressing, at a pressure of 1 to 20 MPa and a temperature of 15 to 30° C. Furthermore, the rolling may be performed under conditions such that the porosity of the electrode (active material layer) after rolling is 20 to 40%, 25 to 35%, 20 to 30%, or 30 to 40%.

[0143] The step of drying the coated slurry may be performed at, for example, 70 to 90°C, or 75 to 85°C, or 80 to 85°C for 10 to 30 minutes, or 15 to 25 minutes, or 20 to 30 minutes, and the drying temperature and time may be appropriately adjusted depending on the type and content of the dispersion medium.

[0144] The dried slurry layer may be rolled and then vacuum dried at a temperature of 100 to 170°C, 120 to 150°C, or 130 to 150°C for about 3 to 10 hours or 5 to 8 hours, and the drying temperature and time may be appropriately adjusted depending on the type and content of the dispersion medium.

[0145] The negative electrode active material layer may have a total binder polymer content (wt %) of 1 to 3 wt %, 1 to 2 wt %, or 2 to 3 wt %.

[0146] Meanwhile, the positive electrode included in the secondary battery of the embodiment may be manufactured by mixing a positive electrode active material, a conductive material, a binder, and a solvent to prepare a slurry, and then directly coating the slurry on a positive electrode current collector, or by casting the slurry on a separate support, peeling the positive electrode active material film from the support, and laminating the film on the positive electrode current collector.

[0147] The positive electrode active material may include a lithium nickel-based transition metal oxide having a nickel content of 80 to 100 mol % based on the total amount of transition metals. In one specific example, the lithium nickel-based transition metal oxide is represented by the following Chemical Formula 1:

[0148] [Chemical formula 1] Li 1+a (Ni b Co c Mn d Al e M f )O2

[0149] In the above Chemical Formula 1, -0.1≦a≦0.2, 0.8≦b≦1.0, 0.01≦c≦0.15, 0.01≦d≦0.15, 0.01≦e≦0.1, 0≦f≦0.05, and M is at least one selected from the group consisting of Mg, Ti, Zr, Nb, and W.

[0150] In a more specific example, the content of nickel in the lithium nickel-based transition metal oxide may be 80 to 100 mol %, or 85 to 100 mol %, or 88 to 100 mol % based on the total amount of transition metals.

[0151] When the nickel content of the lithium nickel-based transition metal oxide is in the range of 80 to 100 mol% relative to the total amount of transition metals, it is possible to control the resistance in the lower SOC region that affects the output of the secondary battery, thereby providing a greater effect in a high-loading electrode where the positive electrode active material is highly coated, making it applicable to the realization of batteries for high-capacity, high-density EVs. However, when the nickel content is low outside this range, there is a problem with capacity expression.

[0152] The positive electrode active material may be composed of single particles having unimodal characteristics in a particle size distribution curve, or may be composed of secondary particles formed by aggregation of primary particles through a granulation process. However, in the enlarged, tableless cylindrical secondary battery, it is more preferable to use a positive electrode active material having the single particle morphology, considering the need to further reduce resistance to improve fast charging characteristics and the need to suppress side reactions and gas generation.

[0153] In this case, the D50 of the single particle type positive electrode active material may be 1 μm to 15 μm, or 2 μm to 8 μm, or 3 to 7 μm, thereby maximizing the effects of reducing the resistance, side reactions, and gas generation.

[0154] The positive electrode current collector is generally manufactured to a thickness of 3 to 300 μm, and is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, it may be one selected from stainless steel, aluminum, nickel, titanium, and aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver, and aluminum is preferably used.

[0155] The positive electrode current collector may have fine irregularities on its surface to enhance adhesion of the positive electrode active material, and may be in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0156] Meanwhile, the conductive material, binder polymer, and dispersion medium may be appropriately selected from those exemplified for the preparation of the negative electrode.

[0157] The separator may be a conventional porous polymer film used in conventional separators, such as a porous polymer film made from a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in combination. Alternatively, an insulating thin film having high ion permeability and mechanical strength may be used. The separator may include a safety reinforced separator (SRS), in which the surface of the separator is thinly coated with a ceramic material. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made from high-melting-point glass fiber or polyethylene terephthalate fiber, may be used, but is not limited to these.

