secondary batteries

Abstract

The present invention provides a secondary battery, which comprises: one or more positive electrodes, each of which comprises a positive electrode active material layer; a plurality of negative electrodes, including a first negative electrode comprising a silicon-based active material and a second negative electrode comprising a carbon-based active material; a separator; and an electrolyte, wherein the positive electrodes and the negative electrodes are alternately stacked with the separator interposed therebetween, and the weight ratio of the silicon-based active material contained in the first negative electrode to the carbon-based active material contained in the second negative electrode is 40:60 to 90:10.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of Korean Patent Application No. 10-2020-0060532, filed on May 20, 2020, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a secondary battery, and more particularly, to a secondary battery having improved capacity characteristics, lifespan characteristics, and rapid charging characteristics. Background Art

[0005] Recently, with the rapid popularization of electronic devices using batteries such as mobile phones, notebook computers and electric vehicles, the demand for secondary batteries with small size, light weight and relatively high capacity has been increasing sharply. In particular, lithium secondary batteries have attracted attention as driving power sources for portable devices due to their light weight and high energy density. Therefore, efforts have been made to research and develop lithium secondary batteries to improve their performance.

[0006] The lithium secondary battery generally includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, an electrolyte, an organic solvent, and the like. Furthermore, an active material layer containing a positive electrode active material or a negative electrode active material may be formed on a current collector in the positive electrode and the negative electrode. Typically, a lithium-containing metal oxide such as LiCoO2 or LiMn2O4 is used as the positive electrode active material in the positive electrode, whereas a lithium-free carbon-based active material or a silicon-based active material is used as the negative electrode active material in the negative electrode.

[0007] Among negative electrode active materials, silicon-based active materials have attracted attention due to their capacity, which is approximately 10 times higher than that of carbon-based active materials, and their excellent fast-charging characteristics. However, silicon-based active materials suffer from volume expansion caused by charging and discharging, which in turn degrades their lifespan. Furthermore, the use of large amounts of binders to mitigate these issues makes it difficult to achieve the desired high-capacity electrodes. Consequently, silicon-based active materials are not currently widely used.

[0008] On the other hand, carbon-based active materials such as artificial graphite and natural graphite exhibit relatively stable life characteristics compared to silicon-based active materials, but when carbon-based active materials are applied in a large thickness to manufacture high-capacity electrodes, problems such as electrode cracking, warping, detachment, etc. may occur.

[0009] To overcome the shortcomings of carbon-based and silicon-based active materials, technologies for manufacturing electrodes containing a combination of carbon-based and silicon-based active materials have been developed. However, to mitigate the effects of the silicon-based active material's volume expansion, the proportion of the silicon-based active material in the negative electrode cannot be increased to the desired level, making it difficult to improve the negative electrode's capacity and rapid charging characteristics. Furthermore, increasing the proportion of the silicon-based active material also requires increasing the amount of binder used to suppress the active material's volume expansion, making it difficult to achieve a high-capacity negative electrode.

[0010] Korean Patent Registration No. 10-0794192 relates to a method of manufacturing a carbon-coated silicon-graphite composite negative electrode material for a lithium secondary battery and a method of manufacturing a secondary battery including the negative electrode material, but these methods have limitations in solving the above-mentioned problems.

[0011] Related technical literature

[0012] [Patent Document]

[0013] Korean Patent Registration No. 10-0794192 Summary of the Invention

[0014] [Technical Issues]

[0015] The present invention is directed to providing a secondary battery having improved capacity characteristics, lifespan characteristics, and rapid charging characteristics.

[0016] [Technical solution]

[0017] One aspect of the present invention provides a secondary battery, comprising: one or more positive electrodes, the positive electrode comprising a positive electrode active material layer; a plurality of negative electrodes, including a first negative electrode comprising a silicon-based active material and a second negative electrode comprising a carbon-based active material; a separator; and an electrolyte, wherein the positive electrodes and the negative electrodes are alternately stacked with the separator interposed therebetween, and the weight ratio of the silicon-based active material contained in the first negative electrode to the carbon-based active material contained in the second negative electrode is in the range of 40:60 to 90:10.

[0018] Beneficial effects

[0019] The secondary battery of the present invention includes a first negative electrode containing a silicon-based active material and a second negative electrode containing a carbon-based active material, wherein the weight ratio of the silicon-based active material contained in the first negative electrode to the carbon-based active material contained in the second negative electrode is adjusted to a specific ratio. According to the secondary battery of the present invention, because separate negative electrodes containing different types of active materials having different characteristics are used, a binder and a conductive material suitable for the characteristics of each active material can be used in desired amounts, maximizing the respective advantages of the silicon-based active material and the carbon-based active material, thereby achieving a secondary battery with excellent capacity and lifespan characteristics.

[0020] Furthermore, in the secondary battery of the present invention, the weight ratio of the silicon-based active material contained in the first negative electrode to the carbon-based active material contained in the second negative electrode is adjusted to a specific weight ratio. Therefore, since the charge potential of the silicon-based active material can be lowered to an appropriate level, the stress level of the silicon-based active material caused by rapid voltage changes during charge and discharge can be reduced, and rapid charging characteristics can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is an image schematically showing the secondary battery of the present invention. DETAILED DESCRIPTION

[0022] The terms and words used in this specification and claims should not be construed as limited to the commonly used meanings or meanings in dictionaries, but should be construed using meanings and concepts consistent with the technical spirit of the present invention, based on the principle that inventors can appropriately define the concepts of the terms to describe their invention in the best manner.

[0023] The terms used in this specification are for the purpose of describing exemplary embodiments only and are not intended to limit the present invention. Unless the context clearly indicates otherwise, expressions in the singular also include expressions in the plural.

[0024] It should be understood that when used in this specification, terms such as "comprises", "including", "comprising", "containing", "having" or "having" specify the existence of the stated features, quantities, steps, elements or a combination thereof, but do not exclude the possibility of the existence or addition of one or more other features, quantities, steps, elements or a combination thereof.

[0025] In the present invention, the average particle size (D 50 ) is defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size (D) can be measured using, for example, laser diffraction. 50). The laser diffraction method generally enables the measurement of particle sizes ranging from submicron to several millimeters and can produce results with high reproducibility and high resolution.

[0026] Hereinafter, the secondary battery of the present invention will be described in detail with reference to the accompanying drawings. In describing the present invention, when it is considered that detailed description of related known configurations or functions may obscure the gist of the present invention, the detailed description may be omitted.

[0027] <Secondary Battery>

[0028] The present invention relates to a secondary battery 1, and more particularly, to a lithium secondary battery.

[0029] like Figure 1 As shown, the secondary battery 1 of the present invention includes: one or more positive electrodes 100, the positive electrode 100 including a positive electrode active material layer 120; a plurality of negative electrodes 200, 300, including a first negative electrode 200 including a silicon-based active material and a second negative electrode 300 including a carbon-based active material; a separator 400; and an electrolyte (not shown), wherein the positive electrode 100 and the negative electrodes 200, 300 are alternately stacked with the separator 400 interposed therebetween, and the weight ratio of the silicon-based active material included in the first negative electrode 200 to the carbon-based active material included in the second negative electrode 300 is in the range of 40:60 to 90:10.

[0030] Conventionally, silicon-based active materials have the advantages of high capacity and rapid charging characteristics. However, due to the large degree of volume expansion and contraction caused by charging and discharging, rapid deterioration of lifespan characteristics is a problem. Meanwhile, although negative electrodes containing a combination of silicon-based and carbon-based active materials have been developed, achieving high capacity and rapid charging characteristics with these silicon-based active materials is difficult because the proportion of silicon-based active materials used in the negative electrode must be reduced to control their volume expansion.

[0031] To address this problem, in the secondary battery of the present invention, the weight ratio of the silicon-based active material contained in the first negative electrode 200 to the carbon-based active material contained in the second negative electrode 300 is adjusted to within the above-mentioned range. In the secondary battery of the present invention, since the silicon-based active material in the first negative electrode 200 and the carbon-based active material in the second negative electrode 300 are used in the above-mentioned weight ratio, the carbon-based active material can lower the charge potential of the silicon-based active material to a desired level. Consequently, the volume expansion / contraction of the silicon-based active material during charge and discharge can be reduced, and the high capacity and rapid charging characteristics of the silicon-based active material can be fully utilized.

[0032] Furthermore, the secondary battery of the present invention includes the first negative electrode 200 and the second negative electrode 300, each containing the silicon-based active material and the carbon-based active material, respectively. Specifically, because the secondary battery of the present invention uses different types of negative electrodes containing different active materials, the negative electrode components can be included in a composition suitable for each negative electrode. Furthermore, even when the proportion of the silicon-based active material used in the secondary battery increases, the high capacity and fast charging characteristics of the silicon-based active material can be fully utilized without degrading the lifespan characteristics.

