Negative electrode for rechargeable lithium battery and rechargeable lithium battery comprising the same

Abstract

Disclosed is a negative electrode for a rechargeable lithium battery and a rechargeable lithium battery including the same, and the negative electrode includes: a current collector; a first negative electrode active material layer disposed on the current collector and including a first negative electrode active material; and a second negative electrode active material layer disposed on the first negative electrode active material layer and including a second negative electrode active material, wherein, when XRD is measured by using CuKα rays, a peak intensity ratio (I (002) / I (110) ) of the first negative electrode active material layer and the second negative electrode active material layer at a (002) plane to a peak intensity at a (110) plane is 150 or less.

Description

TECHNICAL FIELD

[0001] The present invention relates to a negative electrode for a rechargeable lithium battery and a rechargeable lithium battery including the same. BACKGROUND

[0002] Rechargeable lithium batteries, which use an organic electrolyte solution, have recently attracted attention as power sources for small portable electronic devices, and have a discharge voltage twice or more that of conventional batteries using an alkaline aqueous solution, and thus have a high energy density.

[0003] For a positive electrode active material of a rechargeable lithium battery, oxides including lithium and transition metals having a structure capable of intercalating / deintercalating lithium ions, such as LiCoO2, LiMn2O4, LiNi 1-x Co x O2(0 < x < 1), etc., have been mainly used.

[0004] For a negative electrode active material, various carbon-based materials capable of intercalating / deintercalating lithium ions, such as artificial graphite, natural graphite, hard carbon, etc., have been used, and recently, non-carbon-based negative electrode active materials, such as silicon or tin, have been researched to obtain a high capacity.

[0005] The above information disclosed in this Background section is only for enhancing the understanding of the background of the invention, and therefore it can contain information that does not constitute prior art that is already known in this country to those of ordinary skill in the art. SUMMARY

[0006] One embodiment provides a negative electrode for a rechargeable lithium battery, which exhibits reduced resistance and good cycle life characteristics.

[0007] Another embodiment provides a rechargeable lithium battery including the same.

[0008] One embodiment provides a negative electrode for a rechargeable lithium battery, the negative electrode including: a current collector; a first negative electrode active material layer disposed on the current collector and including a first negative electrode active material; and a second negative electrode active material layer disposed on the first negative electrode active material layer and including a second negative electrode active material, wherein, when XRD is measured by using Cu Kα rays, a peak intensity ratio (I (002) / I (110) ) of the first negative electrode active material layer and the second negative electrode active material layer at a (002) plane to a peak intensity at a (110) plane is 150 or less.

[0009] The peak intensity ratio (I (002) / I (110) ) can be about 1 to about 150.

[0010] Peak intensity ratio (I) (002) / I (110) The first and second negative electrode active material layers can be obtained after coating the composition for the first negative electrode active material layer and the composition for the second negative electrode active material layer onto the current collector to prepare the first and second layers, applying a magnetic field to the resulting product, and drying and pressing to prepare the first and second negative electrode active material layers.

[0011] In one embodiment, the peak intensity ratio (I) (002) / I (110) The first and second negative electrode active material layers can be obtained by coating a composition for the first negative electrode active material layer onto a current collector to form a first layer, coating a composition for the second negative electrode active material layer onto the first layer to form a second layer, applying a magnetic field to the resulting product, and drying and pressing it to prepare the first and second negative electrode active material layers.

[0012] The first negative electrode active material layer and the second negative electrode active material layer can be orientation layers, in which the first negative electrode active material and the second negative electrode active material are oriented relative to the current collector.

[0013] When XRD is measured using CuKα rays, the first and second negative electrode active material layers can have a peak intensity ratio (I002) of the peak intensity at the (002) plane relative to the peak intensity at the (110) plane. (002) / I (110) The peak intensity ratio (I) (002) / I (110) ) can correspond to the peak intensity ratio (I) of the unoriented layer (002) / I (110) The non-oriented layer comprises approximately 90% or less of the first negative electrode active material layer and the second negative electrode active material layer, and has the same composition and thickness as the first negative electrode active material layer and the second negative electrode active material layer.

[0014] The ratio of the peel strength of the first negative electrode active material layer to the peel strength of the second negative electrode active material layer can be from about 70% to about 90%.

[0015] The first and second negative electrode active materials can be the same or different from each other, and can include crystalline carbon-based materials. The crystalline carbon-based materials can be artificial graphite, natural graphite, or a combination thereof.

[0016] The first and second negative electrode active materials may further include at least one selected from Si-based negative electrode active materials, Sn-based negative electrode active materials, and lithium vanadium oxide negative electrode active materials.

[0017] The first negative electrode active material layer may have a thickness of about 20 μm to about 125 μm, and the second negative electrode active material layer may have a thickness of about 20 μm to about 125 μm.

[0018] The peak intensity ratio can be a peak integral area value ratio obtained from a peak integral area intensity value at the (002) plane / a peak integral area intensity value at the (110) plane.

[0019] Another embodiment provides a rechargeable lithium battery including: a negative electrode; a positive electrode including a positive electrode active material; and an electrolyte.

[0020] Other embodiments include those described in the following detailed description.

[0021] The negative electrode for a rechargeable lithium battery can exhibit reduced electrical resistance and excellent cycle life characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a schematic diagram showing the orientation according to an embodiment of the present application.

[0023] Figure 2 is an exploded perspective view of a rechargeable lithium battery according to an embodiment.

[0024] Figure 3 shows an SEM photograph showing a cross section of a negative electrode prepared from Example 1 and Comparative Example 1 before pressing.

[0025] Figure 4 is a graph showing the peak intensity I (002) and the peak intensity I (110) and the peak intensity ratio (I (002) / I (110) ) related to a negative electrode precursor prepared by coating a negative electrode active material layer slurry in the process of Example 1 and Comparative Example 1, measured by XRD using CuKα rays.

[0026] Figure 5 is a graph showing the peak intensity I (002) and the peak intensity I (110) and the peak intensity ratio (I (002) / I (110) ) related to a negative electrode prepared by coating a negative electrode active material layer slurry and pressing in the process of Example 1 and Comparative Example 1, measured by XRD using CuKα rays.

[0027] Figure 6 is a graph comparing the peak intensity ratio (I (002) / I (110) ) of the negative electrode obtained from pressing according to Example 1 to Example 3 and Comparative Example 1 and Comparative Example 2.

[0028] Figure 7is a graph showing peel strength ratio (%) of the upper and lower parts of the negative electrode according to Example 1, Comparative Example 1, and Comparative Example 2.

[0029] Figure 8 is a graph showing ion resistance (R ion ) of the negative electrode according to Example 1 and Comparative Example 1.

[0030] Figure 9 is a graph showing DC internal resistance (DCIR) of the negative electrode according to Example 1 and Comparative Example 1 at various discharge depths.

[0031] Figure 10 is a graph showing cycle life characteristics of the negative electrode according to Example 1, Comparative Example 1, and Comparative Example 2 at room temperature.