[0158] The electrolytic solution contains a lithium salt as an electrolyte and an organic solvent for dissolving the lithium salt.

[0159] The lithium salt can be any one that is commonly used in electrolytes for secondary batteries. For example, the anion of the lithium salt can be F - , Cl - , I- , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One selected from the group consisting of:

[0160] The organic solvent contained in the electrolytic solution may be any commonly used organic solvent without limitation, and typically, one or more selected from the group consisting of propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfide, and tetrahydrofuran may be used.

[0161] In particular, among the carbonate organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because they are highly viscous organic solvents with a high dielectric constant and thus easily dissociate lithium salts in the electrolyte. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte solution having high electrical conductivity can be prepared, and therefore, such an electrolyte solution can be preferably used.

[0162] Optionally, the electrolyte may further contain additives such as an overcharge inhibitor that are commonly contained in electrolytes.

[0163] Meanwhile, a cylindrical secondary battery according to an embodiment of the present invention may be manufactured by forming an electrode assembly 10 by disposing a separator 15 between a positive electrode 13 and a negative electrode 14, placing the electrode assembly 10 in a battery can, and then injecting an electrolyte.

[0164] Meanwhile, an example of the overall configuration of a cylindrical secondary battery according to an embodiment is schematically shown in Figure 3. As described above with reference to Figures 1 and 2, the cylindrical secondary battery according to an embodiment basically includes a jelly-roll-shaped electrode assembly 10 in which a positive electrode 13 and a negative electrode 14, each of which has an electrode tab 11, 12 defined by a segment of an uncoated portion 16, and a separator 15 interposed therebetween, are wound in one direction.

[0165] Referring to FIG. 3 , the cylindrical secondary battery may further include a battery can 20 that accommodates the electrode assembly 10 and is electrically connected to the electrode assembly 10, a through-terminal 40 that penetrates one side of the battery can 20 and is electrically connected to the electrode assembly 10, and a cap plate 30 configured to cover the opening of the battery can 20.

[0166] In addition, the through terminal 40 may be electrically connected to a segmented electrode tab 11 having a positive polarity, and the battery can 20 may be electrically connected to a segmented electrode tab 12 having a negative polarity.

[0167] The battery pack may further include an insulating gasket 50 interposed between the battery can 20 and the through terminal 40 to insulate the through terminal 40 from the battery can 20, and may further include first and second current collecting plates 60 and 80 that electrically connect the electrode tabs 11 and 12 to the through terminal 40 and the battery can 20. Furthermore, the battery pack may further include an insulator 70 interposed between the first current collecting plate and the battery can.

[0168] In a battery having such a structure, the electrode tab 12 having a negative polarity is electrically connected to the battery can 20 via the second current collecting plate 80 having a large area, and the electrode tab 11 having a positive polarity is electrically connected to the through terminal 40 via the first current collecting plate 60 having a large area, and electric charges and currents can move through the battery can 20 and the through terminal 40.

[0169] Therefore, since the cylindrical secondary battery according to an embodiment has such a tablet-type battery structure, the current / charge transfer path during charging and discharging is minimized, thereby enabling improved fast charging characteristics.

[0170] The cylindrical secondary battery may be, for example, a cylindrical secondary battery having a form factor ratio (defined as the value obtained by dividing the diameter of the secondary battery, defined from the maximum diameter and maximum height of a cylindrical battery can, by the height, i.e., the ratio of the diameter Φ to the height H) of greater than about 0.4. For example, the cylindrical secondary battery may have a diameter of 35 mm or more and a height of 75 mm or more.

[0171] Here, the form factor refers to values ​​indicating the diameter and height of a cylindrical secondary battery. In a more specific embodiment, the cylindrical secondary battery may be, for example, a 46110 cell, a 48750 cell, a 48110 cell, a 48800 cell, or a 46800 cell. In the numerical value indicating the form factor, the first two digits indicate the diameter of the cell, the next two digits indicate the height of the cell, and the last digit 0 indicates that the cross section of the cell is circular.

