Lithium-ion batteries with improved safety

Abstract

The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the same. The negative electrode for a lithium secondary battery has the following advantages: i) it contains a silicon-based material as a negative electrode active material and has high energy density; and ii) it has an individual composite layer made of a silicon-based material having relatively low electrical conductivity at the outermost shell of a multi-layered negative electrode composite layer, thereby reducing the amount of current and the amount of heat generated when an internal short circuit occurs, thereby improving safety of the secondary battery against internal short circuit.

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery with improved safety against internal short circuits.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0090334 filed on July 21, 2022, and all the contents disclosed in the literature of the Korean patent application are included as part of this specification.

Background Art

[0003] In recent years, lithium secondary batteries have been widely applied not only to small devices such as portable electronic devices but also to medium and large-sized devices such as battery packs or power storage devices for hybrid vehicles and electric vehicles.

[0004] In order to apply such secondary batteries to medium and large-sized devices, a high energy density is required. Therefore, a ternary compound containing nickel (Ni), cobalt (Co), manganese (Mn), etc., specifically, LiNi with a nickel (Ni) content of 60% or more a Co b Mn c O2 (0.6 < a ≦ 0.9, a + b + c = 1), a layered lithium nickel metal oxide is used as a positive electrode active material to achieve high capacity. However, lithium nickel metal oxide shows low chemical and structural stability although the capacity increases as the nickel (Ni) content increases, so a heat generation reaction is likely to occur. Especially in the case of lithium nickel metal oxide, the heat generation onset point is low, and once the heat generation reaction starts, it can rapidly increase the temperature inside the battery and induce ignition or explosion, so there is a problem of low safety.

[0005] The exothermic reaction of the positive electrode active material can be induced when a short-circuit current flows inside the battery, that is, when an internal short circuit occurs. More specifically, the short-circuit current can mainly occur when a short circuit occurs inside the secondary battery due to the penetration of a needle-shaped object or the like, or when a short circuit occurs in an electronic device connected to the secondary battery. Further, when a short-circuit phenomenon occurs in a lithium secondary battery, rapid electrochemical reactions occur at the positive and negative electrodes, generating heat. The heat thus generated is conducted to surrounding substances, and due to such heat conduction, the temperature of the secondary battery cell rises at a high speed, and as a result, ignition of the cell occurs. In particular, in the case of a battery pack including a large number of lithium secondary battery cells, the heat generated in any one cell propagates to the surrounding cells and affects other cells, resulting in ignition of the battery pack.

[0006] Therefore, there is a demand for the development of a battery that exhibits a high energy density while improving safety problems due to internal short circuits.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] Therefore, an object of the present invention is to provide a negative electrode having a high energy density and improved safety problems due to internal short circuits, a lithium secondary battery including the same, and a secondary battery module including the same.

Means for Solving the Problems

[0009] In order to solve the above-described problems, In one embodiment of the present invention, A first negative electrode composite layer or an nth negative electrode composite layer (where n≧2) is placed on the negative electrode current collector. The above-mentioned first negative electrode composite layer or the n-1 negative electrode composite layer comprises a first negative electrode active material containing a carbon-based material and a second negative electrode active material containing a silicon-based material. The above-mentioned nth negative electrode composite layer provides a negative electrode for a lithium secondary battery containing a second negative electrode active material containing a silicon-based substance.

[0010] In this case, the content or proportion of the second negative electrode active material may increase in the first negative electrode composite layer or the (n-1)th negative electrode composite layer as the position of the individual negative electrode composite layers changes from the first negative electrode composite layer to the (n-1)th negative electrode composite layer.

[0011] Furthermore, the carbon-based material of the first negative electrode active material may include one or more selected from the group consisting of soft carbon, hard carbon, natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, carbon black, acetylene black, Ketjen black, carbon nanotubes, fullerenes, activated carbon, graphene, and carbon fibers.

[0012] Furthermore, the silicon-based material of the second negative electrode active material mentioned above is silicon (Si), silicon carbide (SiC), and silicon oxide (SiO₂). q However, this may include one or more of the following (0.8 ≤ q ≤ 2.5).

