Anode for secondary battery and lithium secondary battery including same

Abstract

An anode for a secondary battery according to an embodiment comprises: an anode current collector; a first anode mixture layer formed on at least one surface of the anode current collector and comprising a first silicon-based active material; and a second anode mixture layer formed on the first anode mixture layer and comprising a second silicon-based active material, wherein the second silicon-based active material may comprise an active material in which a carbon coating layer is formed on composite particles that comprise silicon, and the amount of the second silicon-based active material included, based on the total weight of the second anode mixture layer, may exceed the amount of the first silicon-based active material included, based on the total weight of the first anode mixture layer.

Description

Anode for secondary battery and lithium secondary battery containing the same

[0001] The present disclosure relates to a negative electrode for a secondary battery and a lithium secondary battery including the same.

[0002] Recently, extensive research has been conducted on electric vehicles (EVs) as a potential replacement for fossil fuel-powered vehicles like gasoline and diesel, a major source of air pollution. Lithium secondary batteries, with their high discharge voltage and output stability, are primarily used as power sources for these EVs. Consequently, the need for high-energy-density lithium secondary batteries is increasing, and active research and development is underway to develop high-capacity cathodes for these batteries.

[0003] To realize high-capacity and high-energy-density secondary batteries, active technology development is being conducted to apply silicon-based active materials, which have a higher discharge capacity (approximately 1500 mAh / g) than graphite (approximately 350 mAh / g), to secondary battery anodes. When silicon-based active materials with such high discharge capacities are applied together with carbon-based active materials such as graphite, the loading weight (LW) of the anode composite layer can be reduced, enabling the production of anodes with superior energy density and rapid charging characteristics.

[0004] However, silicon-based active materials exhibit significantly greater shrinkage and expansion during charge / discharge compared to carbon-based active materials, which increases the likelihood of cracking between silicon particles within the anode composite layer, leading to relatively lower lifespan characteristics. Therefore, there is a growing need for a secondary battery anode with superior capacity, rapid charging, and lifespan characteristics.

[0005] The purpose of one embodiment is to provide a multilayer structured secondary battery negative electrode and a lithium secondary battery having excellent rapid charging characteristics, energy density, etc.

[0006] Another object of the present invention is to provide a multilayer structured negative electrode for a secondary battery and a lithium secondary battery having excellent capacity characteristics, resistance characteristics, etc.

[0007] An embodiment of the present disclosure provides a negative electrode for a secondary battery, comprising: a negative electrode current collector; a first negative electrode mixture layer formed on at least one surface of the negative electrode current collector and including a first silicon-based active material; and a second negative electrode mixture layer formed on the first negative electrode mixture layer and including a second silicon-based active material, wherein the second silicon-based active material includes an active material having a carbon coating layer formed on a composite particle including silicon, and wherein the content of the second silicon-based active material included based on the total weight of the second negative electrode mixture layer exceeds the content of the first silicon-based active material included based on the total weight of the first negative electrode mixture layer.

[0008] The composite particles containing the above silicon may include carbon particles and a silicon-containing coating formed on the surface of the carbon particles.

[0009] The above carbon coating layer may be at least one selected from the group consisting of amorphous carbon, carbon nanotubes, carbon nanofibers, graphite, graphene, graphene oxide, and reduced graphene oxide.

[0010] The above first silicon-based active material may include composite particles including silicon.

[0011] The above first silicon-based active material may be included in an amount of 0.01 to 20 wt% based on the total weight of the first negative electrode composite layer.

[0012] The second silicon-based active material may be included in an amount of 0.01 to 20 wt% based on the total weight of the second negative electrode composite layer.

[0013] The first negative electrode composite layer may further include a first carbon-based active material, and the second negative electrode composite layer may further include a second carbon-based active material.

[0014] The above second carbon-based active material may have an OI value according to Equation 1 below that of the first carbon-based active material.

[0015] [Formula 1]

[0016] OI = I 004 / I 110

[0017] In the above formula 1, OI is the crystal orientation index according to XRD measurement, and I 004 is the peak intensity of the (004) plane in XRD measurement for carbon-based active materials, and I 110 It may be the peak intensity of the (110) plane in XRD measurements on carbon-based active materials.

[0018] The first carbon-based active material may have an OI value of 1 to 10, and the second carbon-based active material may have an OI value of 1 to 3.

[0019] The thickness of the second cathode composite layer may be greater than or equal to the thickness of the first cathode composite layer.

[0020] The thickness ratio of the first cathode composite layer and the second cathode composite layer may be 1:9 to 5:5.

[0021] The first cathode composite layer and the second cathode composite layer may further include a binder.

[0022] The above binder may include at least one selected from the group consisting of styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene propylene rubber, butyl acrylate rubber, butadiene rubber, isoprene rubber, acrylonitrile rubber, acrylic rubber, silane-based rubber, carboxymethylcellulose (CMC), hydroxypropylmethylcellulose, methylcellulose, polyacrylic acid (PAA)-based binder, polyvinyl alcohol (PVA)-based binder, and polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA Copolymer)-based binder.

[0023] Another embodiment of the present disclosure is a lithium secondary battery comprising a positive electrode, the above-described negative electrode for a secondary battery, and a separator interposed between the positive electrode and the negative electrode.

[0024] According to one embodiment, a multilayer structured secondary battery negative electrode and lithium secondary battery having excellent capacity characteristics including a silicon-based active material and excellent rapid charging characteristics by applying different properties of a carbon-based active material to each layer can be provided.

[0025] According to another embodiment, by appropriately controlling the composition of the electrode mixture layer layer by layer, a negative electrode for a secondary battery and a lithium secondary battery having excellent capacity characteristics, resistance characteristics, etc. can be provided.

[0026] Figure 1 is a schematic cross-sectional view showing the structure of a negative electrode for a secondary battery according to one embodiment.

[0027] Figure 2 is a drawing conceptually showing a basal plane and an edge plane formed by the edges of each of the basal planes in a carbon-based active material having a parallel-layered structure.