[0172] That is, according to a specific embodiment, the cylindrical secondary battery according to one embodiment is a substantially cylindrical battery, and may be a 46110 cell having a diameter of about 46 mm, a height of about 110 mm, and a form factor ratio of about 0.418; a 48750 cell having a diameter of about 48 mm, a height of about 75 mm, and a form factor ratio of about 0.640; a 48110 cell having a diameter of about 48 mm, a height of about 110 mm, and a form factor ratio of about 0.436; a 48800 cell having a diameter of about 48 mm, a height of about 80 mm, and a form factor ratio of about 0.600; or a 46800 cell having a diameter of about 46 mm, a height of about 80 mm, and a form factor ratio of about 0.575.

[0173] This battery has a larger form factor than conventional 18650 and 21700 cells, and is a large-sized battery that is advantageous for fast charging, making it suitable for use in medium- to large-sized devices such as automobiles.

[0174] 4, a battery pack 3 according to another embodiment of the present invention includes a secondary battery assembly in which a plurality of cylindrical secondary batteries 1 according to the above-described embodiment are electrically connected, and a pack housing 2 that accommodates the secondary battery assembly. For ease of illustration, components such as bus bars for electrical connection, a cooling unit, and power terminals are omitted from the drawings in this specification.

[0175] 5, a vehicle 5 according to another embodiment of the present invention is, for example, an electric vehicle, a hybrid vehicle, or a plug-in hybrid vehicle, and includes a battery pack 3 according to the other embodiment. The vehicle 5 includes a four-wheeled vehicle and a two-wheeled vehicle. The vehicle 5 operates by receiving power from the battery pack 3 according to the other embodiment.

[0176] The present invention will be described in detail below with reference to examples to aid in understanding the present invention. However, the examples according to the present invention can be modified in various ways, and the scope of the present invention is not limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0177] Example 1: Manufacturing of secondary battery <Production of negative electrodes> A slurry for the lower layer was prepared by mixing 10 parts by weight of SiO (silicon oxide) as a first negative electrode active material, 50 parts by weight of natural graphite, 0.1 parts by weight of CNT as a first conductive material, and 1.4 parts by weight of styrene butadiene rubber (SBR) as a first binder polymer, and adding water.

[0178] 10 parts by weight of SiO (silicon oxide) as a second negative electrode active material, 50 parts by weight of artificial graphite, 0.1 parts by weight of CNT as a second conductive material, and 0.7 parts by weight of core-shell particles as a second binder polymer were mixed, and water was added to produce a slurry for the upper layer.

[0179] The artificial graphite had a form including a carbon coating layer disposed on secondary particles formed by granulating primary particles of the artificial graphite. The primary particles had a D50 of 10 μm, and the second negative electrode active material, which was artificial graphite in the form of secondary particles formed by granulating and agglomerating the primary particles, had an average particle size D50 of 20 μm. The carbon coating layer on the secondary particles in the second negative electrode active material was included in an amount of 4.0 wt % based on the total weight of the second negative electrode active material. The second negative electrode active material had an average particle size D50 of 21 μm.

[0180] The core-shell particles are composed of a core made of styrene-butadiene rubber and a shell made of an acrylic copolymer and surrounding the core.

[0181] Thereafter, using a double slot die, the lower layer slurry was applied to both sides of a 10 μm thick copper (Cu) foil serving as a negative electrode current collector at 2.5 mAh / cm 2After that, the upper layer slurry was continuously (without time intervals) applied on the applied lower layer slurry at a load of 2.5 mAh / cm 2 The coating speed for applying the lower layer slurry and the upper layer slurry was 30 m / min. The current collector coated with the slurry was then dried at 80°C for 20 minutes to remove water from the slurry, and the dried slurry layer was rolled and then vacuum-dried at about 130°C for 8 hours to prepare a negative electrode.

[0182] Electron microscope images of the negative electrode are shown in Figures 6a and 6b. Referring to Figures 6a and 6b, it was confirmed that negative electrode active material layers of approximately 70 μm and 79 μm thick were formed on both sides of the 10 μm-thick current collector, respectively (total thickness: 159 μm). Furthermore, upon examination of the 70 μm-thick active material layer, it was confirmed that it had a porosity of 30% and contained a 25 μm-thick upper layer region (artificial graphite + silicon oxide distributed layer), a 20 μm-thick mixed region (artificial graphite and natural graphite mixed layer + silicon oxide), and a 25 μm-thick lower layer region (natural graphite + silicon oxide distributed layer).