[0013] Furthermore, the above-mentioned second negative electrode active material may be included in an amount of 1 to 20% by weight relative to the total weight of the negative electrode active material.

[0014] Furthermore, the second negative electrode active material has a degree of spheroidization of 0.5 to 1.0, and the degree of spheroidization may decrease as the position of the individual negative electrode material layers changes from the first negative electrode material layer to the nth negative electrode material layer.

[0015] Furthermore, the total thickness of the negative electrode composite layer may be 50 μm to 300 μm, where the thickness of the nth negative electrode composite layer may be 5% to 30% of the total thickness of the positive electrode composite layer.

[0016] Furthermore, in one embodiment of the present invention, Provided is a lithium secondary battery including the negative electrode, the positive electrode, and the separator positioned between the negative electrode and the positive electrode according to the present invention described above.

[0017] At this time, in the positive electrode, a first positive electrode composite layer or an mth positive electrode composite layer (where m ≥ 2) is disposed on a positive electrode current collector, and the first positive electrode composite layer or the mth positive electrode composite layer may include a first positive electrode active material containing a lithium composite metal oxide represented by Chemical Formula 1 and a second positive electrode active material containing an iron phosphate compound represented by the following Chemical Formula 2.

[0018] [Chemical Formula 1] Li x [Ni y Co z Mn w M 1 v O2

[0019] [Chemical Formula 2] LiFe a M 2 1-a XO4

[0020] In Chemical Formulas 1 and 2 above, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, x, y, z, w, and v are respectively 1.0 ≤ x ≤ 1. 30, 0.1 ≤ y < 1, 0 ≤ z ≤ 1, 0 ≤ w ≤ 1, 0 ≤ v ≤ 0.1, and y + z + w + v = 1, M 2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, X is one or more selected from the group consisting of P, Si, S, As, and Sb, a is 0 ≤ a ≤ 0.5.

[0021] Furthermore, the content or proportion of the second positive electrode active material within each positive electrode active material layer may increase as the position of the individual positive electrode active material layers changes from the first positive electrode active material layer to the mth positive electrode active material layer.

[0022] Furthermore, the above-mentioned second positive electrode active material may be included in an amount of less than 10% by weight relative to the total weight of the positive electrode composite layer.

[0023] Furthermore, the total thickness of the positive electrode composite layer can be between 50 μm and 300 μm.

[0024] Furthermore, in one embodiment of the present invention, The present invention provides a secondary battery module including the lithium secondary battery described above. [Effects of the Invention]

[0025] The negative electrode for lithium secondary batteries according to the present invention has the advantage of improving safety in the event of an internal short circuit in secondary batteries, as it contains a silicon-based material as the negative electrode active material, has a high energy density, and the outermost shell of the multilayer negative electrode composite layer is equipped with individual composite material layers made of silicon-based materials with relatively low electrical conductivity, thereby reducing the amount of current and heat generated when an internal short circuit occurs. [Brief explanation of the drawing]

[0026] [Figure 1] This is a cross-sectional view showing the structure of the negative electrode for a lithium secondary battery according to the present invention. [Figure 2] This is a cross-sectional view showing the structure of a lithium secondary battery equipped with a negative electrode according to the present invention. [Modes for carrying out the invention]

[0027] Since the present invention can be modified in various ways and may have a variety of embodiments, specific embodiments will be described in detail in the detailed description.

[0028] However, this should not be understood as limiting the present invention to any particular embodiment, but rather as including all modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0029] In the present invention, terms such as "includes" and "have" are intended to specify the presence of features, numbers, stages, operations, components, parts, or combinations thereof as described in the specification, and should be understood as not preemptively excluding the presence or possibility of adding one or more other features, numbers, stages, operations, components, parts, or combinations thereof.

[0030] Furthermore, in this invention, when a part such as a layer, film, region, or plate is described as being "on top" of another part, this includes not only the case where it is "directly on top" of the other part, but also the case where another part is located in between. Conversely, when a part such as a layer, film, region, or plate is described as being "below" another part, this includes not only the case where it is "directly below" the other part, but also the case where another part is located in between. Also, in this application, being "on top" may include being located not only at the top but also at the bottom.