[0028] Hereinafter, the present disclosure will be described in detail with reference to the attached drawings. However, these are merely exemplary and the present disclosure is not limited to the specific embodiments described as examples.

[0029] In the present disclosure, the 'orientation' of the active material is measured by the peak intensity (I) of the (004) plane according to XRD measurement. 004 ) and peak intensity (I) on the (110) side 110 ) refers to a characteristic represented by the 'Orientation Index (OI)' value determined by the peak intensity ratio, which is the ratio between the two. For example, a relatively smaller OI value of an active material means a low-oriented active material with a small orientation, and a relatively larger OI value means a high-oriented active material with a large orientation.

[0030] Cathode for secondary battery (100)

[0031] Hereinafter, the negative electrode according to the present disclosure will be described in more detail with reference to the drawings. FIG. 1 is a schematic cross-sectional view showing the structure of a negative electrode for a secondary battery according to one embodiment of the present disclosure. As shown in FIG. 1, the negative electrode (100) according to the present disclosure includes a negative electrode mixture layer (20) formed on at least one surface of a negative electrode current collector (10), and the negative electrode mixture layer (20) includes a first negative electrode mixture layer (21) on the negative electrode current collector (10) and a second negative electrode mixture layer (22) formed on the first negative electrode current collector (21).

[0032] According to one embodiment, a negative electrode (100) for a secondary battery may include: a negative electrode current collector (10); a first negative electrode mixture layer (21) formed on at least one surface of the negative electrode current collector and including a first silicon-based active material; and a second negative electrode mixture layer (22) formed on the first negative electrode mixture layer (21) and including a second silicon-based active material. The second silicon-based active material may be an active material having a carbon coating layer formed on silicon-based active material particles, and the content of the second silicon-based active material included based on the total weight of the second negative electrode mixture layer may exceed the content of the first silicon-based active material included based on the total weight of the first negative electrode mixture layer.

[0033] Non-limiting examples of the negative electrode current collector (10) include copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, and a polymer substrate coated with a conductive metal. The negative electrode current collector is not limited thereto, but may have a thickness of, for example, 10 to 50 μm.

[0034] The composite particles containing the silicon may include carbon particles and a silicon-containing coating formed on the surface of the carbon particles.

[0035] Composite particles containing silicon may contain silicon (Si). For example, the cross-section of the carbon-based particles may vary randomly from a circular shape. Furthermore, the silicon-containing coating may be partially formed on the pores and surface of the carbon-based particles, or may be formed as a plurality of discontinuous islands or patterns.

[0036] The composite particles may include carbon particles containing a plurality of pores and a silicon-containing coating.

[0037] According to some embodiments, the volume expansion of silicon included in the silicon-containing coating can be mitigated by the pores of the carbon-based particles. Accordingly, cracking due to the difference in the volume expansion rate of carbon (e.g., about 150 vol. %) and silicon (e.g., about 400 vol. %) during charge and discharge can be prevented while utilizing the relatively high capacity characteristics of silicon. Accordingly, gas generation due to side reactions between the negative active material and the electrolyte can be suppressed, and the life characteristics of the secondary battery can be improved.

[0038] The pores of the carbon particles may include a shape that extends from the outermost portion of the carbon particles into the interior of the carbon particles. For example, the pores may include open pores that are open to the exterior of the carbon particles.

[0039] The terms "surface of a carbon-based particle" and / or "surface of a carbon-based particle" as used herein may refer to the outer surface of a carbon-based particle, the inner surface of a pore, or the outer surface of a carbon-based particle and the inner surface of a pore. The inner surface of a pore may refer to the surface of a pore indented into the interior of a carbon-based particle.

[0040] For example, a silicon-containing coating can be formed on at least a portion of the outer surface of the carbon-based particles.

[0041] For example, a silicon-containing coating can be formed on at least a portion of the inner surface of the pores of the carbon-based particles.

[0042] For example, a silicon-containing coating can be formed on at least a portion of the outer surface of the carbon-based particles and at least a portion of the inner surface of the pores.

[0043] In some embodiments, the carbon-based particles may include activated carbon, carbon nanotubes, carbon nanowires, graphene, carbon fibers, carbon black, graphite, porous carbon, pyrolyzed cryogel, pyrolyzed xerogel, pyrolyzed aerogel, and the like. These may be used alone or in combination of two or more.

[0044] In some embodiments, the carbon-based particles described above may include an amorphous structure or a crystalline structure.

[0045] In some embodiments, the pore size of the carbon particles may be 0.1 nm to 20 nm, 0.5 nm to 15 nm, or 1 nm to 10 nm. Within this range, excessive deposition of silicon can be prevented and cracking of the negative active material during charge and discharge of the secondary battery can be further suppressed.

[0046] The pore size can refer to the diameter of the pore entrance formed on the surface of the carbon particles.

[0047] The composite particles containing the silicon can have high capacity and low resistance characteristics compared to existing silicon oxide-based active materials. In addition, the composite particles containing the silicon can mitigate the phenomenon of electrode cracks occurring due to volume expansion of the silicon-based active material and secure conductivity because silicon exists within the porous carbon structure.

[0048] The first negative electrode composite layer (21) may include a first silicon-based active material, and the first silicon-based active material may include composite particles containing silicon. The present disclosure applies the first negative electrode composite layer (21) including the first silicon-based active material including composite particles containing silicon as a lower layer, thereby minimizing the resistance occurring between the negative electrode current collector (10) and the negative electrode composite layer (20), and obtaining advantageous effects on cell performance such as rapid charging.

[0049] The first silicon-based active material may be included in an amount of 0.01 to 20 wt% based on the total weight of the first negative electrode mixture layer (21), specifically 4 to 16 wt%, and more specifically 6 to 9 wt%. By including the first silicon-based active material in the above-described range, the resistance occurring between the negative electrode current collector (10) and the negative electrode mixture layer (20) can be minimized, and advantageous effects on cell performance such as rapid charging can be obtained.