[0183] <Production of positive electrodes> The positive electrode active material is a lithium-ion battery with a Ni content of 86 mol% of the total transition metals, with the chemical formula Li[Ni 0.86 Mn 0.05 Co 0.07 ]Al 0.2 97 parts by weight of O2, 1.4 parts by weight of polyvinylidene fluoride (PVdF) as a binder polymer, and 0.4 parts by weight of CNT as a conductive material were mixed in N-methylpyrrolidone (NMP), and then applied to a 20 μm thick Al foil at 5 mAh / cm 2 The mixture was vacuum dried at about 130°C for 8 hours and rolled to a porosity of 30% to prepare a positive electrode.

[0184] <Manufacturing lithium secondary batteries> A non-aqueous electrolyte solution was prepared by dissolving LiPF6 to a concentration of 1.0 M in an organic solvent made by mixing ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) in a volume ratio of 3:3:4.

[0185] A porous polyethylene separator was interposed between the cathode and anode to prepare an electrode assembly, which was then placed in a cylindrical case and the electrolyte was injected to prepare a lithium secondary battery (cylindrical battery). The form factor of the cylindrical battery was a 46800 cylindrical cell with a maximum diameter of 46 mm and a maximum height of 80 mm.

[0186] Comparative Example 1: Manufacturing of secondary battery 10 parts by weight of SiO (silicon oxide), 50 parts by weight of natural graphite, and 50 parts by weight of artificial graphite were mixed as a first negative electrode active material, 0.1 parts by weight of CNT as a conductive material, and 1.2 parts by weight of styrene butadiene rubber (SBR) as a binder polymer, and water was added to prepare a slurry.

[0187] The artificial graphite had a carbon coating layer disposed on secondary particles formed by granulating primary particles of the artificial graphite. The primary particles had a D50 of 10 μm, and the first negative electrode active material, which was artificial graphite in the form of secondary particles formed by granulating and agglomerating the primary particles, had an average particle size D50 of 20 μm. In the second negative electrode active material, the carbon coating layer on the secondary particles was included in an amount of 4.0 wt % based on the total weight of the second negative electrode active material. The second negative electrode active material had an average particle size D50 of 21 μm.

[0188] The slurry was applied to both sides of a 10 μm-thick copper (Cu) foil negative electrode current collector at a loading of 5 mAh / cm2. The coating speed was 30 m / min. The coated current collector was vacuum dried at approximately 130°C for 8 hours and rolled to a porosity of 30% to fabricate a single-layer negative electrode. An electron microscope image of the negative electrode is shown in Figure 6c. Referring to Figure 6c, it was confirmed that the negative electrode active material layers formed on both sides of the current collector consisted entirely of regions containing a mixture of natural graphite, artificial graphite, and silicon oxide.

[0189] A positive electrode and a secondary battery (cylindrical battery) were manufactured in the same manner as in Example 1, except that the negative electrode thus manufactured was used.

[0190] Comparative Example 2: Manufacturing of secondary battery 10 parts by weight of SiO (silicon oxide) as a first negative electrode active material, 50 parts by weight of natural graphite, 50 parts by weight of artificial graphite, 0.1 parts by weight of CNT as a conductive material, and 1.05 parts by weight of styrene butadiene rubber (SBR) as a binder polymer were mixed, and water was added to prepare a slurry.

[0191] The artificial graphite had a carbon coating layer disposed on secondary particles formed by granulating primary particles of the artificial graphite. The primary particles had a D50 of 10 μm, and the first negative electrode active material, which was artificial graphite in the form of secondary particles formed by granulating and agglomerating the primary particles, had an average particle size D50 of 20 μm. In the second negative electrode active material, the carbon coating layer on the secondary particles was included in an amount of 4.0 wt % based on the total weight of the second negative electrode active material. The second negative electrode active material had an average particle size D50 of 21 μm.