[0032] Figure 11 is a graph showing cycle life characteristics of the negative electrode according to Example 1, Comparative Example 1, and Comparative Example 2 at low temperature. DETAILED DESCRIPTION

[0033] Hereinafter, embodiments of the present application are described in detail. However, these embodiments are exemplary, the present application is not limited thereto, and the present application is defined by the scope of claims.

[0034] A negative electrode for a rechargeable lithium battery according to one embodiment includes: a current collector; a first negative electrode active material layer disposed on the current collector and including a first negative electrode active material; and a second negative electrode active material layer disposed on the first negative electrode active material layer and including a second negative electrode active material. When XRD is measured by using CuKα rays, a peak intensity ratio (I (002) / I (110) ) of the first negative electrode active material layer and the second negative electrode active material layer at a (002) plane to a peak intensity at a (110) plane is 150 or less. In one embodiment, the peak intensity ratio (I (002) / I (110) ) can be about 1 to about 150.

[0035] Generally, the peak intensity indicates a height of a peak or an integrated area of a peak, and according to an embodiment, the peak intensity indicates an integrated area of a peak. Further, this value is maintained after charging and discharging a rechargeable lithium battery including the negative electrode active material.

[0036] The peak intensity ratio can be a value obtained after simultaneously coating a composition for preparing the first negative electrode active material layer and a composition for preparing the second negative electrode active material layer on a current collector to form a first layer and a second layer on the first layer, applying a magnetic field to the resulting product, and drying and pressing to prepare the first negative electrode active material layer and the second negative electrode active material layer.

[0037] In one embodiment, the peak intensity ratio can also be a value obtained after coating a composition for preparing the first negative electrode active material layer on a current collector to form a first layer, coating a composition for preparing the second negative electrode active material layer on the first layer to form a second layer, applying a magnetic field to the resultant product, and drying and pressing to prepare the first negative electrode active material layer and the second negative electrode active material layer.

[0038] That is, the first negative electrode active material layer and the second negative electrode active material layer can be orientation layers, and each can have a peak intensity ratio (I (002) / I (110) ) of about 150 or less or about 1 to about 150 after pressing.Such a peak intensity ratio (I (002) / I (110) ) is about 150 or less, thereby shortening a distance for transporting lithium ions and reducing ionic resistance.

[0039] Further, a peak intensity ratio (I (002) / I (110) ) of the first negative electrode coating layer and the second negative electrode coating layer prepared by coating a composition before pressing can be about 50 or less or about 1 to about 50. As such, when the peak intensity ratios (I (002) / I (110) ) before and after pressing are within the above-described ranges, a distance for transporting lithium ions can be shortened and ionic resistance can be reduced.

[0040] Therefore, the peak intensity ratio (I (002) / I (110) ) is a peak intensity ratio of an orientation layer. Here, the orientation layer means that, as described above, a composition for a negative electrode active material layer is coated on a current collector while a magnetic field is applied, so that the negative electrode active material is oriented on the current collector, more specifically, the negative electrode active material is oriented at a predetermined angle. That is, as briefly shown in FIG. 1, it indicates that the negative electrode active material 3 is oriented to one side of the current collector 1 at an angle a. Therefore, a negative electrode active material layer prepared by coating without applying a magnetic field refers to a non-orientation layer. Figure 1

[0041] A peak intensity ratio (I (002) / I (110) ) of a non-orientation layer can generally be about 150 or more after coating and before pressing, and can be about 300 to about 600 after pressing, which is very high compared to the peak intensity ratios (I (002) / I (110) ) of the first negative electrode active material layer and the second negative electrode active material layer according to one embodiment.

[0042] In one embodiment, the first negative active material layer and the second negative active material layer are orientation layers, and a peak intensity ratio (I (002) / I (110) ) of the orientation layers can be about 90% or less or about 1% to about 90% of a peak intensity ratio (I (002) / I (110) ) of a non-orientation layer. When the peak intensity ratio (I (002) / I (110) ) of the first negative active material layer and the second negative active material layer is about 90% or less of the peak intensity ratio (I (002) / I (110) ) of the non-orientation layer, a distance for transporting lithium ions can be shortened and an ionic resistance can be reduced. In one embodiment, the non-orientation layer used to calculate the ratio related to the orientation layer can have substantially the same composition and thickness as the first negative active material layer and the second negative active material layer.

[0043] In one embodiment, the XRD measurement can be measured by using CuKα ray as a target ray, a New Bruker D8 XRD apparatus, and an area method using FullProf. Here, the measurement is performed under conditions of 2θ = 10° to 80°, 0.02 s / step to 0.08 s / step, and a step length of 0.01° / step to 0.03° / step.

[0044] In one embodiment, the first negative active material layer can have a peak intensity ratio (I (002) / I (110) ) substantially the same as a peak intensity ratio (I (002) / I (110) ) of the second negative active material layer (i.e., a difference of 0), or in another embodiment, the first negative active material layer can have a peak intensity ratio (I (002) / I (110) ) greater than the peak intensity ratio (I (002) / I (110) ) of the second negative active material layer, and the difference can be at most about 50. Further, a peak intensity ratio (I (002) / I (110) ) of the first negative coating layer before pressing can also be the same as a peak intensity ratio (I (002) / I (110) ) of the second negative coating layer, or the peak intensity ratio (I (002) / I (110) ) of the first negative coating layer can be greater than the peak intensity ratio (I (002) / I (110) ) of the second negative coating layer, and the difference can be at most about 20.

[0045] In one embodiment, the first negative active material layer and the second negative active material layer can be formed on one side or both sides of the current collector.

[0046] The thickness of the first negative active material layer can be about 20 µm to about 125 µm based on the cross section, and the thickness of the second negative active material can be about 20 µm to about 125 µm based on the cross section. In addition, the total thickness of the first negative active material layer and the second negative active material layer can be about 40 µm to about 250 µm based on the cross section. Thus, if the first negative active material layer and the second negative active material layer are formed on both sides of the current collector, the total thickness of the negative active material layer can reach about 500 µm, which is much greater than the conventional maximum thickness of 200 µm of both sides of the negative active material layer. In one embodiment, the peak intensity ratio (I (002) / I (110) ) of the first negative active material layer and the second negative active material layer is controlled to improve impregnation of the electrolyte, and thus, even if a thick layer is formed, rapid charge and discharge can be effectively performed, and thus, can be appropriately applied to a high-power battery.

[0047] The thickness of the first negative active material layer and the thickness of the second negative active material layer indicate the thickness after drying and pressing during the preparation of the negative electrode.

[0048] In one embodiment, the peak intensity ratio (I (002) / I (110) ) is obtained by performing charge and discharge on a rechargeable lithium battery including a negative electrode and disassembling the battery at full discharge to obtain the negative electrode and measuring the negative electrode by XRD. In addition, the peak intensity ratio (I (002) / I (110) ) of the first negative active material layer is obtained by peeling off the negative active material layer after charge and discharge by using a tape and measuring the active material layer attached to the current collector by XRD.