[0050] The second silicon-based active material may include an active material having a carbon coating layer formed on a composite particle containing silicon. Specifically, the second negative electrode composite layer (22) may include a second silicon-based active material, and specifically, the second silicon-based active material may include an active material having a carbon coating layer formed on a composite particle containing silicon.

[0051] The carbon coating layer can improve the output and rapid life of the active material, and can prevent the second silicon-based active material particles from contacting moisture in the air and / or water in the negative electrode slurry.

[0052] One embodiment of the present disclosure can improve the output and rapid life of the active material as well as suppress the decrease in the discharge capacity of the secondary battery by disposing a second negative electrode composite layer (22) including a second silicon-based active material on the upper layer.

[0053] The above carbon coating layer may be at least one selected from the group consisting of amorphous carbon, carbon nanotubes, carbon nanofibers, graphite, graphene, graphene oxide, reduced graphene oxide, etc., and may specifically be amorphous carbon.

[0054] The content of the second silicon-based active material included based on the total weight of the second negative electrode mixture layer (22) may exceed the content of the first silicon-based active material included based on the total weight of the first negative electrode mixture layer (21). When the content of the silicon-based active material included in the second negative electrode mixture layer (upper layer) adjacent to the electrolyte in which a large amount of lithium ions exist is relatively large, the silicon-based active material having a relatively slow diffusion rate of lithium ions may face a large amount of lithium ions, facilitating the entry and exit of the ions. Accordingly, while maintaining the rapid charging characteristics of the negative electrode for a secondary battery at an excellent level, the content of the silicon-based active material based on the entire negative electrode may be increased to further improve the capacity characteristics.

[0055] The ratio between the content of the first silicon-based active material of the first negative electrode mixture layer (21) and the content of the second silicon-based active material of the second negative electrode mixture layer (22) may be 0:100 to 49:51. Specifically, the ratio between the content of the silicon-based active material of the first negative electrode mixture layer (21) and the content of the silicon-based active material of the second negative electrode mixture layer (22) may be 10:90 to 49:51, and may be 30:70 to 49:51.

[0056] The content of the second silicon-based active material of the second negative electrode mixture layer (22) may be 0.01 to 20 wt%, specifically, the content of the second silicon-based active material of the second negative electrode mixture layer (22) may be 4 to 18 wt%, more specifically, 8 to 16 wt%. When the content of the second silicon-based active material is included within the above-described range, the rapid charging characteristics of the negative electrode for a secondary battery can be maintained at an excellent level, while the content of the silicon-based active material based on the entire negative electrode can be increased to further improve the capacity characteristics.

[0057] When the content and ratio of the silicon-based active material included in the first negative electrode mixture layer (21) and the second negative electrode mixture layer (22) are within the above-described range, the content of the silicon-based active material in the first negative electrode mixture layer (lower layer) formed on one surface adjacent to the current collector can be adjusted relatively low to alleviate problems such as detachment of the mixture layer due to volume expansion, and to improve life characteristics, etc. In addition, the content of the silicon-based active material, which has a low lithium ion diffusion rate compared to the carbon-based active material, can be adjusted relatively high in the second negative electrode mixture layer (upper layer) to secure high-capacity characteristics, rapid-charge characteristics, etc. of the electrode.

[0058] The first negative electrode composite layer (21) and the second negative electrode composite layer (22) included in the above negative electrode composite layer (20) may further include a first carbon-based active material and a second carbon-based active material, respectively. Hereinafter, the crystal orientation index (OI) of the carbon-based active material included in the negative electrode composite layer (20) will be described in more detail.

[0059] The above crystal orientation index (OI) is the peak intensity (I) shown on the (004) plane when measuring the XRD for the active material. 004 ) and peak intensity (I) appearing on the (110) plane 110 ) as a ratio between the crystal orientation index (OI). Specifically, I- 004It can be the peak intensity value of the (004) plane that appears at an angle of 2θ = 54.7±0.2° when measuring XRD using CuKα rays for the active material, and I 110 It can be the peak intensity value of the (110) plane that appears at an angle of 2θ = 77.5±0.2° when measuring XRD using CuKα rays for the active material. In general, the peak intensity value means the height value of the peak or the integrated area value of the peak, and the above I 004 and I 110 can be calculated as the integral area value of the peak.

[0060] Meanwhile, Fig. 2 is a diagram conceptually showing a basal plane and an edge plane formed by the edges of each of the basal planes in a carbon-based active material having a parallel-stacked structure. Referring to Fig. 2, a carbon-based active material (1) such as artificial graphite generally includes carbon layers in which hexagonal rings composed of six carbons are connected in a plane, and the carbon layers are stacked in parallel to each other. In such a carbon-based active material, the basal plane (2) corresponds to the basal plane in the carbon layer having a parallel-stacked structure, and the edge plane (3) means a plane formed by the edges of each of the basal planes.

[0061] During the charging / discharging process of a secondary battery, the intercalation and deintercalation phenomena, in which lithium ions are stored and released in carbon-based active materials, primarily occur through edge planes (3). Therefore, as the number of these edge planes (3) increases, the intercalation and deintercalation of lithium ions during the charging process become easier, and the rapid charging characteristics can also be improved.

[0062] In addition, when charging a battery, carbon-based active materials primarily expand in the direction of their orientation. The higher the orientation of a carbon-based active material, the more concentrated the expansion is in a single direction, so the expansion rate of the active material can be relatively high. On the other hand, the lower the orientation of a carbon-based active material, the less likely it is to be oriented, so the expansion direction is not biased, so the expansion rate of the active material can be relatively low.

[0063] In this regard, the crystal orientation index (OI) value determined by XRD measurement of the carbon-based active material means the peak intensity of the (110) plane relative to the (004) plane. The smaller the OI value of the carbon-based active material, the more disorder in the crystal arrangement, so that the number of edge planes (3) through which lithium ions can enter and exit is relatively larger than the base planes (2), and thus the carbon-based active material may be a low-orientation carbon-based active material having a spherical shape with a non-single orientation direction during charging. On the other hand, the larger the OI value of the carbon-based active material, the less disorder in the crystal arrangement, so that the number of base planes (2) is relatively larger than the edge planes (3), and thus the carbon-based active material may be a highly oriented carbon-based active material having a wide plate-like shape with a single orientation direction.