[0031] The present invention will be described in more detail below.

[0032] <Negative electrode for lithium secondary batteries> In one embodiment, the present invention is described as follows: A first negative electrode composite layer or an nth negative electrode composite layer (where n≧2) is placed on the negative electrode current collector. The above-mentioned first negative electrode composite layer or the n-1 negative electrode composite layer comprises a first negative electrode active material containing a carbon-based material and a second negative electrode active material containing a silicon-based material. The above-mentioned nth negative electrode composite layer provides a negative electrode for a lithium secondary battery containing a second negative electrode active material containing a silicon-based substance.

[0033] The negative electrode for a lithium secondary battery according to the present invention comprises a negative electrode composite layer manufactured by applying, drying, and pressing a slurry containing a negative electrode active material onto a negative electrode current collector, wherein the slurry may optionally further selectively contain conductive materials, binders, and other additives.

[0034] Here, the negative electrode includes a negative electrode current collector and a multilayer negative electrode composite layer in which two or more individual composite layers are laminated on the negative electrode current collector.

[0035] Specifically, as shown in Figure 1, the negative electrode composite layer has a structure in which n (where n≧2) individual negative electrode composite layers 121 are stacked on the negative electrode current collector 11. In this case, the negative electrode composite layer stacked on the surface in contact with the negative electrode current collector 11 is the first negative electrode composite layer 121a, and the second negative electrode composite layer or the nth negative electrode composite layer 121n is sequentially stacked on the first negative electrode composite layer 121a, so that n individual negative electrode composite layers 121 are located on the negative electrode current collector 11.

[0036] The negative electrode composite layer described above is not particularly limited in number of layers as long as it has a structure of two or more layers (where n≧2), but specifically it can be 2 to 10 layers, 2 to 8 layers, 2 to 6 layers, or 2 to 4 layers. By adjusting the number of layers of the negative electrode composite layer within the above range, the present invention makes it possible to easily adjust the internal composition of the negative electrode composite layer according to its position, for example, the composition of the negative electrode composite layer relatively adjacent to the positive electrode, while preventing a decrease in the manufacturing efficiency of the negative electrode.

[0037] Furthermore, the above-mentioned negative electrode composite layer may use negative electrode active materials that are commonly used in the industry, but preferably it may contain both a first negative electrode active material containing a carbon-based material and a second negative electrode active material containing a silicon-based material.

[0038] Specifically, the first negative electrode active material described above may be composed of a carbon-based material mainly composed of carbon (C). Such carbon-based materials may include one or more of the following: graphite with a completely layered crystalline structure like natural graphite; soft carbon having a low-crystalline layered crystalline structure (graphene structure; a structure in which hexagonal honeycomb-patterned planes of carbon are arranged in layers); hard carbon in which these structures are mixed with amorphous parts; artificial graphite; expanded graphite; non-graphitizable carbon; carbon black; acetylene black; Ketjenblack; carbon nanotubes; fullerenes; activated carbon; graphene; and carbon fibers.

[0039] Furthermore, the above-mentioned second negative electrode active material may be composed of a silicon-based material mainly composed of silicon (Si). The above-mentioned silicon-based material may contain one or more of silicon (Si), silicon carbide (SiC), silicon monoxide (SiO), and silicon dioxide (SiO2). Here, when the above-mentioned silicon-based material is contained in the negative electrode composite layer in which silicon monoxide (SiO) and silicon dioxide (SiO2) are uniformly mixed or compounded, these are silicon oxide (SiO2). q However, this can be expressed as 0.8 ≤ q ≤ 2.5.

[0040] Furthermore, the overall negative electrode active material, which includes both the first negative electrode active material and the second negative electrode active material, may be present in an amount of 90-99% by weight relative to the total weight of the negative electrode composite layer, specifically in an amount of 92-98% by weight or 95-99% by weight.