[0049] The charge and discharge is performed once to twice at about 0.1 C to about 2.0 C.

[0050] In one embodiment, the first and second negative active materials included in the first and second negative active material layers can be the same as or different from each other, and can include a crystalline carbon-based active material (also referred to as a "crystalline carbon-based material" or a "crystalline carbon-based negative active material"). The crystalline carbon-based negative active material can be artificial graphite, natural graphite, or a mixture of artificial graphite and natural graphite. When the negative active material includes a crystalline carbon-based material such as artificial graphite, natural graphite, or a mixture of natural graphite and artificial graphite, the orientation characteristics of the carbon material in the electrode with respect to an external magnetic field can be further improved due to the crystalline carbon-based material having more significant crystalline characteristics than amorphous carbon-based active materials. The artificial graphite or the natural graphite can be unassigned shape, flaky, flake-like, spherical, fibrous, or a combination thereof, without specific limitation. In addition, the artificial graphite and the natural graphite are mixed in a ratio of about 70 wt%:30 wt% to about 95 wt%:5 wt%.

[0051] In addition, the negative active material layer can include at least one non-carbon-based material from among Si-based negative active materials, Sn-based negative active materials, and lithium vanadium oxide negative active materials. When the negative active material layer further includes these materials (i.e., a carbon-based negative active material as the first negative active material and a non-carbon-based material as the second negative active material), the first and second negative active materials can be mixed in a weight ratio of about 50:50 to about 99:1.

[0052] The Si-based negative active material can be selected from Si, Si-C composite, SiO x (0 < x < 2) and Si-Q alloy (where Q is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, but not Si), and the Sn-based negative active material is selected from Sn, SnO2, Sn-R alloy (where R is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, but not Si), etc., and the Sn-based negative active material is also selected from a mixture of at least one of them and SiO2. The elements Q and R can 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, Tl, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0053] According to one embodiment, the negative active material can be a Si-carbon composite, and the Si-carbon composite can include silicon particles and crystalline carbon. The silicon particles can have an average particle diameter (D50) of about 10 nm to about 200 nm. The Si-C composite can include an amorphous carbon layer formed at least partially thereon. In the specification, unless otherwise defined herein, the average particle diameter (D50) represents the diameter of the particles having 50 vol% of the cumulative volume in the particle size distribution. Further, the mixing ratio of the silicon particles and the crystalline carbon can be about 1:99 to about 90:10 by weight, and if the amorphous carbon layer is further included, the amount of the amorphous carbon layer can be about 1 part by weight to about 20 parts by weight based on a total of 100 parts by weight of the Si-carbon composite material.

[0054] In the first negative active material layer, the amount of the first negative active material can be about 90 wt% to about 98 wt% based on the total weight of the first negative active material layer, and in the second negative active material layer, the amount of the second negative active material can be about 90 wt% to about 99 wt% based on the total weight of the second negative active material layer.

[0055] The first negative active material layer and the second negative active material layer include a binder, and can further include a conductive material. In the first negative active material layer or the second negative active material layer, the amount of the binder can be about 1 wt% to about 5 wt% based on the total weight of the first negative active material layer or the second negative active material layer. Further, when the conductive material is further included, the first negative active material layer can include about 85 wt% to about 97 wt% of the negative active material, about 1.0 wt% to about 7.5 wt% of the binder, and about 1.0 wt% to about 7.5 wt% of the conductive material, and the second negative active material layer can include about 90 wt% to about 98 wt% of the negative active material, about 1.0 wt% to about 5 wt% of the binder, and about 1.0 wt% to about 5 wt% of the conductive material.

[0056] Generally, the binder in the negative active material layer is mainly distributed in the upper portion not in contact with the current collector, and less distributed in the bottom portion in contact with the current collector. That is, the binder is unevenly distributed in the active material layer.

[0057] In one embodiment, the active material layer is formed as two layers of the first negative active material layer and the second negative active material layer, so that the binder can be uniformly distributed in the active material layer.

[0058] The amount of the binder in the first negative active material layer can be substantially similar to the amount of the binder in the second negative active material layer, and can be known by measuring a peel strength. The ratio of the peel strength of the first negative active material layer to the peel strength of the second negative active material layer can be about 70% to about 90%. The ratio of the peel strength in this range indicates a full distribution and uniform distribution of the binder in the active material layer, so that the negative electrode can exhibit excellent mechanical properties.

[0059] If the amount of the binder in the first negative active material layer is substantially dissimilar to the amount of the binder in the second negative active material layer, and particularly, when the amount of the binder in the second negative active material layer is greater than the amount of the binder in the first negative active material layer, the ratio of the peel strength can be less than about 70%, for example, about 50% to about 60%.

[0060] In one embodiment, the first negative active material layer and the second negative active material layer can be separated from the negative electrode and obtained by using a SAICAS (Surface and Interface Cutting Analysis System) apparatus, and the peel strength can be obtained by measuring each.

[0061] In addition, if the first negative active material layer and the second negative active material layer are formed on both sides of the current collector, the ratio of the peel strength can be similar. This will be described in detail below.

[0062] When one side of the current collector is referred to as an A plane and the other side of the current collector opposite to the one side is referred to as a B plane, the ratio of the peel strength of the first negative active material layer formed on the A plane to the peel strength of the second negative active material layer formed on the A plane refers to a value a, and the ratio of the peel strength of the first negative active material layer formed on the B plane to the peel strength of the second negative active material layer formed on the B plane refers to a value b.

[0063] According to one embodiment, the value a and the value b can be the same, and even if there is a difference, the difference can be about 10% or less. If the value a is not the same as the value b, the value a can be greater than the value b by at most about 10%, or conversely, the value b can be greater than the value a by at most about 10%.

[0064] The binder improves the binding properties of the negative active material particles to each other and the binding properties of the negative active material particles to the current collector. The binder can be a non-aqueous binder, an aqueous binder, or a combination thereof.

[0065] The non-aqueous binder can be an ethylene propylene copolymer, a polyacrylonitrile, a polystyrene, a polyvinyl chloride, a carboxylated polyvinyl chloride, a polyvinyl fluoride, a polyurethane, a polytetrafluoroethylene, a polyvinylidene fluoride, a polyethylene, a polypropylene, a polyamide imide, a polyimide, or a combination thereof.

[0066] The aqueous binder can be styrene-butadiene rubber (SBR), acrylated styrene-butadiene rubber (ABR), acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluoro rubber, ethylene oxide-containing polymer, polyvinylpyrrolidone, polypropylene, polyepichlorohydrin, polyphosphazene, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol, or a combination thereof.

[0067] When the aqueous binder is used as a negative electrode binder, a cellulose-based compound can also be used as a thickening agent to provide viscosity. The cellulose-based compound includes one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, and alkali metal salts thereof. The alkali metal can be Na, K, or Li. The thickening agent can be included in an amount of about 0.1 parts by weight to about 3 parts by weight based on 100 parts by weight of the negative electrode active material.