[0192] Then, using a double slot die, the prepared slurry was applied to one side of a copper (Cu) foil, which was a negative electrode current collector with a thickness of 10 μm, at a rate of 2.5 mAh / cm. 2 After applying the slurry at a loading amount of 2.5 mAh / cm 2 onto the applied slurry, the prepared slurry was applied in the same manner. 2The lower layer slurry and the upper layer slurry were applied at a loading amount of 100 μm. The coating speed for applying the lower layer slurry and the upper layer slurry was 30 m / min, respectively. The current collector with the applied slurry was then dried at 80°C for 20 minutes to remove water from the slurry, and the dried slurry was rolled and then vacuum-dried at about 130°C for 8 hours to prepare a negative electrode. The prepared negative electrode had a porosity of 30% and a double-layer structure with a 50 μm-thick upper layer region and a 50 μm-thick lower layer region, and included a negative electrode active material layer with a total thickness of 100 μm.

[0193] A positive electrode and a secondary battery (cylindrical battery) were manufactured in the same manner as in Example 1, except that the negative electrode thus manufactured was used.

[0194] Characterization of secondary batteries and negative electrodes Experimental example 1: Swelling characteristic evaluation The lithium secondary batteries (cylindrical batteries) of each Example and Comparative Example were initially charged and discharged once using an electrochemical charger / discharger. Charging was performed at a current density of 1.5 C up to a voltage of 4.47 V, and discharging was performed at the same current density down to 3.0 V.

[0195] The swelling characteristics of each secondary battery that had undergone one charge-discharge cycle were evaluated, and were calculated as the percentage of the diameter of the secondary battery that had changed after charge-discharge to the initial diameter of the secondary battery before charge-discharge, using the following equation:

[0196] Swelling (%) = [(diameter of secondary battery after charging / discharging) - (diameter of initial secondary battery)] / (diameter of initial secondary battery) × 100 The results are shown in the following Table 1. The results of X-ray CT photography of the cylindrical secondary batteries of Example 1, Comparative Example 1 and Comparative Example 2 are shown in Figs.

[0197] [Table 1]

[0198] 7 to 9, cracks occurred due to swelling of the negative electrode after charge-discharge cycles. However, when the modified electrode structure of the example was used, the cracks in the core region were improved, significantly reducing the swelling. That is, in Figures 7 to 9, the central region is the center of the jelly roll (the region where the core is removed), and the cylindrical secondary batteries of Comparative Examples 1 and 2, which used negative electrodes having only a single layer or a double layer of a mixture of natural graphite and artificial graphite, showed significant swelling and distorted the circular shape of the central region. However, in the cylindrical secondary battery of Example 1 (where natural graphite and a silicon-based compound were distributed in the lower layer region, natural graphite, artificial graphite, and a silicon-based compound were distributed in the mixed region, and artificial graphite and a silicon-based compound were distributed in the upper layer region), swelling was suppressed and the circular shape was maintained.

[0199] 7 to 9 show X-ray CT photographs of the cylindrical batteries of Example 1, Comparative Example 1, and Comparative Example 2 after one charge / discharge cycle, respectively.

[0200] 7 to 9, the central portions are the central portions of the jelly rolls of the cylindrical batteries (the portions where the cores have been removed). In the cylindrical battery of Example 1 in FIG. 7, the swelling phenomenon was suppressed and the circular shape was maintained. However, in the cylindrical batteries of Comparative Examples 1 and 2 in FIGS. 8 and 9, only regions where artificial graphite and natural graphite were mixed were present in each negative electrode active material layer, which resulted in significant swelling and distorted circularity of the central portions of the jelly rolls.

[0201] Experimental Example 2: Evaluation of lithium deposition phenomenon The lithium secondary batteries (cylindrical batteries) of each Example and Comparative Example were subjected to one charge-discharge cycle under the conditions of Experimental Example 1, and then disassembled. As a result, photographs of the surface of the negative electrode active material layer of each secondary battery and the negative electrode active material layer attached to the separator are shown in FIGS. 10 to 12, respectively.

[0202] 10 to 12, the upper photographs show the surface of the negative electrode active material layer, and the lower photographs show the negative electrode active material layer attached to the separator. In the secondary batteries of Comparative Examples 1 and 2 in FIGS. 11 and 12, black lithium deposits were observed on the surface of the negative electrode active material layer and on the negative electrode active material layer attached to the separator. On the other hand, no lithium deposits were observed in the secondary battery of Example 1 in FIG. 10.