[0064] Accordingly, the lower the orientation index (OI) of the carbon-based active material included in the negative electrode, the easier it is for lithium ions to enter and exit through many edge planes (3), which can provide excellent rapid charging characteristics. In addition, the orientation direction of the active material is not biased, and a large number of sites that can act as buffers during charging / discharging can exist, effectively alleviating the expansion of the active material, so that the lifespan characteristics of the active material can be further improved.

[0065] However, as the orientation decreases, the hardness of the carbon-based active material increases, making it practically difficult to roll low-orientation carbon-based active materials at high densities. Consequently, increasing the rolling density of anodes containing low-orientation carbon-based active materials is difficult, and there are limitations in manufacturing high-energy-density anodes for secondary batteries.

[0066] On the other hand, a highly oriented carbon-based active material with a relatively high OI value has a relatively low hardness, so it can be rolled at a high density, and thus can contribute to manufacturing a negative electrode with a high energy density.

[0067] Accordingly, the negative electrode (100) for a secondary battery according to one embodiment has a multilayer structure, and includes carbon-based active materials with different orientations in each layer in an appropriate distribution, so that both rapid charging characteristics and energy density can be excellent.

[0068] Specifically, the second carbon-based active material may have an OI value according to Equation 1 below that of the first carbon-based active material.

[0069] [Formula 1]

[0070] OI = I 004 / I 110

[0071] In the above formula 1, OI is the crystal orientation index according to XRD measurement, and I 004 is the peak intensity of the (004) plane in XRD measurement for carbon-based active materials, and I 110 is the peak intensity of the (110) plane in XRD measurements for carbon-based active materials.

[0072] More specifically, a large amount of a first carbon-based active material having a relatively high crystal orientation index (OI) value can be distributed in the first negative electrode composite layer (21), which is the lower layer, and a large amount of a second carbon-based active material having a relatively low crystal orientation index (OI) value can be distributed in the second negative electrode composite layer (22), which is the upper layer.

[0073] That is, the content of the first carbon-based active material of the first negative electrode mixture layer (21) may be greater than the content of the first carbon-based active material of the second negative electrode mixture layer (22), and the content of the second carbon-based active material of the first negative electrode mixture layer (21) may be less than the content of the second carbon-based active material of the second negative electrode mixture layer (22).

[0074] In this regard, according to one embodiment, the first negative electrode mixture layer (21) may include only the first carbon-based active material as the carbon-based active material, and the second negative electrode mixture layer (22) may include only the second carbon-based active material as the carbon-based active material. According to another embodiment, the first negative electrode mixture layer (21) and the second negative electrode mixture layer (22) may each include both the first carbon-based active material and the second carbon-based active material. In this case, the content of the first carbon-based active material of the first negative electrode mixture layer (21) may be equal to or greater than the content of the second carbon-based active material and greater than the content of the first carbon-based active material of the second negative electrode mixture layer (22). In addition, the content of the second carbon-based active material of the second negative electrode mixture layer (22) may be equal to or greater than the content of the first carbon-based active material and greater than the content of the second carbon-based active material of the first negative electrode mixture layer (21).

[0075] The above secondary battery negative electrode (100) can secure excellent rapid charging characteristics by easily allowing lithium ions to enter and exit through the second negative electrode composite layer (22), which is the upper layer, by including a large amount of a second carbon-based active material with low orientation, and can contribute to suppressing electrical short-circuiting between the negative electrode current collector and the negative electrode composite layer by alleviating expansion of the active material.

[0076] In addition, the lower first negative electrode composite layer (21) can be rolled to a high density by including a first carbon-based active material with high orientation, thereby contributing to securing a high energy density.

[0077] The first carbon-based active material may have an OI value of 1 to 10, and the second carbon-based active material may have an OI value of 1 to 3. Specifically, the first carbon-based active material of the negative electrode for the secondary battery may have an OI value of 2 to 8, and the second carbon-based active material may have an OI value of 1.5 to 3.

[0078] When the crystal orientation index (OI) values ​​of the first carbon-based active material and the second carbon-based active material are each within the above-described range, the orientation, hardness, etc. of the carbon-based active material included in each composite layer in the multilayer structured negative electrode can be appropriately controlled within different ranges, so that the rapid charging characteristics, energy density, etc. of the negative electrode can all be excellent.

[0079] The first carbon-based active material and the second carbon-based active material are not particularly limited as long as they can satisfy the above-described OI values, etc. For example, the first carbon-based active material and the second carbon-based active material may be at least one carbon-based active material selected from artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, acetylene black, Ketjen black, Super P, graphene, and fibrous carbon. Specifically, considering the ease of controlling the orientation of the active material, etc., the first carbon-based active material and the second carbon-based active material may each include artificial graphite.

[0080] The content of the first carbon-based active material in the first negative electrode composite layer (21) may be 80 to 95 wt%. In addition, the content of the second carbon-based active material in the second negative electrode composite layer (22) may be 70 to 90 wt%.

[0081] The thickness of the second negative electrode mixture layer (22) may be greater than or equal to the thickness of the first negative electrode mixture layer (21). Specifically, the thickness ratio of the first negative electrode mixture layer and the second negative electrode mixture layer may be 1:9 to 5:5.

[0082] The above first negative electrode composite layer (21) may be a mixture of a silicon-based active material and a first carbon-based active material including a porous structure, as well as a negative electrode binder, a conductive material, and / or a dispersing agent.

[0083] The second negative electrode composite layer (22) may be a mixture of a silicon-based active material and a second carbon-based active material including a porous structure, as well as a negative electrode binder, a conductive material, and / or a dispersing agent.