[0033] The secondary battery 1 of the present invention includes one or more positive electrodes 100 . The positive electrode 100 includes a positive electrode active material layer 120 .

[0034] Specifically, the positive electrode 100 may include a positive electrode current collector 110 and a positive electrode active material layer 120 formed on one or more surfaces of the positive electrode current collector 110 .

[0035] The positive electrode current collector 110 is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the positive electrode current collector 110 may include one or more selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy.

[0036] The positive electrode current collector 110 may generally have a thickness of 3 μm to 500 μm.

[0037] The positive electrode current collector 110 may have fine irregularities formed in its surface to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector 110 may be used in any of various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, and a non-woven fabric.

[0038] The positive electrode active material layer 120 is formed on one or more surfaces of the positive electrode current collector 110. Specifically, the positive electrode active material layer 120 may be formed on one or both surfaces of the positive electrode current collector 110.

[0039] The positive active material layer 120 may include a positive active material.

[0040] The positive electrode active material may include a compound capable of reversibly inserting and deinserting lithium, specifically, may include a lithium-transition metal composite oxide containing lithium and one or more selected from the group consisting of nickel, cobalt, manganese and aluminum, preferably a lithium-transition metal composite oxide containing lithium and a transition metal selected from the group consisting of nickel, cobalt and manganese.

[0041] More specifically, the lithium-transition metal composite oxide can be, for example, a lithium-manganese-based oxide (such as LiMnO2, LiMn2O4), a lithium-cobalt-based oxide (such as LiCoO2), a lithium-nickel-based oxide (such as LiNiO2), a lithium-nickel-manganese-based oxide (such as LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni Z O4 (where 0 < Z < 2)), a lithium-nickel-cobalt-based oxide (such as LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1)), a lithium-manganese-cobalt-based oxide (such as LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2)), a lithium-nickel-manganese-cobalt-based oxide (such as Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1 and p + q + r1 = 1), Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2 and p1 + q1 + r2 = 2)) or a lithium-nickel-cobalt-transition metal (M) oxide (such as Li(Ni p2 Co q2 Mn r3 M s2 )O2 (where M is selected from the group consisting of: Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r3 and s2 represent the atomic fractions of the respective independent elements, and satisfy 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1 and p2 + q2 + r3 + s2 = 1)) or a combination of one or more thereof. Among them, in order to improve the capacity characteristics and stability of the battery, the lithium-transition metal composite oxide can be LiCoO2, LiMnO2, LiNiO2, a lithium-nickel-manganese-cobalt oxide (such as Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, Li(Ni 0.8 Mn 0.1 Co0.1 )O2), lithium-nickel-cobalt-aluminum oxide (e.g. Li(Ni 0.8 Co 0.15 Al 0.05 )O2) etc., and considering that the capacity characteristics and stability of the battery can be significantly improved by controlling the type and content ratio of the elements constituting the lithium-transition metal composite oxide, the lithium-transition metal composite oxide can be Li(Ni 0.6 Mn 0.2 Co 0.2 )O2、Li(Ni 0.5 Mn 0.3 Co 0.2 )O2、Li(Ni 0.7 Mn 0.15 Co 0.15 )O2、Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc., and these compounds can be used alone or in combination of two or more thereof.

[0042] The content of the positive electrode active material in the positive electrode active material layer may be 80 wt % to 99 wt %, preferably 92 wt % to 98.5 wt %, so that the capacity of the positive electrode active material can be fully realized.

[0043] In addition to the positive electrode active material, the positive electrode active material layer 120 may further include a positive electrode binder and / or a positive electrode conductive material.

[0044] The positive electrode binder is a component that helps to bond the active material to the conductive material, etc. and to the collector. Specifically, it can include one or more selected from the group consisting of: polyvinylidene fluoride, polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber and fluororubber, and preferably polyvinylidene fluoride.

[0045] In view of sufficiently ensuring bonding between components such as the positive electrode active material, the content of the positive electrode binder in the positive electrode active material layer may be 1 wt % to 20 wt %, preferably 1.2 wt % to 10 wt %.

[0046] The positive electrode conductive material can be used to assist and improve the conductivity of the secondary battery, and there is no particular limitation as long as it does not cause chemical changes and has conductivity. Specifically, the positive electrode conductive material may include one or more selected from the group consisting of: graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives, and preferably includes carbon black from the perspective of improving conductivity.

[0047] In terms of promoting the dispersion of the positive electrode conductive material and further improving the conductivity when preparing the slurry for forming the positive electrode active material layer, the specific surface area of the positive electrode conductive material can be 80 m 2 / g~200m 2 / g, preferably 100m 2 / g~150m 2 / g.

[0048] In terms of sufficiently ensuring conductivity, the content of the positive electrode conductive material in the positive electrode active material layer may be 1 wt % to 20 wt %, preferably 1.2 wt % to 10 wt %.

[0049] The thickness of the positive active material layer 120 may be in the range of 30 μm to 400 μm, preferably 40 μm to 110 μm. In this specification, the thickness of the positive active material layer may be the thickness of a positive active material layer formed on one surface of the positive current collector 110 .

[0050] The porosity of the positive electrode 100 may be in the range of 20% to 35%, preferably 20% to 28%. The porosity of the positive electrode 100 may be determined using the porosity determination method of the following formula 1.

[0051] [Mathematical formula 1]

[0052] Porosity of positive electrode (%) = {1-(electrode density of positive electrode / true density of positive electrode)} × 100

[0053] In the above mathematical formula 1, the true density of the positive electrode is the density of the positive electrode active material layer measured when the positive electrode provided in a predetermined size is pressed using a pressing device until the thickness of the positive electrode no longer changes, and the electrode density of the positive electrode is the density of the positive electrode active material layer measured after the positive electrode is provided in a predetermined size.

[0054] The positive electrode 100 may be manufactured by applying a positive electrode slurry including the positive electrode active material and optionally a binder, a conductive material, and a solvent for forming a positive electrode slurry onto the positive electrode current collector 110 , followed by roll pressing and drying.

[0055] The positive electrode slurry-forming solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that has a preferred viscosity when containing the positive electrode active material and optional binder, conductive material, etc. For example, the positive electrode slurry-forming solvent may be included in the positive electrode slurry in such a manner that the solid content concentration containing the positive electrode active material and optional binder and conductive material is within a range of 50% by weight to 95% by weight, preferably 70% by weight to 90% by weight.

[0056] There are a plurality of negative electrodes, and the negative electrode includes a first negative electrode 200 and a second negative electrode 300. The first negative electrode 200 and the second negative electrode 300 included in the negative electrode may be one or more, specifically, a plurality.

[0057] The first negative electrode 200 contains a silicon-based active material, and the second negative electrode 300 contains a carbon-based active material. The weight ratio of the silicon-based active material contained in the first negative electrode 200 to the carbon-based active material contained in the second negative electrode 300 is within a range of 40:60 to 90:10. Specifically, when there are multiple first negative electrodes 200, the "weight of the silicon-based active material contained in the first negative electrode 200" may refer to the total weight of the silicon-based active material contained in the multiple first negative electrodes 200, and when there are multiple second negative electrodes 300, the "weight of the carbon-based active material contained in the second negative electrode 300" may refer to the total weight of the carbon-based active material contained in the multiple second negative electrodes 300. In this case, the weight ratio of the silicon-based active material contained in the first negative electrode 200 to the carbon-based active material contained in the second negative electrode 300 may refer to the ratio of the total weight of the silicon-based active material contained in the multiple first negative electrodes 200 to the total weight of the carbon-based active material contained in the multiple second negative electrodes 300.

[0058] In the secondary battery of the present invention, the weight ratio of the silicon-based active material contained in the first negative electrode 200 to the carbon-based active material contained in the second negative electrode 300 is adjusted to be within the above range. Generally, it is known that silicon-based active materials have a higher charge potential than carbon-based active materials and have excellent fast charging characteristics. However, due to the high charge potential of the silicon-based active material, the silicon-based active material undergoes a large voltage change during charge and discharge, so rapid volume expansion / contraction may occur, and the life characteristics may decline rapidly. In the secondary battery of the present invention, the silicon-based active material in the first negative electrode 200 and the carbon-based active material in the second negative electrode 300 are used in the above weight ratio. Since the charge potential of the silicon-based active material can be reduced to a desired level by the carbon-based active material, the volume expansion / contraction degree of the silicon-based active material during charge and discharge can be reduced, and the high capacity and fast charging characteristics of the silicon-based active material can be fully demonstrated.