[0068] The conductive material is included to provide electrode conductivity, and any electrically conductive material can be used as the conductive material, except for those that cause chemical changes. Examples of the conductive material can be: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, etc.; metal-based materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0069] The current collector can include one selected from a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, and combinations thereof, but is not limited thereto.

[0070] The negative electrode having the peak intensity ratio (I (002) / I (110) ) can include a negative electrode active material included in the negative electrode active material layer, which is oriented at a predetermined angle. To achieve these, conditions for applying a magnetic field and viscosity of the active material composition are appropriately adjusted during coating of the negative electrode active material composition on the current collector.

[0071] The negative electrode according to one embodiment can be prepared by the following steps.

[0072] The negative electrode active material layer can be prepared by simultaneously coating the composition for the first negative electrode active material layer and the composition for the second negative electrode active material layer via double die coating or coating the composition for the first negative electrode active material layer and then coating the composition for the second negative electrode active material layer, and drying.

[0073] When the negative active material layer preparation is explained in more detail, the composition for the first negative active material layer and the composition for the second negative active material layer are simultaneously coated or separately coated to form the first layer and the second layer, and a magnetic field is applied to the resulting product. The application of the magnetic field can be performed by disposing a magnet under the current collector and moving the current collector sequentially formed with the first layer and the second layer. Here, the positioning of the magnet can allow the magnetic field to be applied to the first negative active material layer and the second negative active material layer.

[0074] If the first negative active material layer and the second negative active material layer are formed on both sides of the current collector, the first negative active material layer and the second negative active material layer are formed on one side of the current collector, and then the first negative active material layer and the second negative active material layer are formed on the other side of the current collector, on which the first negative active material layer and the second negative active material layer are not formed, in the same manner.

[0075] The magnet can have a magnetic field strength of about 4000 Gauss or more or about 4000 Gauss to about 20000 Gauss. In addition, the first negative active material composition and the second negative active material composition can be coated on the current collector and maintained for about 1 second or more and about 5 seconds or less, that is, the first negative active material composition and the second negative active material composition can be exposed to the magnetic field for about 1 second to about 5 seconds. Furthermore, the magnet can be appropriately positioned to be spaced apart from the current collector by about 3 mm to about 50 mm. If the spacing exceeds this range, the magnetic field applied to the negative active material layer is too weak or too strong, so that it can be impossible to prepare a negative electrode having desired physical properties.

[0076] When such a magnetic field application is performed, particularly when the coating process is performed while moving the current collector sequentially formed with the first layer and the second layer, the magnetic field (magnetic flux) generated by the magnet can be formed perpendicular to the current collector, but since the magnetic field according to the coating speed (the speed of moving the current collector) is formed at a predetermined angle as a vector function, the first negative active material and the second negative active material included in the first composition and the second composition can stand on the surface of the current collector, that is, be oriented at a predetermined angle on the surface of the current collector.

[0077] The composition for the first negative active material layer and the composition for the second negative active material layer can have a suitable viscosity of about 1500 cP to about 3500 cP at room temperature (about 20℃ to about 25℃). Accordingly, when the magnetic field strength and the time of exposure to the magnetic field are satisfied, a negative electrode having a peak intensity ratio (I (002) / I (110) ) of the first negative active material layer and the second negative active material layer of about 150 or less can be obtained.

[0078] Further, even if the same magnetic field is applied to the first negative electrode active material layer and the second negative electrode active material layer, the viscosity of the first negative electrode active material layer composition and the second negative electrode active material layer composition can be adjusted to form the first layer and the second layer having different peak intensity ratios (I (002) / I (110) ) of the first layer and the second layer. That is, the viscosity difference between the first negative electrode active material layer composition and the second negative electrode active material layer composition can be about 1500 cP or less or about 10 cP to about 1500 cP at room temperature (about 20°C to about 25°C). For example, the viscosity of the first negative electrode active material layer composition can be about 1500 cP to about 3500 cP or about 2000 cP to about 3500 cP at room temperature (about 20°C to about 25°C). The viscosity of the second negative electrode active material layer composition can be about 1500 cP to about 3500 cP or about 1500 cP to about 3000 cP at room temperature (about 20°C to about 25°C). The viscosity of the first negative electrode active material layer composition and the second negative electrode active material layer composition can be controlled within the range. If a composition having a viscosity exceeding the range is used, it can be impossible to obtain the peak intensity ratio according to one embodiment even if a magnetic field of about 4000 Gauss or more is applied.

[0079] When the viscosity of the first negative electrode active material layer composition and the second negative electrode active material layer composition satisfies the above range, the first negative electrode active material layer and the second negative electrode active material layer having a desired peak intensity ratio can be obtained. The viscosity of the first negative electrode active material layer composition lower than the range results in a great increase in the perpendicularity (i.e., the angle a) of the first carbon-based negative electrode active material included in the first negative electrode active material layer, thereby causing poor particle contact of the negative electrode active material, and thus the electron transport resistance can increase. And the viscosity higher than the range can not be oriented, i.e., the first negative electrode active material included in the first negative electrode active material layer can be positioned substantially horizontally with respect to the current collector. Figure 1

[0080] If the second negative electrode active material layer composition has a viscosity smaller than the range, the second negative electrode active material included in the second negative electrode active material layer has a very high perpendicularity, which results in poor particle contact of the negative electrode active material, and thus the electron transport resistance can increase. And if the viscosity is greater than the range, the orientation can not occur sufficiently and the electrolyte impregnation is deteriorated.

[0081] ​The first layer composition and the second layer composition can be prepared by mixing the negative active material, the binder, and the optional conductive material in a solvent, respectively. In one embodiment, the amount of the binder in the first negative active material layer composition can be greater than the amount of the binder in the second negative active material layer composition, and for example, the amount of the binder in the first negative active material layer composition can be about 0.5 wt% to about 4 wt% higher than the amount of the binder in the second negative active material layer composition. If the amount of the binder in the first negative active material layer composition is greater than the amount of the binder in the second negative active material layer composition, particularly within the range, the binder included in the first negative active material layer composition can move to the second negative active material layer composition during electrode preparation, and thus the amount of the binder in the first negative active material layer and the amount of the binder in the second negative active material layer can be substantially similar in the finally prepared negative electrode, thereby preparing a uniform electrode.

[0082] The negative active material, the binder, and the conductive material can be the same as described above.

[0083] Thereafter, the current collector on which the first layer and the second layer are formed can be dried and pressed to prepare the first negative active material layer and the second negative active material layer. The drying and the pressing can be performed under negative electrode preparation conditions generally known in the related art, but are not limited thereto.

[0084] A rechargeable lithium battery according to another embodiment includes a negative electrode, a positive electrode, and an electrolyte.

[0085] The rechargeable lithium battery can be a battery for high power applications. In other words, the rechargeable lithium battery can be effectively applied to electronic devices requiring high power, such as power tools, electric vehicles, vacuum cleaners, etc. The reason is that the rechargeable lithium battery including the negative electrode according to the embodiment can easily release heat generated during charge and discharge, particularly when it is applied to high capacity batteries and high power electronic devices, thus can suppress deterioration due to heat, and is effectively used as a high power battery. In addition, the rechargeable lithium battery can easily release heat according to charge and discharge, and effectively suppress an increase in battery temperature, thus effectively improving cycle life characteristics, particularly cycle life characteristics at high rates.