[0203] Experimental example 3: Evaluation of rapid charging characteristics A rapid charge / discharge test was carried out on each of the batteries of Example 1 and Comparative Example 1 under the conditions of 2.5C (4.1V, 0.05C) / 0.5C (3.0V), and the test results are shown in FIGS. 13a to 13d, respectively.

[0204] For reference, in Figures 13a to 13d, the solid lines show the evaluation results for Comparative Example 1, and the dotted lines show the evaluation results for Example 1. Figure 13a shows the change in capacity characteristics with each cycle during rapid charge / discharge, and Figure 13b shows a 2.5C charge profile. Furthermore, Figures 13c and 13d are DCIR profiles showing the change in voltage and resistance with time during rapid charge at 50% SOC.

[0205] 13a, it was confirmed that Example 1 exhibited excellent capacity retention rate during cycles in the rapid charge-discharge test compared to Comparative Example 1. Furthermore, referring to FIGS. 13c and 13d, it was confirmed that Example 1 reduced overvoltage and suppressed an increase in interfacial resistance compared to Comparative Example 1.

[0206] This confirms that the battery of Example 1 exhibits improved rapid charging characteristics compared to Comparative Example 1.

[0207] Experimental Example 4: Evaluation of the distribution characteristics (QBR) of the binder polymer of the negative electrode The negative electrode prepared in Example 1 was prepared in a size of 1 cm x 1 cm, placed in a container containing OsO4, and sealed. After 3 hours, the negative electrode was removed and placed in a vacuum oven to dry for 48 hours. The binder polymer contained in the negative electrode active material layer was dyed with OsO4. In this case, since the negative electrode had negative electrode active material layers coated on both sides of the current collector, the negative electrode active material layer formed on the upper surface of the current collector was designated as the first negative electrode active material layer, and the negative electrode active material layer formed on the lower surface of the current collector was designated as the second negative electrode active material layer.

[0208] A cross section of the dyed negative electrode was then prepared using argon ion milling, and the components in the first and second negative electrode active material layers of the prepared negative electrode cross section were then EDS mapped using an energy dispersive X-ray spectroscopy (EDS) detector equipped with a scanning electron microscope (SEM).

[0209] From the EDS mapping results, line profiles were extracted in the thickness direction of the first and second negative electrode active material layers. From the extracted line profile results, the average value Bs of the Os atomic ratio of the Os-stained binder polymer in the surface regions of the first and second negative electrode active material layers and the average value Bf of the Os atomic ratio of the Os-stained binder polymer in the bottom regions of the first and second negative electrode active material layers were extracted, and the QBR values ​​were calculated using the following formula. The results are shown in Table 2.

[0210] QBR=Bs / Bf

[0211] In this case, the surface regions of the first and second negative electrode active material layers refer to regions extending from the outermost surfaces of the first and second negative electrode active material layers in the thickness direction thereof to within 15% of the total thickness of the first and second negative electrode active material layers, and the bottom regions of the first and second negative electrode active material layers refer to regions extending from the interfaces of the first and second negative electrode active material layers facing the current collector to within 15% of the total thickness of the first and second negative electrode active material layers.

[0212] [Table 2]

[0213] 15a is a graph showing changes in normalized intensity of the Os component stained by the binder polymer of the first negative electrode active material layer extracted and analyzed by EDS mapping along the distance from the surface of the first negative electrode active material layer toward the current collector for the negative electrode of Example 1. FIG. 15b is a graph showing changes in normalized intensity of the Os component stained by the binder polymer of the second negative electrode active material layer extracted and analyzed by EDS mapping along the distance from the surface of the second negative electrode active material layer toward the current collector for the negative electrode of Example 1.