[0059] Furthermore, the secondary battery of the present invention includes the first negative electrode 200 and the second negative electrode 300, each comprising a silicon-based active material and a carbon-based active material, respectively. Specifically, because the secondary battery of the present invention uses different types of negative electrodes comprising different active materials, the negative electrode components can be comprised in a composition suitable for each negative electrode. Furthermore, even when the proportion of the silicon-based active material used in the secondary battery is increased, the high capacity and fast charging characteristics of the silicon-based active material can be fully utilized without degrading the lifespan characteristics.

[0060] When the weight of the silicon-based active material is less than 40 wt % and the weight of the carbon-based active material is greater than 60 wt % based on the total weight of the silicon-based active material contained in the first negative electrode 200 and the carbon-based active material contained in the second negative electrode 300, the proportion of the carbon-based active material used in the entire negative electrode is increased. In this case, since the proportion of the carbon-based active material used in the entire negative electrode is increased, the capacity of the negative electrode is reduced, and the overall N / P ratio of the secondary battery (i.e., the ratio N / P below) is reduced. 总和 / P 总和 ) is reduced, and thus the proportion of the silicon-based active material used during the charge and discharge of the negative electrode increases. In this case, because the volume expansion / contraction of the silicon-based active material increases the influence on the secondary battery, there is a risk that the life characteristics may be reduced. In addition, because the potential of the carbon-based active material during rapid charging is very low and, for example, a negative (-) potential may be formed, when the proportion of the carbon-based active material used in the entire negative electrode is increased, lithium (Li) plating may occur, which is also disadvantageous in terms of the life characteristics during rapid charging.

[0061] When the weight of the silicon-based active material is greater than 90% by weight and the weight of the carbon-based active material is less than 10% by weight, based on the combined weight of the silicon-based active material contained in the first negative electrode 200 and the carbon-based active material contained in the second negative electrode 300, the proportion of the silicon-based active material increases, exacerbating the problem of volume expansion / contraction due to charge and discharge, and potentially reducing lifespan characteristics. Furthermore, when the proportion of the silicon-based active material used in the entire negative electrode is too high, the positive electrode 100 may form a high potential due to the high potential of the silicon-based active material during battery operation, thereby increasing the resistance of the positive electrode 100 and potentially reducing lifespan characteristics.

[0062] In the secondary battery of the present invention, the weight ratio of the silicon-based active material contained in the first negative electrode 200 to the carbon-based active material contained in the second negative electrode 300 can be in the range of 40:60 to 90:10, preferably 60:40 to 75:25, and when this range is met, the fast charging characteristics, capacity and life characteristics of the secondary battery can be further improved.

[0063] In the secondary battery of the present invention, by adjusting the amount of active materials contained in the first negative electrode 200 and the second negative electrode 300 or adjusting the number of the first negative electrode 200 and the second negative electrode 300, the weight ratio of the silicon-based active material contained in the first negative electrode 200 to the carbon-based active material contained in the second negative electrode 300 can be achieved, such as Figure 1 It should be noted that Figure 1 This is merely an exemplary illustration of the first negative electrode 200 and the second negative electrode 300, and the number of the first negative electrode and the second negative electrode is not limited to Figure 1 The quantity shown in .

[0064] The first negative electrode 200 includes a silicon-based active material. Specifically, the first negative electrode 200 may include only a silicon-based active material as an active material and may not include a carbon-based active material.

[0065] The first negative electrode 200 includes a first negative current collector 210 and a first negative active material layer 220 formed on one or more surfaces of the first negative current collector 210 . The first negative active material layer 220 may include the silicon-based active material.

[0066] The first negative electrode current collector 210 may include one or more selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, and preferably includes copper.

[0067] The thickness of the first negative electrode current collector 210 can generally be 3 μm to 500 μm, preferably 5 μm to 30 μm.

[0068] The first negative electrode current collector 210 can have fine irregularities formed on its surface to increase the adhesion of the negative electrode active material. For example, the first negative electrode current collector can be used in any of a variety of forms such as films, sheets, foils, meshes, porous materials, foams, and non-woven fabrics.

[0069] The first negative electrode active material layer 220 is formed on one or more surfaces of the current collector of the first negative electrode 200. Specifically, as Figure 1 shown, the first negative electrode active material layer 220 can be formed on one or two surfaces of the first negative electrode current collector 210.

[0070] The first negative electrode active material layer 220 can contain the above-mentioned silicon-based active material.

[0071] The silicon-based active material can contain a compound represented as SiO x (0 ≤ x < 2). Since SiO2 does not react with lithium ions and thus cannot store lithium, it is preferred that x satisfies the above range.

[0072] Specifically, the silicon-based active material can contain Si. Although Si is generally advantageous because it has a capacity about 2.5 to 3 times higher than that of silicon oxides (such as SiO x (0 < x < 2)), Si undergoes more severe volume expansion / contraction during charging and discharging than silicon oxides, and thus commercialization is relatively difficult. However, in the case of the secondary battery of the present invention, although a silicon-based active material containing Si is used, since the charging potential of the active material can be reduced to a desired level, deterioration of the life characteristics can be prevented while the high capacity and fast charging characteristics of the silicon-based active material can be improved. Specifically, the silicon-based active material can be mainly formed of Si, and more specifically, the silicon-based active material can be formed of Si.

[0073] In terms of ensuring the structural stability of the active material during charging and discharging, enabling a more stable formation of a conductive network for maintaining conductivity, or further improving the accessibility of an adhesive for bonding the active material and the current collector, the average particle size (D 50 ) of the silicon-based active material can be in the range of 1 μm to 10 μm, preferably 2 μm to 6 μm.

[0074] In order to minimize the impact of volume expansion / contraction of the silicon-based active material in the battery and fully realize the high capacity of the silicon-based active material in the secondary battery, the content of the silicon-based active material in the first negative electrode active material layer can be 60 weight% to 90 weight%, preferably 70 weight% to 80 weight%.

[0075] In addition to including the silicon-based active material, the first negative electrode active material layer 220 may further include a first negative electrode binder and a first negative electrode conductive material.

[0076] The first negative electrode binder may be used to improve adhesion between the silicon-based active material and the negative electrode current collector or enhance bonding between the silicon-based active materials.

[0077] Specifically, the first negative electrode binder may include one or more selected from the group consisting of styrene-butadiene rubber (SBR), nitrile rubber, acrylic rubber, butyl rubber, fluororubber, polyvinyl alcohol (PVA), CMC, starch, hydroxypropyl cellulose, regenerated cellulose, polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN) and polyacrylamide (PAM).

[0078] Preferably, the first negative electrode binder comprises one or more selected from the group consisting of PVA, PAA, PAN, and PAM, preferably PVA and PAA, in order to have high strength, excellent resistance to volume expansion / contraction of the silicon-based active material, and be able to prevent deformation or warping of the electrode by imparting excellent flexibility to the negative electrode. When the first negative electrode binder comprises PVA and PAA, the first negative electrode binder may be a copolymer of PVA and PAA, specifically a copolymer of PVA and PAA having units derived from vinyl alcohol and units derived from acrylic acid in a weight ratio of 50:50 to 90:10, preferably 55:45 to 80:20.

[0079] In terms of ensuring better dispersibility in an aqueous solvent such as water when preparing a slurry for forming a negative electrode active material layer and improving binding by more smoothly coating the active material, the first negative electrode binder may include a first negative electrode binder in which hydrogen is replaced by Li, Na, Ca, etc.

[0080] In the first negative electrode active material layer, the content of the first negative electrode binder can be 5 weight% to 30 weight%, preferably 10 weight% to 20 weight%, and when the above range is met, because the silicon-based active material can be smoothly combined, the problem of volume expansion of the active material can be minimized. At the same time, during the preparation of the slurry for forming the negative electrode active material layer, the negative electrode binder can be easily dispersed and the coating ability and the phase stability of the slurry can be improved.

[0081] The first negative electrode conductive material can be used to assist and improve the conductivity of the secondary battery, and there is no particular limitation as long as it does not cause chemical changes and has conductivity. Specifically, the first negative electrode conductive material may include one or more selected from the group consisting of: graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives, and preferably includes carbon black in terms of achieving conductivity.

[0082] In terms of promoting the dispersion of the negative electrode conductive material and further improving the conductivity during the preparation of the slurry for forming the negative electrode active material layer, the specific surface area of the first negative electrode conductive material may be 80 m 2 / g~200m 2 / g, preferably 100m 2 / g~150m 2 / g.

[0083] In the first negative electrode active material layer, the content of the first negative electrode conductive material may be 5 wt % to 20 wt %, preferably 7 wt % to 15 wt %, and is preferred when the above range is satisfied because an excellent conductive network can be formed while reducing the increase in resistance caused by the negative electrode binder.