[0086] The high power battery can be a cylindrical battery or a pouch battery. Further, the cylindrical battery can be a 18650 battery (diameter of 18 mm, height of 65 mm) and a 21700 battery (diameter of 21 mm, height of 70 mm), but is not limited thereto.

[0087] The positive electrode can include a positive current collector and a positive active material layer formed on the positive current collector. The positive active material can include a lithiated intercalation compound that reversibly intercalates and deintercalates lithium ions. Specifically, one or more complex oxides of metals selected from cobalt, manganese, nickel, and combinations thereof can be used with lithium. More specifically, a compound represented by one of the following chemical formulas can be used. Li a A 1-b X b D2(0.90≤a≤1.8, 0≤b≤0.5); Li a A 1-b X b O 2-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a E 1-b X b O 2-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a E 2-b X b O 4-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a Ni 1-b-c Co b X c D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2); Li a Ni 1-b-c Co b X c O 2-α T α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α<2); Li a Ni 1-b-c Co b X c O 2-α T2(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α<2); Li a Ni 1-b- c Mn b X c D α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α≤2); Li a Ni 1-b-c Mn b Xc O 2-α T α (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α<2); Li a Ni 1-b-c Mn b X c O 2-α T2(0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.5, 0<α<2); Li a Ni b E c G d O2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1); Li a Ni b Co c Mn d G e O2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0≤d≤0.5, 0.001≤e≤0.1); Li a NiG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a CoG b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-b G b O2(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn2G b O4(0.90≤a≤1.8, 0.001≤b≤0.1); Li a Mn 1-g G g PO4(0.90≤a≤1.8, 0≤g≤0.5); QO2; QS2; LiQS2; V2O5; LiV2O5; LiZO2; LiNiVO4 Li (3-f) J2(PO4)3(0≤f≤2); Li (3-f) Fe2(PO4)3(0≤f≤2); and Li a FePO4(0.90≤a≤1.8).

[0088] In the above formula, A is selected from the group consisting of Ni, Co, Mn, and combinations thereof; X is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements, and combinations thereof; D is selected from the group consisting of O, F, S, P, and combinations thereof; E is selected from the group consisting of Co, Mn, and combinations thereof; T is selected from the group consisting of F, S, P, and combinations thereof; G is selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and combinations thereof; Q is selected from the group consisting of Ti, Mo, Mn, and combinations thereof; Z is selected from the group consisting of Cr, V, Fe, Sc, Y, and combinations thereof; and J is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

[0089] Further, the compound can have a coating layer on its surface, or can be mixed with another compound having a coating layer. The coating layer can include at least one coating element compound selected from the group consisting of an oxide of a coating element, a hydroxide of a coating element, a hydroxy oxide of a coating element, an oxycarbonate of a coating element, and a hydroxycarbonate of a coating element. The compound for the coating layer can be amorphous or crystalline. The coating element included in the coating layer can include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer can be provided by using these elements in the compound in a method that does not adversely affect the properties of the positive electrode active material, for example, the method can include any coating method such as spraying, dipping, etc., but is not described in more detail since it is well known in the related art.

[0090] In the positive electrode, the amount of the positive electrode active material can be about 90 wt% to about 98 wt% based on the total weight of the positive electrode active material layer.

[0091] In one embodiment, the positive electrode active material layer can further include a binder and a conductive material. Here, the binder and the conductive material can be included in an amount of about 1 wt% to about 5 wt% each based on the total amount of the positive electrode active material layer.

[0092] The binder improves the adhesion properties of the positive electrode active material particles to each other and the adhesion properties of the positive electrode active material particles to the current collector. Examples of the binder can be polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, oxygen-containing ethylene-based polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, nylon, etc., but are not limited thereto.

[0093] The conductive material is included to provide electrode conductivity, and any electrically conductive material can be used as the conductive material, unless it causes a chemical change. Examples of the conductive material include: carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, etc.; metal-based materials including metal powder or metal fiber of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0094] The positive electrode active material layer can further include oxalic acid to control the viscosity of the positive electrode active material composition. When the oxalic acid is further included, the amount of the oxalic acid can be about 0.01 parts by weight to about 2.0 parts by weight, based on 100 parts by weight of the positive electrode active material layer.

[0095] In addition, the positive electrode active material layer can further include a nitrile additive to improve safety. The nitrile additive can be a compound including a -C≡N functional group or a nitrile rubber. The compound including the -C≡N functional group can be a cyanalcohol, acetonitrile, cyanoacrylate, or a combination thereof. When the nitrile additive is further included, the amount of the nitrile additive can be about 0.01 parts by weight to about 1 part by weight, based on 100 parts by weight of the positive electrode active material layer.

[0096] The current collector can use Al, but is not limited thereto.

[0097] The electrolyte includes a non-aqueous organic solvent and a lithium salt.

[0098] The non-aqueous organic solvent serves as a medium for transporting ions involved in the electrochemical reaction of the battery.

[0099] The non-aqueous organic solvent can include a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, or an aprotic solvent.

[0100] The carbonate-based solvent can include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. The ester-based solvent can include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, methyl hydroxy valerate, hexanolide, etc. The ether-based solvent can include dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxy ethane, 2-methyl tetrahydrofuran, tetrahydrofuran, etc. In addition, the ketone-based solvent can include cyclohexanone, etc. The alcohol-based solvent can include ethanol, isopropyl alcohol, etc., and examples of the aprotic solvent include a nitrile such as R-CN (wherein R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, and can include a double bond, an aromatic ring, or an ether bond), an amide such as dimethylformamide, a dioxolane such as 1,3-dioxolane, sulfolane, etc.

[0101] The organic solvent can be used alone or in a mixture. When the organic solvent is used in a mixture, the mixing ratio can be controlled according to the desired battery performance, and this can be well known to those skilled in the relevant art.

[0102] In addition, the carbonate-based solvent can include a mixture of cyclic carbonates and chain carbonates. The cyclic carbonates and the chain carbonates are mixed together in a volume ratio of about 1:1 to about 1:9, and when the mixture is used as an electrolyte, it can have enhanced performance.

[0103] In addition to the carbonate-based solvent, the organic solvent can also include an aromatic hydrocarbon-based solvent. Here, the carbonate-based solvent and the aromatic hydrocarbon-based solvent can be mixed together in a volume ratio of about 1:1 to about 30:1.

[0104] The aromatic hydrocarbon-based solvent can be an aromatic hydrocarbon-based compound represented by Chemical Formula 3.

[0105] [Chemical Formula 3]

[0106]

[0107] (In Chemical Formula 3, R1 to R6 are the same or different, and are selected from hydrogen, halogen, C1 to C10 alkyl, halogenated alkyl, and combinations thereof.)