[0214] In Figures 15a and 15b, the binder lines show the intensity in each depth direction when the entire Os component of the actually measured Os-stained binder polymer is normalized to 1. The trend lines show the tendency of the binder lines, and are lines smoothed using the LOWESS smoothing method, i.e., the locally weighted scatter plot smoothing method. The Avg wt% line is a line that always shows a value of 1. [Explanation of symbols]

[0215] 1 Cylindrical secondary battery 2-pack housing 3 Battery Pack 5. Automobiles 10 Electrode assembly 11, 12 Electrode (positive and negative) tabs 13 Positive electrode 14 Negative electrode 15 Separation membrane 16 Uncoated section 17 Current collector 18 Active material layer 20 Battery can 30 Cap Plate 40 Feed-through terminal 50 Insulation gasket 60 First current collecting plate 70 Insulator 80 Second current collecting plate

Claims

1. A cylindrical secondary battery comprising: a jelly-roll-shaped electrode assembly in which a positive electrode including a positive electrode active material layer on a positive electrode current collector, a negative electrode including a negative electrode active material layer on a negative electrode current collector, and a separator interposed between the positive electrode and the negative electrode are wound up; and a battery can accommodating the electrode assembly, the negative electrode active material layer includes, in order from the negative electrode current collector side, a lower layer region, a mixed region, and an upper layer region, the lower layer region contains a silicon-based compound and natural graphite as active materials, the mixed region contains a silicon-based compound, natural graphite, and artificial graphite as active materials; the mixed region has an active material distribution gradient in which the distribution ratio of the natural graphite decreases and the distribution ratio of the artificial graphite increases toward the upper layer region, the upper layer region contains a silicon-based compound and artificial graphite as active materials, A cylindrical secondary battery having a diameter of 35 mm or more and a height of 75 mm or more based on the maximum diameter and maximum height of the battery can.

2. 2. The cylindrical secondary battery according to claim 1, wherein the negative electrode current collector is in surface contact with the lower layer region, the lower layer region is in surface contact with the mixed region, and the mixed region is in surface contact with the upper layer region.

3. 10. The cylindrical secondary battery of claim 1, wherein the positive electrode active material layer contains, as an active material, a lithium nickel-based transition metal oxide having a nickel content of 80 to 100 mol % based on the total amount of transition metals.

4. 4. The cylindrical secondary battery according to claim 3, wherein the lithium nickel-based transition metal oxide is represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ａ (N ｂ Co ｃ Mn ｄ Al ｅ M ｆ )O ２ In the formula 1, -0.1≦a≦0.2, 0.8≦b≦1.0, 0.01≦c≦0.15, 0.01≦d≦0.15, 0.01≦e≦0.1, and 0≦f≦0.05, and M is at least one selected from the group consisting of Mg, Ti, Zr, Nb, and W.

5. 2. The cylindrical secondary battery according to claim 1, wherein the negative electrode active material layer has a thickness of 40 μm to 200 μm.

6. In the negative electrode active material layer, based on the cross-sectional thickness of the portion where the mixed region is formed at the thickest, 10. The cylindrical secondary battery of claim 1, wherein the mixed region has a thickness of 20% to 80% of the total thickness of the negative electrode active material layer.

7. In the negative electrode active material layer, based on the cross-sectional thickness of the portion where the mixed region is formed at the thickest, 7. The cylindrical secondary battery of claim 6, wherein the lower layer region has a thickness of 10% to 50% of the thickness of the entire negative electrode active material layer.

8. In the negative electrode active material layer, based on the cross-sectional thickness of the portion where the mixed region is formed at the thickest, 7. The cylindrical secondary battery of claim 6, wherein the upper layer region has a thickness of 10% to 50% of the thickness of the entire negative electrode active material layer.

9. The cylindrical secondary battery according to claim 1, wherein the mixed region contains the natural graphite and the artificial graphite in a weight ratio of 2:8 to 8:

2.

10. 2. The cylindrical secondary battery according to claim 1, wherein the natural graphite has a particle shape exhibiting a sphericity of more than 0.91 and an average particle size D50 of 5 μm to 30 μm.

11. 2. The cylindrical secondary battery according to claim 1, wherein the artificial graphite comprises secondary particles formed by agglomeration of primary particles, and a carbon coating layer formed on the surface of the secondary particles.

12. The cylindrical secondary battery of claim 11, wherein the carbon coating layer is included in an amount of 0.5 wt % to 10 wt % based on the total weight of the artificial graphite.