[0084] The thickness of the first negative active material layer 220 may be in the range of 10 μm to 100 μm, preferably 20 μm to 50 μm. In this specification, the thickness of the first negative active material layer 220 may be the thickness of a first negative active material layer 220 formed on one surface of the first negative current collector 210 .

[0085] The porosity of the first negative electrode 200 may be in the range of 28% to 50%, preferably 32% to 40%, and is preferred when this range is met because the volume expansion / contraction of the silicon-based active material can be properly accommodated, and at the same time, because an appropriate level of contact between the active materials can be maintained, the conductivity can be improved.

[0086] In this specification, the porosity of the first negative electrode 200 can be calculated by the following Mathematical Formula 2.

[0087] [Mathematical formula 2]

[0088] Porosity of the first negative electrode (%) = {1-(electrode density of the first negative electrode / true density of the first negative electrode)}×100

[0089] In Mathematical Formula 2, the true density of the first negative electrode is the density of the first negative electrode active material layer measured when the negative electrode provided in a predetermined size is pressed using a pressing device until the thickness of the negative electrode no longer changes, and the electrode density of the first negative electrode is the density of the first negative electrode active material layer measured after the first negative electrode is provided in a predetermined size.

[0090] In the first negative electrode 200 , the N1 / P ratio calculated by the following formula a may be in the range of 2.2 to 3.0, preferably 2.4 to 2.8.

[0091] [Mathematical formula a]

[0092] N1 / P ratio={(discharge capacity per unit area of one first negative electrode active material layer in the first negative electrode) / (discharge capacity per unit area of one positive electrode active material layer in the positive electrode)}

[0093] When the N1 / P ratio of the first negative electrode is adjusted within the above range, the proportion of the silicon-based active material used during operation of the first negative electrode can be reduced to an appropriate level, thereby minimizing the volume expansion effect of the silicon-based active material in the secondary battery.

[0094] The N1 / P ratio can be calculated by manufacturing half cells of each of the first negative electrode 200 and the positive electrode 100 and determining the discharge capacity per unit area of the first negative electrode 200 and the positive electrode 100 , and substituting the results into Formula a.

[0095] The first negative electrode 200 may be manufactured by applying a negative electrode slurry including a silicon-based active material and optionally a first negative electrode binder, a first negative electrode conductive material, and a solvent for forming a negative electrode slurry to the first negative electrode current collector, and performing roll pressing and drying.

[0096] In terms of promoting dispersion of the negative electrode active material, the negative electrode binder and / or the negative electrode conductive material, the negative electrode slurry-forming solvent may include, for example, one or more selected from the group consisting of distilled water, ethanol, methanol and isopropyl alcohol, and preferably distilled water.

[0097] Taking into account the viscosity, coating ability, dispersibility, etc. of the negative electrode slurry, the negative electrode slurry may contain the negative electrode slurry-forming solvent in an amount such that the solid content concentration in the negative electrode slurry is within a range of 15 wt % to 45 wt %, preferably within a range of 20 wt % to 30 wt %.

[0098] The second negative electrode 300 includes a carbon-based active material. Specifically, the second negative electrode 300 may include only a carbon-based active material as an active material and may not include a silicon-based active material. Because the second negative electrode 300 includes the carbon-based active material, the total charge potential of the negative electrode can be reduced, and sudden volume expansion caused by the silicon-based active material in the first negative electrode can be prevented.

[0099] Specifically, the second negative electrode 300 may include a second negative electrode current collector 310 and a second negative electrode active material layer 320 formed on one or more surfaces of the second negative electrode current collector 310 , and the second negative electrode active material layer 320 may include the carbon-based active material.

[0100] The second negative electrode current collector 310 may include one or more selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy, and preferably includes copper.

[0101] The thickness of the second negative electrode current collector 310 may generally be in the range of 3 μm to 500 μm, preferably 5 μm to 30 μm.

[0102] The second negative electrode current collector 310 may have fine irregularities formed in its surface to increase the adhesion of the negative electrode active material. For example, the second negative electrode current collector may be used in any of various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, and a non-woven fabric.

[0103] The second negative active material layer 320 is formed on one or more surfaces of the second negative current collector 310. Specifically, the second negative active material layer 320 may be formed on one or both surfaces of the second negative current collector 310.

[0104] The second negative electrode active material layer 320 may include the aforementioned carbon-based active material.

[0105] Specifically, the carbon-based active material may include one or more selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, preferably one or more of artificial graphite and natural graphite, more preferably artificial graphite and natural graphite.

[0106] In terms of ensuring structural stability during charge and discharge and reducing side reactions with the electrolyte, the average particle size (D 50 ) can be in the range of 10 μm to 30 μm, preferably 15 μm to 25 μm.

[0107] In order to fully utilize the capacity of the carbon-based active material, the content of the carbon-based active material in the second negative electrode active material layer may be 90 wt % to 99 wt %, preferably 92 wt % to 98 wt %.

[0108] In addition to including the carbon-based active material, the second negative electrode active material layer 320 may further include a second negative electrode binder and a second negative electrode conductive material.

[0109] The second negative electrode binder may be used to improve adhesion between the carbon-based active material and the negative electrode current collector or enhance bonding between the carbon-based active materials.

[0110] Specifically, the second negative electrode binder may include one or more selected from the group consisting of: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, poly(methyl methacrylate), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM) rubber, styrene-butadiene rubber (SBR) and fluororubber, and SBR is preferred in terms of improving the adhesion between the carbon-based active material and the current collector.

[0111] In the second negative electrode active material layer, the content of the second negative electrode binder may be 0.5 wt % to 10 wt %, preferably 1 wt % to 5 wt %, and more preferably 1.5 wt % to 3.5 wt %.

[0112] There is no particular limitation on the second negative electrode conductive material as long as it does not cause chemical changes in the battery and has conductivity, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black or thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0113] The content of the second negative electrode conductive material in the second negative electrode active material layer may be 0.5 wt % to 5 wt %.

[0114] In terms of promoting the dispersion of the negative electrode conductive material and further improving the conductivity during the preparation of the slurry for forming the negative electrode active material layer, the specific surface area of the second negative electrode conductive material may be 80 m 2 / g~200m 2 / g, preferably 100m 2 / g~150m 2 / g.

[0115] The thickness of the second negative active material layer 320 may be in the range of 20 μm to 200 μm, preferably 60 μm to 120 μm. In this specification, the thickness of the second negative active material layer 320 may be the thickness of one second negative active material layer 320 formed on one surface of the negative current collector 31 .

[0116] The porosity of the second negative electrode 300 may be in the range of 20% to 40%, preferably 21% to 30%. This range is preferred because the impregnation performance of the electrolyte can be improved while also improving the energy density.

[0117] The porosity of the second negative electrode 300 may be calculated by applying Math Formula 2, which is a formula for determining the porosity of the first negative electrode 200 .

[0118] In the second negative electrode 300 , the N2 / P ratio calculated by the following formula (b) may be in the range of 1.0 to 1.3, preferably 1.05 to 1.1.

[0119] [Formula b]

[0120] N2 / P ratio={(discharge capacity per unit area of one second negative electrode active material layer in the second negative electrode) / (discharge capacity per unit area of one positive electrode active material layer in the positive electrode)}

[0121] When the N2 / P ratio of the second negative electrode is adjusted to be within the above range, it is preferable because stable performance can be obtained while using the entire potential range of the second negative electrode 300 and the positive electrode 100 .

[0122] The N2 / P ratio can be calculated by manufacturing half cells of the second negative electrode 300 and the positive electrode 100, determining the discharge capacity per unit area of the second negative electrode 300 and the positive electrode 100, and substituting the results into Formula (b).

[0123] In the secondary battery of the present invention, the positive electrode 100 and the negative electrodes 200 and 300 may be stacked alternately. Figure 1 As shown, the positive electrode 100 may be disposed between a plurality of negative electrodes 200 and 300 , and in this case, the first negative electrodes 200 and the second negative electrodes 300 may be stacked alternately or randomly.

[0124] In the secondary battery of the present invention, N is calculated by the following mathematical formula c: 总和 / P 总和The ratio can be in the range of 2 to 3, preferably 2.2 to 2.8, and more preferably 2.40 to 2.55, and when this range is met, the life characteristics of the secondary battery can be further improved because the high capacity and fast charging characteristics of the silicon-based active material can be exhibited and the influence of the volume expansion / contraction of the silicon-based active material can be minimized.

[0125] [Mathematical formula c]

[0126] N 总和 / P 总和 Ratio={(the total discharge capacity per unit area of the negative electrodes present in the secondary battery) / (the total discharge capacity per unit area of the positive electrodes present in the secondary battery)}

[0127] Specifically, the N can be obtained by measuring the discharge capacity per unit area of each of the multiple negative electrodes present in the secondary battery and calculating the sum thereof, measuring the discharge capacity per unit area of each of the multiple positive electrodes present in the secondary battery and calculating the sum thereof, and then substituting the above-obtained values into the mathematical formula c. 总和 / P 总和 Compare.