[0084] The above binder is a compound that serves to adhere the components within the negative electrode mixture layer (20) well to each other and to adhere the negative electrode mixture layer (20) well to the current collector, and may be at least one selected from the group consisting of, for example, at least one rubber-based binder selected from the group consisting of styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene propylene rubber, butyl acrylate rubber, butadiene rubber, isoprene rubber, acrylonitrile rubber, acrylic rubber, and silane-based rubber; a cellulose-based binder such as carboxymethylcellulose (CMC), hydroxypropylmethylcellulose, methylcellulose, or an alkali metal salt thereof; a water-soluble polymer-based binder such as a polyacrylic acid (PAA)-based binder, a polyvinyl alcohol (PVA)-based binder, and a polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA Copolymer)-based binder. Specifically, the first negative electrode mixture layer (21) and the second negative electrode mixture layer (22) may each further include a rubber-based binder. More specifically, the first negative electrode mixture layer (21) and the second negative electrode mixture layer (22) may independently further include a rubber-based binder and a cellulose-based binder.

[0085] The binder content of the first negative electrode mixture layer (21) may be greater than or equal to the binder content of the second negative electrode mixture layer (22). Specifically, the ratio between the binder content of the first negative electrode mixture layer (21) and the binder content of the second negative electrode mixture layer (22) may be 9:1 to 5:5. More specifically, the ratio between the binder content of the first negative electrode mixture layer (21) and the binder content of the second negative electrode mixture layer (22) may be 8.5:1.5 to 6:4.

[0086] If the content of the above-mentioned binder is too low, the adhesive strength of the first negative electrode mixture layer (21) adjacent to the current collector may be reduced, which may cause problems such as scrap generation and mixture layer detachment during the notching process. On the other hand, if the content of the binder is too high throughout the negative electrode, the electrical resistance may increase, which may deteriorate the battery characteristics. Accordingly, if the content of the binder included in the first negative electrode mixture layer (21), which is the lower layer, is adjusted to be relatively high, the binder content throughout the negative electrode can be lowered while substantially alleviating the above-mentioned problems, thereby also alleviating the increase in electrical resistance.

[0087] The binder content of the first negative electrode mixture layer (21) may be 1.0 to 5.0 wt%. In addition, the binder content of the second negative electrode mixture layer (22) may be 0.1 to 2.5 wt%. Specifically, the binder content of the first negative electrode mixture layer (21) may be 2.0 to 4.0 wt%, and the binder content of the second negative electrode mixture layer (22) may be 0.4 to 2.0 wt%. When the binder content of each negative electrode mixture layer is within the above-described range, the multilayer electrode may have excellent flexibility, adhesiveness, etc., so that problems such as electrode detachment during the process or cracking or electrode detachment during the charge / discharge process can be substantially alleviated, and low resistance characteristics can also be secured.

[0088] The conductive material is used to provide conductivity to the electrode and maintain the structure of the electrode, and can be used as a conductive material that does not cause side reactions with other elements of the secondary battery. For example, graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc., and one of these may be used alone or as a mixture of two or more. Specifically, the conductive material may include carbon nanotubes (CNTs). Carbon nanotubes (CNTs) have higher electron mobility than existing conductive materials such as carbon black, so they can realize high energy density even in a small amount, and have high strength due to their stable structure, and can substantially alleviate volume expansion of silicon-based active materials. Therefore, when the conductive material includes carbon nanotubes (CNTs), the energy density, life characteristics, and resistance characteristics of the electrode may be further improved.

[0089] The loading weight (LW) ratio of the first negative electrode mixture layer (21) and the second negative electrode mixture layer (22) may be 2:8 to 8:2. In addition, the loading weight of the first negative electrode mixture layer (21) may be 1.5 to 9.5 mg / cm 2 It can be, and the loading weight of the second cathode composite layer (22) is 1.5 to 9.5 mg / cm 2 It could be.

[0090] The above loading weight (LW) refers to the amount of the negative electrode composite layer formed on the current collector, i.e., the layer including the active material, binder, conductive material, etc., expressed in units of weight per area. Here, the area is based on the area of ​​the current collector, and the weight is based on the weight of the entire formed negative electrode composite layer.

[0091] When the loading weight (LW) value, ratio, etc. of the first negative electrode composite layer (21) and the second negative electrode composite layer (22) are within the above-described range, a negative electrode having a multilayer structure with excellent capacity characteristics, life characteristics, and rapid charging characteristics can be provided.

[0092] The method for manufacturing the above-described negative electrode for a secondary battery is not particularly limited, and may be performed by methods such as (1) a 'two-coating' process in which the slurries of the upper and lower layers are sequentially applied, and (2) a 'simultaneous coating' process in which the slurries of the upper and lower layers are simultaneously applied. For example, the 'two-coating' process may be performed by forming a first negative electrode mixture layer (21) by applying a first negative electrode slurry including a first solvent, a first carbon-based active material, a silicon-based active material, a binder, and a conductive material on a current collector by a method such as bar coating, casting, or spraying, and drying at 70 to 100°C, and then applying a second negative electrode slurry including a second solvent, a second carbon-based active material, a silicon-based active material, a binder, and a conductive material on the first negative electrode mixture layer (21) by a method such as bar coating, casting, or spraying, and drying at 70 to 100°C.

[0093] The solvent may be, for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. The amount of the solvent used is sufficient to dissolve or disperse the active material, conductive material, and binder, taking into account the coating thickness and manufacturing yield of the composition for forming the negative electrode mixture layer, and to have a viscosity that can exhibit excellent thickness uniformity when applied thereafter to form the negative electrode mixture layer.

[0094] lithium secondary battery

[0095] A lithium secondary battery according to one embodiment may include a positive electrode, a negative electrode for a secondary battery, and a separator interposed between the positive electrode and the negative electrode. The negative electrode may be a negative electrode for a secondary battery according to any one of the embodiments described above.

[0096] The positive electrode may include a positive electrode current collector and a positive electrode composite layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector may include stainless steel, nickel, aluminum, titanium, or an alloy thereof. The positive electrode current collector may also include aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The positive electrode current collector may have a thickness of, but is not limited to, 10 to 50 μm, for example.