[0041] Furthermore, the second negative electrode active material may be present in an amount of 1 to 20% by weight relative to the total weight of the negative electrode active material, specifically in amounts of 1 to 9% by weight, 3 to 7% by weight, 5 to 15% by weight, 11 to 19% by weight, or 13 to 17% by weight relative to the total weight of the negative electrode active material. By adjusting the content of the second negative electrode active material within the above ranges, the present invention can minimize the rate of volume change of the battery due to charging and discharging, and at the same time improve the charging capacity per unit mass while reducing lithium consumption and irreversible capacity loss during the initial charging and discharging of the battery.

[0042] Furthermore, the first negative electrode active material and the second negative electrode active material are both contained in n-1 negative electrode composite layers, and as the position of the individual negative electrode composite layers changes from the first negative electrode composite layer in contact with the negative electrode current collector to the (n-1)th negative electrode composite layer furthest from the negative electrode current collector, the amount of the second negative electrode active material contained in the individual negative electrode composite layer, or the ratio of the amount of the second negative electrode active material to the weight of the individual negative electrode composite layer, may increase.

[0043] As one example, the second anode active material may be included in the first anode composite layer in an amount of 1 to 45% by weight relative to the total weight of the second anode active material, and in an amount of 55 to 99% by weight relative to the total weight of the second anode composite layer.

[0044] As another example, the second anode active material may be included in the first anode composite layer at a weight of 1 to 10% of the total weight of the second anode active material, in the second anode composite layer at a weight of 10 to 40% of the total weight, and in the third anode composite layer at a weight of 40 to 89% of the total weight.

[0045] As another example, the second anode active material may be included in the first anode composite layer at a weight of 1-5% of the total weight of the second anode active material, in the second anode composite layer at a weight of 5-15% of the total weight, in the third anode composite layer at a weight of 15-30% of the total weight, and in the fourth anode composite layer at a weight of 30-79% of the total weight.

[0046] The first negative electrode active material described above contains a carbon-based material and therefore exhibits excellent electrical properties such as electrical conductivity. However, when a short-circuit current flows inside the secondary battery, for example, when a short circuit occurs inside the secondary battery due to the penetration of a needle-shaped object, the carbon-based material with high electrical conductivity increases the amount of short-circuit current between the positive and negative electrodes. As a result, the secondary battery generates significant short-circuit heat internally, which can accelerate the heat-generating reaction of the secondary battery. However, the present invention provides an individual composite layer composed of a second negative electrode active material containing a silicon-based material as the outermost layer of the negative electrode composite layer, i.e., the nth negative electrode composite layer, and simultaneously ii) increasing the concentration of the second negative electrode active material containing a silicon-based material, which has relatively lower electrical conductivity than the carbon-based material, from the innermost to the outermost layer of the negative electrode composite layer, i.e., from the first negative electrode composite layer to the (n-1)th negative electrode composite layer, thereby reducing the amount of short-circuit current when the secondary battery experiences an internal short circuit. This has the advantage that the secondary battery can reduce and / or delay heat generation.

[0047] Furthermore, the second anode active material may show a tendency for the degree of sphericity of the active material to decrease as it progresses from the first anode composite layer to the nth anode composite layer. Here, "degree of sphericity" can mean the ratio of the shortest diameter (minor axis) to the longest diameter (major axis) among any diameters passing through the center of the particle, and a degree of sphericity of 1 means that the particle's shape is spherical. The above degree of sphericity can be measured using a particle shape analyzer.

[0048] Specifically, the second negative electrode active material may have a degree of spheroidization of 0.5 to 1.0, and this degree of spheroidization may decrease as it progresses from the first negative electrode composite layer to the nth negative electrode composite layer, thus having a constant gradient of spheroidization.

[0049] As one example, the second negative electrode active material contained in the first negative electrode composite layer may have a degree of spheroidization of 0.8 to 1.0, and the second negative electrode active material contained in the second negative electrode composite layer may have a degree of spheroidization of 0.5 to 0.7.