[0108] Specific examples of the aromatic hydrocarbon-based solvent can be selected from benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

[0109] The electrolyte can also include an additive of vinylene carbonate or a vinylene carbonate-based compound represented by Chemical Formula 4 to improve the cycle life.

[0110] [Chemical Formula 4]

[0111]

[0112] (In Chemical Formula 4, R7 and R8 are the same or different, and each can be independently hydrogen, halogen, cyano (CN), nitro (NO2), or fluoro C1 to C5 alkyl, provided that at least one of R7 and R8 is halogen, cyano (CN), nitro (NO2), or fluoro C1 to C5 alkyl, and R7 and R8 are not hydrogen at the same time.)

[0113] Examples of the ethylene carbonate-based compound include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, or fluoroethylene carbonate. The amount of the additive for improving the cycle life can be flexibly used within a suitable range.

[0114] The non-aqueous organic solvent can further include vinyl ethylene carbonate, hexanetricarbonitrile, lithium tetrafluoroborate, propane sultone, etc. as additives.

[0115] The lithium salt dissolved in the organic solvent supplies lithium ions to the battery, enables the basic operation of the rechargeable lithium battery, and improves the transport of lithium ions between the positive electrode and the negative electrode. Examples of the lithium salt can include at least one or two or more selected from LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2)(wherein x and y are natural numbers, for example, integers from 0 to 20), lithium difluoro(bisoxalato)phosphate, LiCl, LiI, and LiB(C2O4)2(lithium bis(oxalato)borate: LiBOB) and lithium difluoro(oxalato)borate (LiDFOB). The concentration of the lithium salt can be in the range from about 0.1 M to about 2.0 M. When the lithium salt is included in the above concentration range, the electrolyte can have excellent performance and lithium ion mobility due to optimal electrolyte conductivity and viscosity.

[0116] According to the kind of the battery, the rechargeable lithium battery can further include a separator located between the negative electrode and the positive electrode. Examples of suitable separator materials include polyethylene, polypropylene, polyvinylidene fluoride, and their multilayers (such as polyethylene / polypropylene double-layer separator, polyethylene / polypropylene / polyethylene triple-layer separator, and polypropylene / polyethylene / polypropylene triple-layer separator).

[0117] Figure 2 is an exploded perspective view of a rechargeable lithium battery according to an embodiment. The rechargeable lithium battery according to the embodiment can be a cylindrical battery.

[0118] Reference Figure 2 The rechargeable lithium battery 100 is a cylindrical battery, and includes a negative electrode 112, a positive electrode 114, a separator 113, an electrolyte (not shown) impregnated in the negative electrode 112, the positive electrode 114, and the separator 113, a battery case 120, and a sealing member 140 sealing the battery case 120.

[0119] Such a rechargeable lithium battery 100 is manufactured by sequentially stacking the negative electrode 112, the separator 113, and the positive electrode 114, winding them in a spiral form, and accommodating them in the battery case 120.

[0120] Hereinafter, examples and comparative examples of the present application are described. However, these examples will not be construed as limiting the scope of the invention in any sense.

[0121] (Example 1)

[0122] 94.4 wt% of natural graphite, 2.6 wt% of Si-carbon composite, 2.0 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose were mixed with a water solvent to prepare a slurry for a first negative electrode active material layer having a viscosity (at 25°C) of 2568 cP.

[0123] 95.8 wt% of natural graphite, 2.7 wt% of Si-carbon composite, 0.5 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose were mixed with a water solvent to prepare a slurry for a second negative electrode active material layer having a viscosity (at 25°C) of 2362 cP.

[0124] Here, the Si-carbon composite has a core including artificial graphite and silicon particles, and a soft carbon coated on the surface of the core. The soft carbon coating layer has a thickness of 20 nm, and the silicon particles have an average particle diameter (D50) of 100 nm.

[0125] While moving the Cu foil current collector, slurries for the first negative electrode active material layer and the second negative electrode active material layer are coated onto the Cu foil via dual-mode coating to prepare a first layer and a second layer on the Cu foil. The resulting Cu foil (10 μm thick) is then positioned on a magnet with a magnetic field strength of 6000 to 7000 Gauss, spaced 3 mm to 10 mm apart. The Cu foil is then moved along the magnet to expose it to the magnetic field for 1 to 3 seconds. The resulting product is then dried to prepare a first negative electrode active material layer and a second negative electrode active material layer with a cross-sectional thickness of 65 μm. The same steps are then used to form the first and second negative electrode active material layers on the opposite side of the Cu foil on which the first and second negative electrode active material layers are not formed, thereby manufacturing the negative electrode. As a result, the negative electrode has a thickness of 270 μm (including the 10 μm current collector).

[0126] After forming the first and second negative electrode active material layers, pressing is performed to prepare the negative electrode. In this negative electrode, after pressing, the cross-sectional thickness of the first negative electrode active material layer is 42 μm, and the cross-sectional thickness of the second negative electrode active material layer is 42 μm, making the total thickness of the negative electrode 178 μm.

[0127] 97.36wt% LiNi 0.8 Co 0.1 Al 0.1 O2, 1.3 wt% carbon black conductive material, 1.1 wt% polyvinylidene fluoride, 0.1 wt% oxalic acid, and 0.14 wt% nitrile rubber were mixed in N-methylpyrrolidone solvent to prepare a positive electrode active material slurry. The positive electrode active material slurry was coated onto an Al current collector, dried, and pressed to prepare the positive electrode.

[0128] The 21700 cylindrical rechargeable lithium-ion battery cell is manufactured using a negative electrode, a positive electrode, and an electrolyte. This 21700 cylindrical rechargeable lithium-ion battery has a capacity of 4933 mAh and a specific capacitance of 4.72 mAh / cm³. 2 A full cell with a current density of [missing information]. Here, the electrolyte is prepared by using a mixed solvent of ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate (volume ratio 20:10:70) and dissolving 1 M LiPF6 therein.

[0129] (Example 2)

[0130] A slurry for a first negative active material layer having a viscosity (at 25°C) of 2564 cP was prepared by mixing 91.8 wt% of natural graphite, 5.2 wt% of Si-carbon composite, 2.0 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose with an aqueous solvent.

[0131] A slurry for a second negative active material layer having a viscosity (at 25°C) of 2235 cP was prepared by mixing 98.5 wt% of natural graphite, 0.5 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose with an aqueous solvent.

[0132] A negative electrode was prepared by the same steps as those of Example 1, except for using the slurry for the first negative active material layer and the slurry for the second negative active material layer, in which the thickness of the first negative active material layer was 42 μm and the thickness of the second negative active material layer was 42 μm, so that the total thickness of the negative electrode was 178 μm.

[0133] A 21700-type cylindrical rechargeable lithium battery cell was manufactured using the negative electrode and the positive electrode and the electrolyte of Example 1, which is a full cell having a battery capacity of 4933 mAh and a current density of 4.72 mAh / cm 2

[0134] (Example 3)

[0135] A slurry for a first negative active material layer having a viscosity (at 25°C) of 2154 cP was prepared by mixing 94.4 wt% of natural graphite, 2.6 wt% of Si-carbon composite, 2.0 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose with an aqueous solvent.