[0097] The positive electrode active material layer may include a positive electrode active material. The positive electrode active material may include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0098] According to exemplary embodiments, the positive electrode active material may include a lithium-nickel metal oxide. The lithium-nickel metal oxide may further include at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0099] In some embodiments, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or crystal structure represented by the following chemical formula 1.

[0100] [Chemical Formula 1]

[0101] Li x Ni a M b O 2+z

[0102] In Chemical Formula 1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, -0.5≤z≤0.1 may be satisfied. As described above, M may include Co, Mn, and / or Al.

[0103] The chemical structure represented by Chemical Formula 1 represents the bonding relationship included in the layered structure or crystal structure of the positive electrode active material and does not exclude other additional elements. For example, M includes Co and / or Mn, and Co and / or Mn can serve as the main active element of the positive electrode active material together with Ni. Chemical Formula 1 is provided to express the bonding relationship of the above main active elements and should be understood as a formula encompassing the introduction and substitution of additional elements.

[0104] In one embodiment, auxiliary elements may be further included in addition to the main active element to enhance the chemical stability of the positive electrode active material or the layered structure / crystal structure. The auxiliary elements may be incorporated into the layered structure / crystal structure to form bonds, and in this case, it should be understood that they are also included within the chemical structure range represented by Chemical Formula 1.

[0105] The auxiliary element may include, for example, at least one of Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may also act as an auxiliary active element that contributes to the capacity / output activity of the positive electrode active material together with Co or Mn, for example, Al.

[0106] For example, the positive electrode active material or the lithium-nickel metal oxide may include a layered structure or crystal structure represented by the following chemical formula 1-1.

[0107] [Chemical Formula 1-1]

[0108] Li x Ni a M1 b1 M2 b2 O 2+z

[0109] In Chemical Formula 1-1, M1 may include Co, Mn, and / or Al. M2 may include the auxiliary elements described above. In Chemical Formula 1-1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, -0.5≤z≤0.1 may be satisfied.

[0110] The above-described positive electrode active material may further include a coating element or doping element. For example, elements substantially identical to or similar to the above-described auxiliary elements may be used as the coating element or doping element. For example, the above-described elements may be used singly or in combination of two or more.

[0111] The above coating element or doping element may be present on the surface of the lithium-nickel metal oxide particle, or may penetrate through the surface of the lithium-nickel metal composite oxide particle and be included in the bonding structure represented by the above chemical formula 1 or chemical formula 1-1.

[0112] The above positive electrode active material may include a nickel-cobalt-manganese (NCM) lithium oxide. In this case, an NCM lithium oxide with an increased nickel content may be used.

[0113] Ni can be provided as a transition metal associated with the output and capacity of a lithium secondary battery. Therefore, by employing a high-content (High-Ni) composition as described above in the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0114] However, as the Ni content increases, the long-term storage stability and lifespan stability of the cathode or secondary battery may relatively deteriorate, and side reactions with the electrolyte may also increase. However, according to exemplary embodiments, the inclusion of Co can maintain electrical conductivity, while improving lifespan stability and capacity retention characteristics through Mn.

[0115] The content of Ni (e.g., the mole fraction of nickel among the total moles of nickel, cobalt, and manganese) in the NCM-based lithium oxide may be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the content of Ni may be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0116] In some embodiments, the positive electrode active material may include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0117] In some embodiments, the positive electrode active material may include, for example, a Mn-rich active material, an LLO (Li rich layered oxide) / OLO (Over Lithiated Oxide) active material, or a Co-less active material having a chemical structure or crystal structure represented by Chemical Formula 2.

[0118] [Chemical Formula 2]

[0119] p[Li2MnO3]·(1-p)[Li q JO2]

[0120] In chemical formula 2, 0 <p<1이고, 0.9≤q≤1.2이며, J는 Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg 및 B 중 적어도 하나의 원소를 포함할 수 있다.

[0121] A separator may be interposed between the anode and cathode. The separator may be configured to prevent electrical short-circuiting between the anode and cathode and to allow ion flow. Depending on the embodiment, the thickness of the separator may be 10 μm to 20 μm, but the present disclosure is not limited thereto.

[0122] For example, the separator may include a porous polymer film or a porous nonwoven fabric. The porous polymer film may include a polyolefin polymer such as an ethylene polymer, a propylene polymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer. The porous nonwoven fabric may include high-melting-point glass fibers, polyethylene terephthalate fibers, and the like. The separator may also include a ceramic material. For example, inorganic particles may be coated on the polymer film or dispersed within the polymer film to improve heat resistance.

[0123] The separator may have a single-layer or multi-layer structure including the above-described polymer film and / or non-woven fabric.

[0124] Lithium secondary batteries, such as those described above, have excellent rapid charging characteristics, lifespan characteristics, and resistance characteristics, and thus can be very useful as a power source for electric vehicles (EVs).

[0125] Example

[0126] Hereinafter, embodiments of the present disclosure will be further described with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are merely illustrative of the present disclosure and do not limit the scope of the appended claims. It will be apparent to those skilled in the art that various changes and modifications to the examples are possible within the scope and technical idea of ​​the present disclosure, and it is also natural that such changes and modifications fall within the scope of the appended claims.

[0127] Manufacturing example

[0128] 1. Example 1

[0129] (1) Manufacturing of composite particles

[0130] 1) Manufacturing of carbon particles

[0131] i) Synthesis of resol oligomer: Phenol and formaldehyde were mixed in a molar ratio of 1:2, 1.5 wt% of triethylamine was added, and the reaction was performed under the conditions of 85°C, 4 h, and 160 rpm (stirring).

[0132] ii) Suspension stabilization of resol oligomer: 1 g of PVA was dispersed in a water-dispersible medium and then added to the resol oligomer.

[0133] iii) Curing of resol oligomer: 3g of curing agent HMTA was added and reacted under the conditions of 98℃, 12h, 400 rpm (stirring).

[0134] iv) Obtaining carbon material: The above-mentioned cured resol oligomer was classified using a sieve and then washed using H2O.

[0135] v) Unreacted monomers and oligomers were removed from the above-mentioned washed resol oligomer using ethanol and dried.