[0128] like Figure 1 As shown, the separator 400 is interposed between the positive electrode 100 and the negative electrode 200, 300. Specifically, the positive electrode 100 and the negative electrode 200, 300 are alternately stacked with the separator 400 interposed therebetween.

[0129] The separator 400 is used to separate the negative electrode 200, 300 and the positive electrode 100 and provide a channel for the migration of lithium ions, and any separator commonly used in lithium secondary batteries can be used without particular limitation. In particular, a separator with low resistance to the migration of electrolyte ions and excellent electrolyte impregnation capacity is preferred. Specifically, it is possible to use: a porous polymer film, for example, a porous polymer film formed by a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer or an ethylene / methacrylate copolymer; or a stacked structure having two or more layers thereof. In addition, ordinary porous non-woven fabrics can be used, such as non-woven fabrics made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. In addition, in order to ensure heat resistance or mechanical strength, a coated separator comprising a ceramic component or a polymer material and optionally having a single-layer or multi-layer structure can be used.

[0130] Examples of the electrolyte (not shown) include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, inorganic solid electrolytes, and melt-type inorganic electrolytes that can be used to manufacture secondary batteries.

[0131] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0132] As an organic solvent, any organic solvent that can be used as a medium through which ions participating in the electrochemical reaction of the battery can migrate can be used without particular limitation. Specifically, as the organic solvent, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone, etc.; ether solvents such as dibutyl ether, tetrahydrofuran, etc.; ketone solvents such as cyclohexanone, etc.; aromatic hydrocarbon solvents such as benzene, fluorobenzene, etc.; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) or propylene carbonate (PC); alcohol solvents such as ethanol or isopropanol, etc.; nitriles such as R-CN (R is a C2 to C20 hydrocarbon group having a linear, branched or cyclic structure and can contain a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolanes, etc.; or cyclopentane sulfone. Among them, carbonate solvents are preferred, and a combination of a cyclic carbonate (such as EC or PC) having high ion conductivity and high dielectric constant and a linear carbonate compound (such as EMC, DMC or DEC) having low viscosity, which can improve the charge / discharge performance of the battery, is more preferred. In this case, when the cyclic carbonate and the linear carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte can be excellent.

[0133] As the lithium salt, any compound capable of providing lithium ions for lithium secondary batteries can be used without particular limitation. Specifically, as the lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc. can be used. The lithium salt is preferably used in a concentration range of 0.1 to 2.0M. When the concentration of the lithium salt is within the above range, because the electrolyte has appropriate conductivity and viscosity, the electrolyte performance can be excellent, and lithium ions can move efficiently.

[0134] According to a conventional secondary battery manufacturing method, the secondary battery may be manufactured by interposing the separator between the above-mentioned negative electrode and positive electrode and injecting an electrolyte.

[0135] The secondary battery of the present invention can be used in the fields of portable devices such as mobile phones, laptop computers, and digital cameras; and electric vehicles including hybrid electric vehicles (HEVs), and can be particularly preferably used as a constituent battery of medium to large battery modules.

[0136] The medium-to-large battery module can be preferably used as a power source for EVs, HEVs, or power storage devices requiring high output and large capacity.

[0137] Hereinafter, exemplary embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be embodied in many different forms and is not limited to the embodiments described herein.

[0138] Example

[0139] Example 1: Manufacturing of a Secondary Battery

[0140] <Manufacturing of positive electrode>

[0141] The positive electrode active material Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2(average particle size(D 50 ):15μm), carbon black (product name: Super C65, manufacturer: TIMCAL) as a positive electrode conductive material and PVDF as a positive electrode binder were added to NMP used as a solvent for forming a positive electrode slurry to prepare a positive electrode slurry (solid content concentration: 78wt%).

[0142] To form a positive electrode active material layer (thickness: 54 μm), 454 mg / 25 cm 2 The positive electrode slurry was applied to both surfaces of an aluminum current collector (thickness: 12 μm) used as a positive electrode current collector, rolled, and then dried in a vacuum oven at 130° C. for 10 hours, thereby obtaining the positive electrode of Example 1 (positive electrode thickness: 120 μm, porosity: 24%). A plurality (20) of positive electrodes were manufactured.

[0143] <Manufacturing of negative electrode>

[0144] 1. Fabrication of the First Negative Electrode

[0145] The Si (average particle size (D 50 ): 3 μm), carbon black (product name: Super C65, manufacturer: TIMCAL) as a first negative electrode conductive material and a PVA / PAA copolymer (containing units derived from vinyl alcohol and units derived from acrylic acid in a ratio of 66:34; weight average molecular weight: approximately 360,000 g / mol) as a first negative electrode binder were added to distilled water used as a solvent for forming a negative electrode slurry to prepare a negative electrode slurry (solid content concentration: 25 wt%).

[0146] To form the first negative electrode active material layer (thickness: 26.5 μm), 91.8 mg / 25 cm 2 The negative electrode slurry was applied to both surfaces of a copper current collector (thickness: 15 μm) used as a negative electrode current collector, rolled, and then dried in a vacuum oven at 130° C. for 10 hours, thereby obtaining a first negative electrode A (negative electrode thickness: 68 μm, negative electrode porosity: 34%). A plurality (17) of first negative electrodes A were manufactured.

[0147] In addition, the negative electrode slurry was mixed with 91.8 mg / 25 cm 2 A loading amount of 100 μm was applied to one surface of a copper current collector (thickness: 15 μm) used as a negative electrode current collector, rolled, and then dried in a vacuum oven at 130° C. for 10 hours to form a first negative electrode active material layer (thickness: 26.5 μm), thereby obtaining a first negative electrode B (negative electrode thickness: 41.5 μm, negative electrode porosity: 34%). A plurality (two) of first negative electrodes B were manufactured.

[0148] The first negative electrode A and the first negative electrode B are used as the first negative electrode.

[0149] 2. Fabrication of the Second Negative Electrode

[0150] By mixing artificial graphite (average particle size (D 50 ):15μm) and natural graphite (average particle size (D 50 A 90:10 (weight / weight) mixture of 200 μm (0.475 μm) and 18 μm (0.775 μm) was added to distilled water used as a solvent for forming a negative electrode slurry to prepare a negative electrode slurry (solid content concentration: 48 wt%).

[0151] To form the second negative electrode active material layer (thickness: 74 μm), 298 mg / 25 cm 2 The negative electrode slurry was applied to both surfaces of a copper current collector (thickness: 8 μm) used as a negative electrode current collector, roll-pressed, and then dried in a vacuum oven at 130° C. for 10 hours, thereby obtaining a second negative electrode (negative electrode thickness: 156 μm, negative electrode porosity: 27%). A plurality (two) of second negative electrodes were manufactured.

[0152] <Manufacturing of Secondary Batteries>

[0153] 1. Manufacturing of secondary batteries

[0154] A secondary battery is manufactured by alternately stacking the negative electrodes (first and second negative electrodes) and the positive electrodes so that the electrodes are separated from each other with a polyethylene separator interposed therebetween, and injecting an electrolyte. Two first negative electrodes B are arranged at the outermost portion of the secondary battery so that the first negative electrode active material layer is arranged to face the positive electrode active material layer of the positive electrode. The first negative electrode A and the second negative electrode are alternately stacked with the positive electrode so that their negative electrode active material layers face the positive electrode active material layer of the positive electrode.

[0155] As the electrolyte, an electrolyte prepared by adding 3 wt % (based on the total weight of the electrolyte) of vinylene carbonate and 1 M LiPF 6 as a lithium salt to an organic solvent formed of fluoroethylene carbonate (FEC) and DMC in a volume ratio of 30:70 was used.

[0156] The secondary battery includes 17 first negative electrodes A, two first negative electrodes B, two second negative electrodes, and 20 positive electrodes. The weight ratio of the silicon-based active material contained in the first negative electrodes (first negative electrodes A and first negative electrodes B) to the carbon-based active material contained in the second negative electrodes is 68.5:31.5. Furthermore, the ratio of the total weight of the first negative electrodes to the total weight of the second negative electrodes is 90:10.

[0157] 2. N / P ratio

[0158] (1) N1 / P ratio

[0159] A negative electrode for measuring the N / P ratio was manufactured in the same manner as the method for manufacturing the first negative electrode A, except that the negative electrode active material layer was formed only on one surface of the copper current collector. A coin-type half-cell was manufactured, comprising the negative electrode for measuring the N / P ratio, a lithium metal counter electrode opposite to the negative electrode for measuring the N / P ratio, a polyethylene separator interposed between the negative electrode for measuring the N / P ratio and the lithium metal counter electrode, and an electrolyte, and the discharge capacity (unit: mAh / g) was measured. The discharge capacity was calculated by multiplying the discharge capacity by the load (91.8 mg / 25 cm 2 =3.67mg / cm 2 =0.00367g / cm 2 ), find the discharge capacity per unit area of a first negative electrode active material layer in the first negative electrode (unit: mAh / cm 2 ).