13. The cylindrical secondary battery according to claim 11, wherein the artificial graphite has an average particle size D50 of 4 μm to 32 μm.

14. 2. The cylindrical secondary battery according to claim 1, wherein the silicon-based compound contains Si, SiOx (0<x≦2), a Si-Y alloy (Y is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements excluding Si, transition metals, rare earth elements, and combinations thereof), or two or more of these.

15. 10. The cylindrical secondary battery of claim 1, wherein the silicon-based compound is contained in an amount of 10 to 50 wt % based on the total amount of active materials contained in the negative electrode active material layer.

16. the lower layer region, the mixed region, and the upper layer region each contain the active material, the binder polymer, and the conductive material; The cylindrical secondary battery of claim 1 , wherein the lower region contains a binder polymer in a content (wt %) greater than that of the upper region, relative to the total content (wt %) of each region.

17. The lower layer region contains 1 wt % to 1.2 wt % of a binder polymer based on the total content (wt %), 17. The cylindrical secondary battery of claim 16, wherein the upper layer region contains 0.5 wt % to 0.9 wt % of the binder polymer based on the total content (wt %) of the binder polymer.

18. The cylindrical secondary battery according to claim 16, wherein the binder polymer of the lower layer region includes styrene butadiene rubber (SBR) or a mixture of styrene butadiene rubber and an acrylic copolymer.

19. the binder polymer of the lower layer region includes a mixture of styrene butadiene rubber and an acrylic copolymer; The cylindrical secondary battery according to claim 18, wherein the styrene-butadiene rubber is contained in a larger amount than the acrylic copolymer.

20. 17. The cylindrical secondary battery according to claim 16, wherein the binder polymer of the upper layer region includes core-shell particles having a core portion made of styrene-butadiene rubber and a shell portion surrounding the outside of the core portion and made of an acrylic copolymer, or a mixture of the core-shell particles and styrene-butadiene rubber.

21. the binder polymer of the upper layer region includes a mixture of the core-shell particles and styrene-butadiene rubber; The cylindrical secondary battery according to claim 20, wherein the core-shell particles are contained in a content greater than that of the styrene-butadiene rubber.

22. The cylindrical secondary battery according to claim 20, wherein the average particle size D50 of the core-shell particles is 30 nm to 100 nm, and the average particle size of the styrene-butadiene rubber is 200 nm to 350 nm.

23. the negative electrode active material layer has a QBR (Quantified Binder Ratio) of 2.0 or less, The cylindrical secondary battery of claim 1 , wherein the QBR is defined by the following mathematical formula: QBR=(QBR / QBR) / ... QBR = Bs / Bf (In the above mathematical formula, Bs represents the average value of the Os atomic ratio in a surface region of the negative electrode active material layer extending from the outermost surface of the negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer; Bf represents the average value of the Os atomic ratio in a bottom region of the negative electrode active material layer extending from the interface of the negative electrode active material layer facing the negative electrode current collector to within 15% of the total thickness of the negative electrode active material layer; The Os atomic ratio is determined by dividing the cross section of the negative electrode active material layer by OsO ４ After staining with , the sample is analyzed by energy dispersive spectroscopy (EDS).

24. In the positive electrode and the negative electrode, a non-coated portion where an active material layer is not formed is present along an end portion of one side of the current collector in a direction parallel to the winding direction, The cylindrical secondary battery according to claim 1 , wherein at least a portion of the current collector in the uncoated portion defines an electrode tab.

25. 25. The cylindrical secondary battery of claim 24, wherein at least a portion of the current collector defining the electrode tab is fabricated in the form of a plurality of segmented pieces that can be bent independently.

26. 2. The cylindrical secondary battery according to claim 1, wherein a form factor ratio, defined as the diameter divided by the height, is greater than 0.

4.

27. 27. The cylindrical secondary battery according to claim 26, wherein the cylindrical secondary battery is a 46110 cell, a 48750 cell, a 48110 cell, a 48800 cell, or a 46800 cell.

28. A battery pack comprising the cylindrical secondary battery according to any one of claims 1 to 27.

29. 30. A motor vehicle comprising the battery pack of claim 28.

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