[0136] A slurry for a second negative active material layer having a viscosity (at 25°C) of 2465 cP was prepared by mixing 95.8 wt% of natural graphite, 2.7 wt% of Si-carbon composite, 0.5 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose with an aqueous solvent.

[0137] A negative electrode was prepared by the same steps as those of Example 1, except for using the slurry for the first negative active material layer and the slurry for the second negative active material layer, in which the thickness of the first negative active material layer was 42 μm and the thickness of the second negative active material layer was 42 μm, so that the total thickness of the negative electrode was 178 μm.

[0138] ​A 21700-type cylindrical rechargeable lithium battery cell was manufactured using the negative electrode and the positive electrode and electrolyte of Example 1, which is a full cell having a battery capacity of 4933 mAh and a current density of 4.72 mAh / cm 2 .

[0139] (Comparative Example 1)

[0140] The slurry for the first negative electrode active material layer of Example 1 and the slurry for the second negative electrode active material layer of Example 1 were coated on a Cu foil current collector (thickness of 10 µm) via a double die coating and dried to prepare a first negative electrode active material layer having a cross-sectional thickness of 65 µm and a second negative electrode active material layer having a cross-sectional thickness of 65 µm. The first negative electrode active material layer and the second negative electrode active material layer were formed on both sides of the Cu foil, and thus, the total thickness of the resulting product was 270 µm in thickness including the 10 µm of the current collector.

[0141] After the first negative electrode active material layer and the second negative electrode active material layer were formed, pressing was performed to prepare a negative electrode. In the negative electrode, the cross-sectional thickness of the first negative electrode active material layer was 42 µm and the cross-sectional thickness of the second negative electrode active material layer was 42 µm after the pressing, so that the total thickness of the negative electrode after the pressing was 178 µm. A rechargeable lithium battery was manufactured using the negative electrode and the positive electrode and electrolyte of Example 1 by the same steps as those of Example 1.

[0142] (Comparative Example 2)

[0143] 95.1 wt% of natural graphite, 2.6 wt% of Si-carbon composite, 1.3 wt% of styrene butadiene rubber, and 1.0 wt% of carboxymethyl cellulose were mixed with an aqueous solvent to prepare a slurry for a first negative electrode active material layer having a viscosity (at 25°C) of 1856 cP.

[0144] The slurry for the first negative electrode active material layer was coated on a Cu foil via slot die coating while moving the Cu foil current collector (thickness of 10 µm) and dried to prepare a first negative electrode active material layer having a cross-sectional thickness of 130 µm. The first negative electrode active material layer was coated on both sides of the Cu foil, respectively, and thus, the total thickness of both sides of the first negative electrode active material layer was 270 µm including the 10 µm of the current collector.

[0145] After the first negative electrode active material layer was formed, pressing was performed to prepare a negative electrode. In the negative electrode, the cross-sectional thickness of the first negative electrode active material layer was 84 µm after the pressing, so that the total thickness of the negative electrode was 178 µm.

[0146] A rechargeable lithium battery was manufactured by the same steps as those of Example 1 using the negative electrode and the positive electrode and electrolyte of Example 1.

[0147] Experimental Example 1) Measurement of SEM photograph

[0148] Cross-sectional SEM photographs of the negative electrode before pressing were measured according to Example 1 and Comparative Example 1. The results are shown as A and B in Figure 3 In Figure 3 the center white is the current collector, and A and B are the negative active material layers on both sides of the current collector. Figure 3 The thickness values of the active material layers shown in Figure 3 are actual measurement values. For example, in Example 1, the active material layer was formed with a thickness of 130 μm in which the first negative active material layer and the second negative active material layer were added, but actually 128 μm and 124 μm were obtained, respectively, according to the deviation caused during the preparation process. It is also considered that such a deviation would be easily understood in the related art. As shown in Figure 3 , the B side (B of Figure 3 ) and the A side (A of ) of Example 1 and Comparative Example 1 have similar morphologies, but it can be known that Example 1 has a certain degree of perpendicularity rather than Comparative Example 1.

[0149] Through these results, it can be known that the perpendicularity can actually differ due to the application of the magnetic field.

[0150] Experimental Example 2) Measurement of X-ray diffraction characteristics

[0151] After coating, for the negative electrodes of Example 1 and Comparative Example 1, XRD was measured by using CuKα rays as target rays and a New Bruker D8 XRD apparatus by area method using Fullprof. Herein, the measurement was performed under the conditions of 2θ = 10° to 80°, 0.05 s / step, and a step length of 0.026° / step.

[0152] Figure 4 According to the measurement results, the peak intensity I (002) and the peak intensity I (110) were measured. The results are shown in Figure 4 . In addition, according to the measurement results, the peak intensity ratio (I (002) / I (110) ) was measured. The results are also shown in

[0153] The peak intensity I (002) and the peak intensity I (110) of the negative electrodes according to Example 1 and Comparative Example 1 after pressing were measured under the same conditions. The results and the peak intensity ratio (I (002) / I(110) ) in Figure 5 .

[0154] As shown in Figure 4 , the peak intensity ratio (I (002) / I (110) ) of Example 1 before pressing was 24, and the peak intensity ratio (I (002) / I (110) ) of Comparative Example 1 was 160. As shown in Figure 5 , the peak intensity ratio (I (002) / I (110) ) of Example 1 after pressing was 116, and the peak intensity ratio (I (002) / I (110) ) of Comparative Example 1 was 370.

[0155] From these results, it can be seen that the peak intensity ratio (I (002) / I (110) ) of the negative electrode active material layer to which a magnetic field was applied was 150 or less, and it was 90% or less of the peak intensity ratio (I (002) / I (110) ) of the negative electrode active material layer to which a magnetic field was not applied.

[0156] The peak intensity I (002) and the peak intensity I (110) of the negative electrode according to Example 2 and Example 3 and Comparative Example 2 were measured under the same conditions, and the peak intensity ratio (I (002) / I (110) ) was obtained from these results. The results are shown in Figure 6 . For comparison, the peak intensity ratio (I (002) / I (110) ) of Example 1 and Example 2 is also shown in Figure 6 . As shown in Figure 6 , Example 1 was 116, Example 2 was 142, Example 3 was 110, Comparative Example 1 was 370, and Comparative Example 2 was 505.

[0157] According to these results, the peak intensity ratio (I (002) / I (110) ) of Example 1 to Example 3 was 150 or less, but the peak intensity ratio of Comparative Example 1 and Comparative Example 2, which were not oriented by a magnetic field, was much greater than 150. Therefore, it can be seen that the peak intensity ratio (I (002) / I (110) ) is significantly different depending on whether orientation is performed by a magnetic field / whether orientation is not performed by a magnetic field.

[0158] Experimental Example 3) Measurement of peel strength

[0159] The adhesive distribution at the top (second negative electrode active material layer) and bottom (first negative electrode active material layer) of the negative electrode according to Example 1 and Comparative Examples 1 and 2 was confirmed by measuring the peel strength.