[0160] A positive electrode for measuring the N / P ratio was manufactured in the same manner as the method for manufacturing the positive electrode, except that the positive electrode active material layer was formed only on one surface of the aluminum current collector. A coin-type half-cell was manufactured, comprising the positive electrode for measuring the N / P ratio, a lithium metal counter electrode opposite to the positive electrode for measuring the N / P ratio, a polyethylene separator and an electrolyte interposed between the positive electrode for measuring the N / P ratio and the lithium metal counter electrode, and the discharge capacity (unit: mAh / g) was measured. The discharge capacity was calculated by multiplying the discharge capacity by the load (454 mg / 25 cm 2 =18.16mg / cm 2 =0.01816g / cm 2 ), find the “discharge capacity per unit area of one positive electrode active material layer in the positive electrode” (unit: mAh / cm 2 ).

[0161] The N1 / P ratio was determined by the following mathematical formula a, and the N1 / P ratio of the secondary battery of Example 1 was 2.69.

[0162] [Mathematical formula a]

[0163] N1 / P ratio={(discharge capacity per unit area of one first negative electrode active material layer in the first negative electrode) / (discharge capacity per unit area of one positive electrode active material layer in the positive electrode)}

[0164] (2) N2 / P ratio

[0165] A negative electrode for measuring the N / P ratio was manufactured in the same manner as the method for manufacturing the second negative electrode, except that the negative electrode active material layer was formed only on one surface of the copper current collector. A coin-type half-cell was manufactured, comprising the negative electrode for measuring the N / P ratio, a lithium metal counter electrode opposite to the negative electrode for measuring the N / P ratio, a polyethylene separator interposed between the negative electrode for measuring the N / P ratio and the lithium metal counter electrode, and an electrolyte, and the discharge capacity (unit: mAh / g) was measured. The discharge capacity was calculated by multiplying the discharge capacity by the load (298 mg / 25 cm 2 =11.92mg / cm 2 =0.01192g / cm 2 ), find the “discharge capacity per unit area of a second negative electrode active material layer in the second negative electrode” (unit: mAh / cm 2 ).

[0166] A positive electrode for measuring the N / P ratio was manufactured in the same manner as the method for manufacturing the positive electrode, except that the positive electrode active material layer was formed only on one surface of the aluminum current collector. A coin-type half-cell was manufactured, comprising the positive electrode for measuring the N / P ratio, a lithium metal counter electrode opposite to the positive electrode for measuring the N / P ratio, a polyethylene separator and an electrolyte interposed between the positive electrode for measuring the N / P ratio and the lithium metal counter electrode, and the discharge capacity (unit: mAh / g) was measured. The discharge capacity was calculated by multiplying the discharge capacity by the load (454 mg / 25 cm 2 =18.16mg / cm 2 =0.01816g / cm 2 ), find the “discharge capacity per unit area of one positive electrode active material layer in the positive electrode” (unit: mAh / cm 2 ).

[0167] The N2 / P ratio was determined by the following mathematical formula b, and the N2 / P ratio of the secondary battery of Example 1 was 1.08.

[0168] [Formula b]

[0169] N2 / P ratio={(discharge capacity per unit area of one second negative electrode active material layer in the second negative electrode) / (discharge capacity per unit area of one positive electrode active material layer in the positive electrode)}

[0170] (3)N 总和 / P 总和 Compare

[0171] N 总和 / P 总和 The ratio is calculated by the following method.

[0172] The total discharge capacity of the first negative electrode was determined by multiplying the "discharge capacity per unit area of one first negative electrode active material layer in the first negative electrode" determined in (1) by the number of first negative electrode active material layers in the secondary battery of Example 1. The total discharge capacity of the second negative electrodes was determined by multiplying the "discharge capacity per unit area of one second negative electrode active material layer in the second negative electrode" determined in (2) by the number of second negative electrode active material layers in the secondary battery of Example 1. The total discharge capacity of the negative electrodes present in the secondary battery of Example 1 was determined by adding the total discharge capacity of the first negative electrodes to the total discharge capacity of the second negative electrodes.

[0173] By multiplying the “discharge capacity per unit area of one positive electrode active material layer in the positive electrode” determined in (1) by the number of positive electrode active material layers in the secondary battery of Example 1, the total discharge capacity of the positive electrodes present in the secondary battery of Example 1 was determined.

[0174] The N 总和 / P 总和 The ratio is determined by the following mathematical formula c, and N in Example 1 总和 / P 总和 The ratio is 2.53.

[0175] [Mathematical formula c]

[0176] N 总和 / P 总和 Ratio={(the total discharge capacity per unit area of the negative electrodes present in the secondary battery) / (the total discharge capacity per unit area of the positive electrodes present in the secondary battery)}

[0177] Example 2: Manufacturing of Secondary Battery

[0178] A secondary battery of Example 2 was manufactured in the same manner as in Example 1, except that 15 first negative electrodes A, 2 first negative electrodes B, 4 second negative electrodes, and 20 positive electrodes were prepared for manufacturing the secondary battery.

[0179] In Example 2, the weight ratio of the silicon-based active material in the first negative electrode to the carbon-based active material in the second negative electrode is 49.2:50.8, and the ratio of the total weight of the first negative electrode to the total weight of the second negative electrode is 80:20.

[0180] N of the secondary battery of Example 2 总和 / P 总和 The ratio is 2.37.

[0181] Example 3: Manufacturing of Secondary Battery

[0182] A secondary battery of Example 3 was manufactured in the same manner as in Example 1, except that 18 first negative electrodes A, 2 first negative electrodes B, 1 second negative electrode, and 20 positive electrodes were prepared for manufacturing the secondary battery.

[0183] In Example 3, the weight ratio of the silicon-based active material in the first negative electrode to the carbon-based active material in the second negative electrode was 82.1:17.9, and the ratio of the total weight of the first negative electrode to the total weight of the second negative electrode was 95:5.

[0184] N of the secondary battery of Example 3 总和 / P 总和 The ratio is 2.61.

[0185] Comparative Example 1: Production of Secondary Battery

[0186] A secondary battery of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the second negative electrode was not used and 19 first negative electrodes A, 2 first negative electrodes B, and 20 positive electrodes were alternately stacked.

[0187] In Comparative Example 1, the weight ratio of the silicon-based active material in the first negative electrode to the carbon-based active material in the second negative electrode was 100:0, and the ratio of the total weight of the first negative electrode to the total weight of the second negative electrode was 100:0.

[0188] The N of the secondary battery of Comparative Example 1 总和 / P 总和 The ratio is 2.69.

[0189] Comparative Example 2: Production of Secondary Battery

[0190] A secondary battery of Comparative Example 2 was manufactured in the same manner as in Example 1, except that 11 first negative electrodes A, 2 first negative electrodes B, 8 second negative electrodes, and 20 positive electrodes were prepared for manufacturing the secondary battery.

[0191] In Comparative Example 2, the weight ratio of the silicon-based active material in the first negative electrode to the carbon-based active material in the second negative electrode was 26.6:73.4, and the ratio of the total weight of the first negative electrode to the total weight of the second negative electrode was 60:40.

[0192] The N of the secondary battery of Comparative Example 2 总和 / P 总和 The ratio is 2.04.

[0193] Comparative Example 3: Production of Secondary Battery

[0194] A secondary battery of Comparative Example 3 was manufactured in the same manner as in Example 1, except that 7 first negative electrodes A, 2 first negative electrodes B, 12 second negative electrodes, and 20 positive electrodes were prepared for manufacturing the secondary battery.

[0195] In Comparative Example 3, the weight ratio of the silicon-based active material in the first negative electrode to the carbon-based active material in the second negative electrode was 13.9:86.1, and the ratio of the total weight of the first negative electrode to the total weight of the second negative electrode was 40:60.

[0196] The N of the secondary battery of Comparative Example 3 总和 / P 总和 The ratio is 1.72.

[0197] Comparative Example 4: Production of Secondary Battery

[0198] The secondary battery of Comparative Example 4 was manufactured in the same manner as Example 1, except that the following two negative electrodes were arranged at the outermost sides and 19 second negative electrodes and 20 positive electrodes were alternately stacked between the two negative electrodes. The two negative electrodes were manufactured in the same manner as the second negative electrode, except that the first negative electrode was not used and the second negative electrode active material layer was formed only on one surface instead of on both surfaces.