[0050] As another example, the second negative electrode active material contained in the first negative electrode composite layer may have a degree of spheroidization of 0.9 to 1.0, the second negative electrode active material contained in the second negative electrode composite layer may have a degree of spheroidization of 0.7 to 0.8, and the second negative electrode active material contained in the third negative electrode composite layer may have a degree of spheroidization of 0.5 to 0.6.

[0051] The present invention makes it possible to reduce the electrical conductivity on the surface of the negative electrode composite layer in contact with the separation membrane without reducing the energy density of the negative electrode composite layer, by controlling the degree of sphericization of the second negative electrode active material to have a constant gradient according to the position of the negative electrode composite layer containing the second negative electrode active material.

[0052] Furthermore, the total thickness of the negative electrode composite layer is not particularly limited, but it may be 50 μm to 300 μm, and more specifically, it may be 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0053] Furthermore, among the individual anode composite layers constituting the anode composite layer, the nth anode composite layer located in the outermost shell and in contact with the separation membrane can have its thickness adjusted to a certain range. Specifically, the thickness of the nth anode composite layer may be 5% to 30% of the total thickness of the anode composite layer, and more specifically, it may be 5% to 20%, 5% to 15%, 5% to 10%, or 10% to 15% of the total thickness of the anode composite layer.

[0054] The present invention allows for a reduction in electrical conductivity at the negative electrode surface during normal operation of a secondary battery by adjusting the total thickness and individual thicknesses of the negative electrode composite layer within the above range. As a result, the negative electrode of the present invention can not only prevent a reduction in the energy density of the electrode, but also embody a resistance sufficient to reduce the flow of current at the negative electrode surface during an internal short circuit of the secondary battery.

[0055] On the other hand, while the negative electrode composite layer provides adhesion to the negative electrode current collector, it may also contain a binder to allow the negative electrode active material, conductive material, and other additives to bond to each other. Examples of such binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used. The binder may be present in an amount of 1 to 10% by weight based on the weight of the negative electrode composite layer, specifically in an amount of 2 to 8% by weight or 1 to 5% by weight.

[0056] Furthermore, the negative electrode may include a negative electrode current collector that has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, calcined carbon, etc. can be used as the negative electrode current collector, and in the case of copper or stainless steel, those surface-treated with carbon, nickel, titanium, silver, etc. can also be used. In addition, similar to the positive electrode current collector, the negative electrode current collector can have fine irregularities formed on its surface to strengthen the bonding force with the negative electrode active material, and various forms such as film, sheet, foil, net, porous body, foam, and nonwoven fabric are possible. Furthermore, the average thickness of the negative electrode current collector can be suitably applied in the range of 3 to 500 μm, taking into consideration the conductivity and total thickness of the manufactured negative electrode.

[0057] <Lithium-ion secondary battery> Furthermore, in one embodiment of the present invention, The present invention provides a lithium secondary battery comprising the negative electrode, positive electrode, and a separator membrane located between the negative electrode and the positive electrode as described above.

[0058] As shown in Figure 2, the lithium secondary battery according to the present invention includes an electrode assembly comprising a positive electrode 20, a negative electrode 10 according to the present invention, and a separation membrane 30 disposed between the positive electrode and the negative electrode, and has a structure in which an electrolyte composition is injected and sealed after the electrode assembly is inserted into a battery case.

[0059] In this case, since the negative electrode contains a silicon-based material as the negative electrode active material, it has a high energy density. By providing an individual composite material layer made of a silicon-based material with relatively low electrical conductivity at the outermost shell of the multilayer negative electrode composite material layer, the amount of current when an internal short circuit occurs can be reduced, thereby reducing the amount of heat generated. This has the advantage of improving the safety of secondary batteries in the event of an internal short circuit.

[0060] If the negative electrode described above has the same configuration as the negative electrode of the present invention described above, a detailed explanation will be omitted below.

[0061] Furthermore, the positive electrode comprises a positive electrode composite layer manufactured by applying, drying, and pressing a slurry containing positive electrode active material onto a positive electrode current collector, and the slurry may optionally further contain conductive materials, binders, and other additives.