[0160] Peel strength was determined by cutting the negative electrode with a blade using the SAICAS (Surface and Interface Cutting Analysis System) device, separating it into a bottom (first negative electrode active material layer) and a top (second negative electrode active material layer). The peel strength of the obtained top and bottom layers was measured as a percentage of the bottom peel strength to the top peel strength. The results are presented in... Figure 7 As shown in [the image]. Figure 7 In the middle, plane A represents Figure 3 Plane A, and plane B represents Figure 3 Plane B.

[0161] like Figure 7 As shown, the percentage of peel strength at the bottom of plane A and plane B in Example 1 and Comparative Example 1 is 70% or higher (Example 1: plane A - 75%, plane B - 76%, Comparative Example 1: plane A - 78%, plane B - 79%), but the percentage of peel strength at the bottom of plane A and plane B in Comparative Example 2 is approximately 50%.

[0162] Based on the fact that the percentage of peel strength at the bottom of plane A to the percentage of peel strength at the top of plane B differs by 1% from that of plane A in Comparative Example 1, they are essentially similar, and therefore the adhesive distribution ratio can be maintained even when a magnetic field is applied to the active material layer.

[0163] These results show that when the negative electrode active material layer is formed in two layers, the amount of binder in the first negative electrode active material layer is greater than that in the second negative electrode active material layer, which allows the binder to be uniformly distributed throughout the active material layer.

[0164] Experimental Example 4) Measurement of resistance

[0165] For the rechargeable lithium batteries according to Example 1 and Comparative Example 1, the ionic resistance was measured at 25°C using a two-probe method with an impedance analyzer (Solartron 1260A impedance / gain phase analyzer). The results are as follows: Figure 8 As shown in the image.

[0166] like Figure 8 As shown, the ion resistance of Example 1 is significantly lower than that of Comparative Example 1.

[0167] Experimental Example 5) Measurement of DC internal resistance (DCIR: Direct Current Internal Resistance)

[0168] The DC internal resistance (DCIR) of the rechargeable lithium-ion battery for Example 1 and Comparative Example 1 was evaluated by charging and discharging at 0.5C and 25°C, while changing the depth of discharge (the ratio of discharge capacity to rated capacity) to 0%, 10%, 20%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%, and by measuring the voltage drop (V) while current flowed at 0.5C for 10 seconds. The results are in... Figure 9 As shown in the image.

[0169] like Figure 9 As shown, the resistance of Example 1 is significantly lower than that of Comparative Example 1, especially at the initial (0%) depth of discharge, the resistance is significantly lower than that of Comparative Example 1.

[0170] Experimental Example 6) Measurement of cycle life characteristics at room temperature and low temperature

[0171] The rechargeable lithium batteries of Example 1, Comparative Example 1, and Comparative Example 2 were charged and discharged 100 times at 1.0C and at room temperature (RT, 25°C) from 4.2V to 2.5V. The capacity retention based on charge-discharge cycles was evaluated by calculating the discharge capacity ratio of each cycle to the first cycle. Results are presented in... Figure 10 As shown in the image.

[0172] The rechargeable lithium batteries of Example 1, Comparative Example 1, and Comparative Example 2 were charged and discharged 100 times at 1.0C and at a low temperature (10°C) from 4.2V to 2.5V. The capacity retention based on charge-discharge cycles was evaluated by calculating the discharge capacity ratio of each cycle to the first cycle. Results are shown in... Figure 11 As shown in the image.

[0173] like Figure 10 and Figure 11 As shown, Example 1, rather than Comparative Examples 1 and 2, exhibits superior cycle life characteristics at both room temperature and low temperature. In particular, Example 1 demonstrates surprisingly superior low-temperature cycle life characteristics compared to Comparative Examples 1 and 2. Comparative Example 1 exhibits 85% capacity retention after 64 cycles at low temperature, while Comparative Example 2 exhibits 85% capacity retention after 50 cycles or more at low temperature; therefore, the capacity retention of Comparative Examples 1 and 2 is significantly degraded.

[0174] While this disclosure has been described in conjunction with exemplary embodiments now considered to be practical, it will be understood that the invention is not limited to the disclosed embodiments, but rather is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims

1. A negative electrode for a rechargeable lithium battery, the negative electrode comprising: a current collector; a first negative active material layer disposed on the current collector and including a first negative active material; and a second negative active material layer disposed on the first negative active material layer and including a second negative active material, the first negative active material layer and the second negative active material layer being orientation layers in which the first negative active material and the second negative active material are oriented with respect to the current collector. wherein, when XRD is measured by using CuKa rays, the peak intensity ratio I of the peak intensity at the (002) plane to the peak intensity at the (110) plane of the first negative electrode active material layer and the second negative electrode active material layer is 150 or less, and (002) / I (110) 150 or less. wherein the peak intensity ratio I (002) / I (110) In the range of 110 to 150.

2. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, the peak intensity ratio I (002) / (110) is obtained after applying a magnetic field to the resultant product, and drying and pressing to prepare the first negative electrode active material layer and the second negative electrode active material layer after coating the composition for the first negative electrode active material layer and the composition for the second negative electrode active material layer on the current collector to prepare a first layer and a second layer.

3. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The peak intensity ratio I (002) / I (110) is obtained after coating a composition for the first negative electrode active material layer on the current collector to form a first layer, coating a composition for the second negative electrode active material layer on the first layer to form a second layer, applying a magnetic field to the resultant product, and drying and pressing to produce the first negative electrode active material layer and the second negative electrode active material layer.

4. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, A ratio of a peel strength of the first negative active material layer to a peel strength of the second negative active material layer is 70% to 90%.

5. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, the peak intensity ratio I (002) / I (110) the peak intensity ratio I (002) / I (110) 90% or less of the peak intensity ratio I of the non-oriented layer having the same composition and thickness as the first negative electrode active material layer and the second negative electrode active material layer.

6. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The first negative active material and the second negative active material are the same as or different from each other and include a crystalline carbon-based material.

7. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The crystalline carbon-based material is artificial graphite, natural graphite, or a combination thereof.

8. The negative electrode for a rechargeable lithium battery according to claim 7, wherein, The first negative active material and the second negative active material further include at least one selected from a Si-based negative active material, a Sn-based negative active material, and a lithium vanadium oxide negative active material.

9. The negative electrode for a rechargeable lithium battery according to claim 7, wherein, The first negative active material layer has a thickness of 20 μm to 125 μm, and the second negative active material layer has a thickness of 20 μm to 125 μm.

10. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, The peak intensity ratio is a peak integral area value ratio obtained from a peak integral area intensity value at the (002) plane / a peak integral area intensity value at the (110) plane.

11. The negative electrode for a rechargeable lithium battery according to claim 1, wherein, 12.A rechargeable lithium battery, the rechargeable lithium battery comprising: the negative electrode according to any one of claims 1 to 11; a positive electrode; and an electrolyte. ​

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