[0199] In Comparative Example 4, the weight ratio of the silicon-based active material in the first negative electrode to the carbon-based active material in the second negative electrode was 0:100, and the ratio of the total weight of the first negative electrode to the total weight of the second negative electrode was 0:100.

[0200] The N of the secondary battery of Comparative Example 1 总和 / P 总和 The ratio is 1.08.

[0201] Comparative Example 5: Production of Secondary Battery

[0202] <Manufacturing of negative electrode>

[0203] As the negative electrode active material, Si (average particle size (D 50 ):3μm) and a 68.5:31.5 (weight / weight) mixture of carbon-based active materials. In this case, artificial graphite (average particle size (D 50 ):15μm) and natural graphite (average particle size (D 50 A 90:10 (weight / weight) mixture of 1,2-dimethyl-1,4-dioxane (1,2-dioxane, ...

[0204] A negative electrode slurry (solid content concentration: 25 wt %) was prepared by adding the negative electrode active material, carbon black (product name: Super C65, manufacturer: TIMCAL Corporation) as a negative electrode conductive material, and a PVA / PAA copolymer (containing units derived from vinyl alcohol and units derived from acrylic acid in a ratio of 66:34; weight average molecular weight: approximately 360,000 g / mol) as a negative electrode binder in a weight ratio of 75:10:15 to distilled water used as a solvent for forming a negative electrode slurry.

[0205] In order to form a negative electrode active material layer (thickness: 29.5 μm), the negative electrode slurry was mixed at 100.5 mg / 25 cm 2 The coating was applied on both surfaces of a copper current collector (thickness: 15 μm) used as a negative electrode current collector, rolled, and dried in a vacuum oven at 130° C. for 10 hours to obtain a first negative electrode (negative electrode thickness: 44.5 μm, negative electrode porosity: 32.8%). A plurality (21) of first negative electrodes were manufactured.

[0206] <Manufacturing of Secondary Batteries>

[0207] A secondary battery of Comparative Example 5 was manufactured in the same manner as in Example 1, except that the above-manufactured negative electrode was used instead of the first and second negative electrodes and 21 above-manufactured negative electrodes and 20 positive electrodes were prepared for manufacturing a secondary battery.

[0208] The N of the secondary battery of Comparative Example 5 总和 / P 总和 The ratio is 2.68.

[0209] Comparative Example 6: Production of Secondary Battery

[0210] <Manufacturing of negative electrode>

[0211] As the negative electrode active material, Si (average particle size (D 50 ):3μm) and a 68.5:31.5 (weight / weight) mixture of carbon-based active materials. In this case, artificial graphite (average particle size (D 50 ):15μm) and natural graphite (average particle size (D 50 A 90:10 (weight / weight) mixture of 1,2-dimethyl-1,4-dioxane (1,2-dioxane, ...

[0212] A negative electrode slurry (solid content concentration: 48 wt %) was prepared by adding the negative electrode active material, carbon black (product name: Super C65, manufacturer: TIMCAL) as a negative electrode conductive material, SBR as a negative electrode binder, and CMC as a thickener to distilled water used as a solvent for forming a negative electrode slurry at a weight ratio of 95.65:0.75:1.1:2.5.

[0213] In order to form a negative electrode active material layer (thickness: 23 μm), the negative electrode slurry was mixed with 81.8 mg / 25 cm 2 The negative electrode was coated on both surfaces of a copper current collector (thickness: 15 μm) used as a negative electrode current collector, rolled, and then dried in a vacuum oven at 130° C. for 10 hours to obtain a first negative electrode (negative electrode thickness: 61 μm, negative electrode porosity: 35.4%). A plurality of (21) negative electrodes were manufactured.

[0214] <Manufacturing of Secondary Batteries>

[0215] A secondary battery of Comparative Example 6 was manufactured in the same manner as in Example 1, except that the above-manufactured negative electrode was used instead of the first and second negative electrodes and 21 above-manufactured negative electrodes and 20 positive electrodes were prepared for manufacturing a secondary battery.

[0216] The N of the secondary battery of Comparative Example 6总和 / P 总和 The ratio is 2.78.

[0217] [Table 1]

[0218]

[0219] Experimental Example 1: Evaluation of initial efficiency and capacity retention

[0220] The initial efficiency and capacity retention rate of each of the secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 6 were evaluated using an electrochemical charger / discharger.

[0221] The secondary battery was: 1) charged (0.2C, constant current / constant voltage (CC / CV) charge, 4.2V, 0.05C cutoff) and discharged (0.2C, constant current (CC) discharge, 3.2V cutoff) for the first cycle, and then 2) charged (1.0C, CC / CV charge, 4.2V, 0.05C cutoff) and discharged (0.5C, CC discharge, 3.0V cutoff) from the second cycle to the 100th cycle.

[0222] The capacity retention rate was determined by the following formula. The results are shown in Table 2 below.

[0223] Capacity retention (%) = {(discharge capacity at the 100th cycle) / (discharge capacity at the first cycle)} × 100

[0224] [Table 2]

[0225] Capacity retention at the 100th cycle (%) Example 1 94.4 Example 2 91.6 Example 3 92.8 Comparative Example 1 89.1 Comparative Example 2 79.3 Comparative Example 3 85.7 Comparative Example 4 90.9 Comparative Example 5 88.6 Comparative Example 6 35.7

[0226] Referring to Table 2, it can be seen that the secondary batteries of Examples 1 to 3 exhibit excellent capacity, initial efficiency and life characteristics, wherein the secondary batteries contain a first negative electrode comprising a silicon-based active material and a second negative electrode comprising a carbon-based active material and wherein the silicon-based active material contained in the first negative electrode and the carbon-based active material contained in the second negative electrode have a specific weight ratio.

[0227] However, in Comparative Example 1, because the negative electrode contained only a silicon-based active material as the active material, the high potential of the silicon-based active material also resulted in a high potential in the positive electrode active material within the same operating voltage range as the examples. Therefore, it was determined that in Comparative Example 1, because the positive electrode resistance increased as the positive electrode potential increased, Comparative Example 1 exhibited lower lifespan characteristics than the examples.

[0228] In addition, in the case of Comparative Examples 2 and 3, as the ratio of the carbon-based active material used in the negative electrode increases, the overall N / P ratio (N 总和 / P总和 In this case, since the ratio of the negative electrode active material participating in charge and discharge in the negative electrode increases, the volume expansion / contraction of the silicon-based active material increases the influence on the secondary battery, thereby deteriorating the life characteristics.

[0229] In the case of Comparative Example 4, because the negative electrode contains only a carbon-based active material as the active material, the lifespan characteristics are not significantly reduced compared to the examples, but the energy density and rapid charging performance are significantly reduced. Therefore, Comparative Example 4 is not suitable for use as a secondary battery requiring rapid charging characteristics and high energy density.

[0230] In the case of Comparative Example 5, because the binder content of the negative electrode composition is increased, the volume expansion of the silicon-based active material can be controlled to a certain extent. However, due to the increase in the binder content of the negative electrode composition, high resistance is generated in the carbon-based active material, resulting in a decrease in life characteristics. On the other hand, in the case of Comparative Example 6, because the binder content of the negative electrode composition is low, it is difficult to control the volume expansion / contraction of the silicon-based active material, thereby reducing the life characteristics. Therefore, in the secondary batteries of Comparative Examples 5 and 6, because a single negative electrode contains both silicon-based active material and carbon-based active material, it is impossible to adjust the composition of the negative electrode according to the characteristics of each active material. As a result, the secondary batteries of Comparative Examples 5 and 6 have lower life characteristics than the secondary batteries of the examples.

[0231] Experimental Example 2: Fast Charging Life Evaluation

[0232] The capacity retention rate of each secondary battery manufactured in Examples 1 to 3 and Comparative Examples 1 to 6 during rapid charging was evaluated using an electrochemical charger / discharger. The secondary batteries were: 1) charged (0.2C, CC / CV charge, 4.2V, 0.05C cutoff) and discharged (0.2C, CC discharge, 3.2V cutoff) for the first cycle, and then 2) charged (2.0C, CC / CV charge, 4.2V, 0.05C cutoff) and discharged (0.5C, CC discharge, 3.2V cutoff) from the second cycle to the 100th cycle.

[0233] The capacity retention rate was determined by the following formula. The results are shown in Table 3 below.