[0136] vi) Carbonization and activation: The dried resol oligomer was calcined at 900°C for 1 hour under a nitrogen atmosphere. During the calcination, CO2 gas was introduced at a rate of 1 L / min and carbonized at 900°C to produce porous carbon.

[0137] 2) Formation of silicone-containing coating

[0138] Silane gas was injected into a CVD coater at a flow rate of 50 mL / min to 100 mL / min, the temperature was increased to 550°C at a heating rate of 5°C to 20°C, and maintained for about 120 minutes to produce composite particles containing silicon, which are indicated as “composite particles” in Table 2 below.

[0139] (2) Composite particles with a carbon coating layer formed

[0140] An active material was manufactured by coating carbon on the surface of the composite particle containing the silicon manufactured above by a CVD method, and a carbon coating layer was formed on the composite particle containing the silicon, and this was indicated as “carbon coated composite particle” in Table 2 below.

[0141] (3) Carbon-based active materials

[0142] Three types of artificial graphite, namely, first artificial graphite, third artificial graphite, were prepared using carbon-based active materials. The properties of each artificial graphite used are shown in Table 1 below.

[0143] At this time, the peak intensity value for calculating the crystal orientation index (OI) value of the carbon-based active material was measured using an XRD device (PANalytical Empyrean) with CuKα line as the target line. At this time, the measurement conditions were 2θ = 10° to 80°, scan speed (° / S) = 3, and step size = 0.025° / step.

[0144] Crystal orientation index (OI)(I (004) / I (110) ) 1st artificial graphite 2.11 2nd artificial graphite 4.88 3rd artificial graphite 3.06

[0145] (4) Manufacturing of cathode

[0146] A first negative electrode slurry was prepared, including 88.2 wt% of second artificial graphite as a first carbon-based active material, 8 wt% of composite particles as a first silicon-based active material, 2.8 wt% of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as binders, and 1 wt% of single-walled carbon nanotubes (SWCNTs) as conductive materials, based on solid content.

[0147] A second negative electrode slurry was prepared, each containing 80.2 wt% of first artificial graphite as a second carbon-based active material, 16 wt% of carbon-coated composite particles as a second silicon-based active material, 2.8 wt% of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as binders, and 1 wt% of single-walled carbon nanotubes (SWCNTs) as conductive materials, based on solid content.

[0148] Afterwards, the first negative electrode slurry was applied onto the copper foil, which is the negative electrode collector, and dried at 80°C to form a first negative electrode mixture layer. The second negative electrode slurry was applied onto the first negative electrode mixture layer and dried at 80°C to form a second negative electrode mixture layer, thereby manufacturing the negative electrode for a secondary battery of Example 1, which is shown in Table 2. At this time, the loading weight (LW) ratio of the first negative electrode mixture layer and the second negative electrode mixture layer was applied as 5:5.

[0149] (5) Manufacturing of secondary batteries

[0150] Li[Ni], a Li-transition metal complex oxide, on aluminum foil 0.8 Co 0.1 Mn 0.1 ]O2-based active material was applied and dried to manufacture a positive electrode, and a polyolefin separator was interposed between the positive electrode and the negative electrode manufactured above, and the manufactured secondary battery cell was placed in a pouch for a secondary battery, and then an electrolyte solution in which 1 M LiPF6 was dissolved in a solvent mixed with ethylene carbonate (EC) and diethyl carbonate (DEC) was injected into the pouch for a secondary battery and then sealed to manufacture a pouch-type lithium secondary battery, which is shown in Table 2. The manufactured pouch-type lithium secondary battery was applied as a secondary battery sample of Example 1.

[0151] 2. Comparative Examples 1 to 5

[0152] A negative electrode and a secondary battery were manufactured in the same manner as in Example 1, except that the types and contents of the first carbon-based active material, the second carbon-based active material, the first silicon-based active material, and the second silicon-based active material included in the negative electrode composite layer were changed as shown in Table 2 below.

[0153] First negative electrode mixture layer (lower layer) Second negative electrode mixture layer (upper layer) First carbon-based active material First silicon-based active material Second carbon-based active material Second silicon-based active material Type Content (weight %) Type Content (weight %) Type Content (weight %) Type Content (weight %) Example 1 Second artificial graphite 88.2 Composite particle 8 First artificial graphite 80.2 Carbon coated composite particle 16 Comparative example 188.2 880.2 Composite particle 16 Comparative example 284.2 12 84.2 Coated composite particle 12 Comparative example 384.2 12 84.2 Composite particle 12 Comparative example 484.2 12 Third artificial graphite 84.2 Carbon coated composite particle 12 Comparative example 584.2 12 84.2 Composite particle 12

[0154] Evaluation example

[0155] 1. Evaluation of internal resistance (DC-IR) characteristics

[0156] For the secondary batteries manufactured in Example 1 and Comparative Examples 1 to 5, the C-rate was increased or decreased to 0.2C, 0.5C, 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C at the SOC 50% point, and when charging and discharging at the corresponding C-rates were performed for 10 seconds, the terminal points of the voltage were constructed as a straight-line equation, and the slope thereof was adopted as the DCIR. The resulting values ​​are shown in Table 3 below.

[0157] 2. Evaluation of general (room temperature) life characteristics

[0158] The general life characteristics were evaluated in the range of SOC 4-98% for the secondary battery samples according to Example 1 and Comparative Examples 1 to 5 at 25℃. Specifically, the secondary battery samples were charged at 0.3C under constant current / constant voltage (CC / CV) conditions to a voltage corresponding to SOC 98%, cut off at 0.05C, and then discharged at 0.3C under constant current (CC) conditions to a voltage corresponding to SOC 4%. The cycle was performed 300 times, and the discharge capacity retention rate compared to the initial discharge capacity was calculated as a %, and the results are shown in Table 3. At this time, if the capacity retention rate suddenly dropped after a certain number of cycles, it was indicated as ‘sudden drop after ~ cycles.’