[0234] Capacity retention (%) = {(discharge capacity at the 100th cycle) / (discharge capacity at the first cycle)} × 100

[0235] [Table 3]

[0236] Capacity retention at the 100th cycle (%) Example 1 89.0 Example 2 85.2 Example 3 87.9 Comparative Example 1 83.5 Comparative Example 2 61.1 Comparative Example 3 58.4 Comparative Example 4 53.2 Comparative Example 5 73.1 Comparative Example 6 22.2

[0237] Referring to Table 3, it can be seen that the secondary batteries of Examples 1 to 3 exhibit excellent life characteristics during rapid charging, wherein the secondary batteries contain a first negative electrode comprising a silicon-based active material and a second negative electrode comprising a carbon-based active material and wherein the silicon-based active material contained in the first negative electrode and the carbon-based active material contained in the second negative electrode have a specific weight ratio.

[0238] However, in Comparative Example 1, the negative electrode contains only a silicon-based active material as the active material. Therefore, within the same operating voltage range as the examples, the high potential of the silicon-based active material also creates a high potential in the positive electrode active material. Therefore, it can be determined that in Comparative Example 1, because the positive electrode resistance increases with increasing positive electrode potential, Comparative Example 1 has lower rapid charge life characteristics than the examples.

[0239] Furthermore, carbon-based active materials have a very low potential during rapid charging and, for example, may form a negative (-) potential. Therefore, in Comparative Examples 2, 3, and 4, in which carbon-based active materials were used at high rates, lifespan characteristics significantly deteriorated due to the possibility of Li plating, etc., during rapid charging.

[0240] In the case of Comparative Example 5, because the binder content of the negative electrode composition is increased, the volume expansion of the silicon-based active material can be controlled to a certain extent. However, due to the increase in the binder content of the negative electrode composition, high resistance is generated in the carbon-based active material, and thus the fast charge life characteristics are reduced. On the other hand, in the case of Comparative Example 6, because the binder content of the negative electrode composition is low, it is difficult to control the volume expansion / contraction of the silicon-based active material, thereby reducing the fast charge life characteristics. Therefore, in the secondary batteries of Comparative Examples 5 and 6, because one negative electrode contains both a silicon-based active material and a carbon-based active material, it is impossible to adjust the composition of the negative electrode according to the characteristics of each active material, so that the secondary batteries of Comparative Examples 5 and 6 have lower fast charge life characteristics than the secondary batteries of the embodiments.

[0241] Experimental Example 3: Evaluation of Energy Density

[0242] The secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 6 were charged and discharged for one cycle under the following conditions, and the discharge capacities (unit: Wh) of the secondary batteries were measured.

[0243] In this case, after charging was completed, the thickness of each secondary battery was measured, and the volume (unit: liter (L)) of the secondary battery was determined by multiplying the obtained thickness of the secondary battery by the area of the secondary battery.

[0244] The energy density (unit: Wh / L) was determined by dividing the discharge capacity of each secondary battery measured above by the volume of the secondary battery. The results are shown in Table 4 below.

[0245] <Charge and Discharge Conditions>

[0246] Charging: Charge to 4.2V at 0.2C in CC / CV mode, 5% cutoff

[0247] Discharge: Discharge to 3.2V at 0.2C in CC mode

[0248] [Table 4]

[0249] Energy density (Wh / L) Example 1 537 Example 2 521 Example 3 543 Comparative Example 1 554 Comparative Example 2 506 Comparative Example 3 480 Comparative Example 4 428 Comparative Example 5 490 Comparative Example 6 449

[0250] Referring to Table 4, it can be seen that because the secondary batteries of Examples 1 to 3 included the silicon-based active material, the secondary batteries had improved capacity and thus had excellent energy density.

[0251] In the case of Comparative Example 1, because the negative electrode contains only a silicon-based active material as an active material, the capacity is improved and the energy density is high. However, when only a silicon-based active material is used as the negative electrode active material as described above, the high potential of the silicon-based active material forms a high potential in the positive electrode. As a result, it can be seen that the lifespan characteristics of Comparative Example 1 are inferior to those of the examples due to increased resistance.

[0252] In addition, in the case of the secondary batteries of Comparative Examples 2 to 4, since the ratio of the negative electrode containing the carbon-based active material having low energy density was too high, the energy density of the secondary batteries was low.

[0253] In the case of the secondary battery of Comparative Example 5, although the energy density was improved to some extent due to the inclusion of a silicon-based active material in the negative electrode, the high binder content in the negative electrode increased the resistance of the carbon-based active material. In this case, the increased resistance of the carbon-based active material reduced the capacity, thereby reducing the energy density.

[0254] In addition, in the case of Comparative Example 6, it was difficult to control the volume expansion of the silicon-based active material due to the low binder content of the negative electrode composition. In this case, it was difficult to control the thickness change of the negative electrode, so the energy density was reduced.

[0255] [Explanation of symbols]

[0256] 1: Secondary battery

[0257] 100: Positive

[0258] 110: Positive electrode collector

[0259] 120: Positive electrode active material layer

[0260] 200: First negative electrode

[0261] 210: First negative electrode current collector

[0262] 220: First negative electrode active material layer

[0263] 300: Second negative electrode

[0264] 310: Second negative electrode current collector

[0265] 320: Second negative electrode active material layer

[0266] 400: Diaphragm

Claims

1. A secondary battery, comprising: one or more positive electrodes, the positive electrodes comprising a positive electrode active material layer; a plurality of negative electrodes, the negative electrodes including a first negative electrode comprising a silicon-based active material and a second negative electrode comprising a carbon-based active material; diaphragm; and electrolytes, wherein the positive electrode and the negative electrode are alternately stacked with the separator provided therebetween, and the weight ratio of the silicon-based active material contained in the first negative electrode to the carbon-based active material contained in the second negative electrode is in the range of 40:60 to 90:10, wherein When there are multiple first negative electrodes, the weight of the silicon-based active material contained in the first negative electrode refers to the total weight of the silicon-based active materials contained in the multiple first negative electrodes. When there are multiple second negative electrodes, the weight of the carbon-based active material contained in the second negative electrode refers to the total weight of the carbon-based active materials contained in the multiple second negative electrodes. wherein the silicon-based active material comprises Si, and The carbon-based active material comprises at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon. 2 . The secondary battery of claim 1 , wherein the first negative electrode comprises a first negative electrode current collector and a first negative electrode active material layer formed on one or more surfaces of the first negative electrode current collector, and the first negative electrode active material layer comprises the silicon-based active material.

3. The secondary battery of claim 2 , wherein the first negative electrode active material layer further comprises a first negative electrode binder and a first negative electrode conductive material, and the first negative electrode active material layer comprises 60 wt % to 90 wt % of the silicon-based active material, 5 wt % to 30 wt % of the first negative electrode binder, and 5 wt % to 20 wt % of the first negative electrode conductive material.

4. The secondary battery of claim 3, wherein the first negative electrode binder comprises at least one selected from the group consisting of styrene-butadiene rubber, nitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyacrylic acid, polyethylene glycol, polyacrylonitrile, and polyacrylamide.

5. The secondary battery according to claim 2, wherein the N1 / P ratio calculated by the following mathematical formula a is in the range of 2.2 to 3.0: [Mathematical formula a] N1 / P ratio={(discharge capacity per unit area of one first negative electrode active material layer in the first negative electrode) / (discharge capacity per unit area of one positive electrode active material layer in the positive electrode)}.

6. The secondary battery according to claim 1, wherein the average particle size D of the silicon-based active material is 50 In the range of 1μm to 10μm. 7 . The secondary battery of claim 1 , wherein the second negative electrode comprises a second negative electrode current collector and a second negative electrode active material layer formed on one or more surfaces of the second negative electrode current collector, and the second negative electrode active material layer comprises the carbon-based active material.

8. The secondary battery of claim 7, wherein the second negative electrode active material layer further comprises a second negative electrode binder and a second negative electrode conductive material, and the second negative electrode active material layer comprises 90 wt% to 99 wt% of the carbon-based active material, 0.5 wt% to 10 wt% of the second negative electrode binder, and 0.5 wt% to 5 wt% of the second negative electrode conductive material.

9. The secondary battery of claim 8, wherein the second negative electrode binder comprises at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, poly(methyl methacrylate), polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer rubber, styrene-butadiene rubber, and fluororubber.

10. The secondary battery according to claim 1, wherein the average particle size D of the carbon-based active material is 50 In the range of 10μm to 30μm.

11. The secondary battery according to claim 7, wherein the N2 / P ratio calculated by the following mathematical formula b is in the range of 1.0 to 1.3: [Mathematical formula b] N2 / P ratio={(discharge capacity per unit area of one second negative electrode active material layer in the second negative electrode) / (discharge capacity per unit area of one positive electrode active material layer in the positive electrode)}.

12. The secondary battery according to claim 1, wherein N is calculated by the following mathematical formula c: all / P all Ratios in the range of 2 to 3: [Mathematical formula c] N all / P all Ratio={(the total discharge capacity per unit area of the negative electrodes present in the secondary battery) / (the total discharge capacity per unit area of the positive electrodes present in the secondary battery)}.

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