[0159] 3. Evaluation of rapid charging life characteristics

[0160] For the secondary battery samples according to Example 1 and Comparative Examples 1 to 5, a cycle of charging for 25 minutes in the range of SOC 8-80% at 25°C and discharging at 0.3C was repeated 200 times, and the discharge capacity retention rate compared to the initial discharge capacity was calculated as a %, and the results are shown in Table 3.

[0161] Internal resistance characteristics (mΩ) General life characteristics (%) @ 200 cycles Fast charge life characteristics (%) @ 300 cycles Fast charge life characteristics (%) @ 400 cycles Example 10.96 79 79 5.59 3.9 Comparative example 10.98 19 79 4.58 7.5 Comparative example 20.96 99 79 290 Comparative example 30.97 97 9 0.48 0.2 Comparative example 40.97 59 478 Evaluation discontinued Comparative example 50.98 9 3.57 6.5 Evaluation discontinued

[0162] Referring to Table 3 above, it can be confirmed that the resistance characteristics, life characteristics, etc. of the secondary battery are different when the type and content of the silicon-based active material in the multilayer structured negative electrode are different. Specifically, the lithium secondary battery of Example 1, which includes a carbon-based active material with a low crystal orientation index and a second silicon-based active material coated with carbon on the active material particles in the second negative electrode mixture layer (upper layer), and includes a carbon-based active material with a relatively high crystal orientation index and a first silicon-based active material with no carbon coated on the active material particles in the first negative electrode mixture layer (lower layer), and in which the content of the second silicon-based active material included based on the total weight of the second negative electrode mixture layer exceeds the content of the first silicon-based active material included based on the total weight of the first negative electrode mixture layer, has low resistance and excellent general life characteristics and rapid charge life characteristics.

[0163] On the other hand, it can be confirmed that a lithium secondary battery including a second silicon-based active material in which carbon is not coated on the active material particles in the second negative electrode mixture layer (upper layer) (Comparative Examples 1 and 3) or in which the content of the second silicon-based active material included based on the total weight of the second negative electrode mixture layer is the same as the content of the first silicon-based active material included based on the total weight of the first negative electrode mixture layer (Comparative Examples 2 and 3) has inferior rapid charge life characteristics.

[0164] It can be confirmed that the general life characteristics and rapid charge life characteristics of the lithium secondary batteries of Comparative Examples 4 and 5, which include a carbon-based active material with a high crystal orientation index in the second negative electrode composite layer (upper layer) and a silicon-based active material with carbon coated on the active material particles, are inferior.

[0165] The above description is merely an example of applying the principles of the present disclosure, and other configurations may be included without departing from the scope of the present invention.

Claims

1. Negative current collector; A first negative electrode composite layer formed on at least one surface of the negative electrode current collector and including a first silicon-based active material; and A second negative electrode composite layer formed on the first negative electrode composite layer and including a second silicon-based active material; The above second silicon-based active material includes an active material having a carbon coating layer formed on a composite particle including silicon, A negative electrode for a secondary battery, wherein the content of the second silicon-based active material included based on the total weight of the second negative electrode composite layer exceeds the content of the first silicon-based active material included based on the total weight of the first negative electrode composite layer.

2. In paragraph 1, A negative electrode for a secondary battery, wherein the composite particles include carbon particles and a silicon-containing coating formed on the surface of the carbon particles.

3. In paragraph 1, A negative electrode for a secondary battery, wherein the carbon coating layer is at least one selected from the group consisting of amorphous carbon, carbon nanotubes, carbon nanofibers, graphite, graphene, graphene oxide, and reduced graphene oxide.

4. In paragraph 1, A negative electrode for a secondary battery, wherein the first silicon-based active material comprises composite particles containing silicon.

5. In paragraph 1, A negative electrode for a secondary battery, wherein the first silicon-based active material is included in an amount of 0.01 to 20 wt% based on the total weight of the first negative electrode composite layer.

6. In paragraph 1, A negative electrode for a secondary battery, wherein the second silicon-based active material is included in an amount of 0.01 to 20 wt% based on the total weight of the second negative electrode composite layer.

7. In paragraph 1, The above first negative electrode composite layer further includes a first carbon-based active material, A negative electrode for a secondary battery, wherein the second negative electrode composite layer further includes a second carbon-based active material.

8. In paragraph 7, The second carbon-based active material is a negative electrode for a secondary battery, wherein the OI value according to Equation 1 below is less than or equal to the OI value of the first carbon-based active material. [Formula 1] OI = I 004 / I 110 In the above formula 1, OI is the crystal orientation index according to XRD measurement, and I 004 is the peak intensity of the (004) plane in XRD measurement for carbon-based active materials, and I 110 is the peak intensity of the (110) plane in XRD measurements for carbon-based active materials.

9. In paragraph 8, The above first carbon-based active material has an OI value of 1 to 10, The above second carbon-based active material has an OI value of 1 to 3. Cathode for secondary batteries.

10. In paragraph 1, A negative electrode for a lithium secondary battery, wherein the thickness of the second negative electrode mixture layer is greater than or equal to the thickness of the first negative electrode mixture layer.

11. In paragraph 1, A negative electrode for a lithium secondary battery, wherein the thickness ratio of the first negative electrode composite layer and the second negative electrode composite layer is 1:9 to 5:

5.

12. In paragraph 1, A negative electrode for a lithium secondary battery, wherein the first negative electrode composite layer and the second negative electrode composite layer further comprise a binder.

13. In paragraph 12, A negative electrode for a lithium secondary battery, wherein the binder comprises at least one selected from the group consisting of styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene propylene rubber, butyl acrylate rubber, butadiene rubber, isoprene rubber, acrylonitrile rubber, acrylic rubber, silane-based rubber, carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose, polyacrylic acid (PAA)-based binder, polyvinyl alcohol (PVA)-based binder, and polyvinyl alcohol-polyacrylic acid copolymer (PVA-PAA copolymer)-based binder.

14. Bipolar, A negative electrode for a secondary battery according to any one of claims 1 to 13; and Including a separator interposed between the positive and negative electrodes, Lithium secondary battery.

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