[0062] Here, the positive electrode includes a multilayer positive electrode composite layer in which two or more individual composite layers are stacked on a positive electrode current collector. Specifically, referring to Figure 2, the positive electrode 20 has m (where m≧2) positive electrode composite layers 22 arranged on a positive electrode current collector 21, and the first positive electrode composite layer 221a or the mth positive electrode composite layer 221m may include a first positive electrode active material containing a lithium composite metal oxide represented by chemical formula 1, and a second positive electrode active material containing an iron phosphate compound represented by the following chemical formula 2.

[0063] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0064] [Chemical formula 2] LiFea M 2 1-a XO4

[0065] In the above chemical formulas 1 and 2, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. x, y, z, w, and v are such that 1.0 ≤ x ≤ 1.30, 0.1 ≤ y < 1, 0 ≤ z ≤ 1, 0 ≤ w ≤ 1, and 0 ≤ v ≤ 0.1, respectively, and y + z + w + v = 1. M 2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. X is one or more elements selected from the group consisting of P, Si, S, As, and Sb. a is such that 0 ≤ a ≤ 0.5.

[0066] Specifically, the positive electrode composite layer has a structure in which m (where m≧2) individual positive electrode composite layers are stacked on the positive electrode current collector. In this case, the positive electrode composite layer stacked on the surface in contact with the positive electrode current collector is the first positive electrode composite layer, and the second positive electrode composite layer or the mth positive electrode composite layer is sequentially stacked on the first positive electrode composite layer, so that m individual positive electrode composite layers are located on the positive electrode current collector.

[0067] The positive electrode composite layer described above is not particularly limited in number of layers as long as it has a structure of two or more layers (where m≧2), but specifically it can be 2 to 10 layers, 2 to 8 layers, 2 to 6 layers, or 2 to 4 layers. By adjusting the number of layers of the positive electrode composite layer within the above range, the present invention can improve the energy density of the electrode while preventing a decrease in the manufacturing efficiency of the positive electrode, and at the same time effectively dissipate the heat generated during the charging and discharging of the battery to the outside.

[0068] Furthermore, the positive electrode composite layer is manufactured by applying, drying, and pressurizing a slurry containing a positive electrode active material that allows for reversible intercalation and deintercalation of lithium ions during battery charging and discharging, but each layer may contain different types of positive electrode active material.

[0069] Specifically, the positive electrode according to the present invention includes a first positive electrode active material containing a lithium composite metal oxide represented by the following chemical formula 1 in the positive electrode composite material layer, and further includes a second positive electrode active material containing an iron phosphate compound represented by the chemical formula 2 in a positive electrode composite material layer separated from the positive electrode current collector, i.e., a second positive electrode composite material layer or m-th positive electrode composite material layer disposed on the first positive electrode composite material layer.

[0070] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0071] [Chemical formula 2] LiFe a M 2 1-a XO4

[0072] In the above chemical formulas 1 and 2, M 1 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. x, y, z, w, and v are such that 1.0 ≤ x ≤ 1.30, 0.1 ≤ y < 1, 0 ≤ z ≤ 1, 0 ≤ w ≤ 1, and 0 ≤ v ≤ 0.1, respectively, and y + z + w + v = 1. M 2 is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. X is one or more elements selected from the group consisting of P, Si, S, As, and Sb. a is such that 0 ≤ a ≤ 0.5.

[0073] Lithium composite metal oxides, represented by chemical formula 1, are ternary lithium oxides mainly composed of nickel (Ni), cobalt (Co), and manganese (Mn). They have advantages such as high energy density and performance characteristics that make them suitable for medium to large secondary batteries used in transportation applications such as electric vehicles (EVs) and energy storage systems (ESSs). However, while the capacity of these lithium composite metal oxides increases with increasing nickel (Ni) content, they exhibit low chemical and structural stability, making them prone to exothermic reactions and thus increasing the likelihood of ignition.

[0074] The above-mentioned exothermic reaction can be induced when a short-circuit current flows inside the battery, that is, when an internal short circuit occurs. Generally, a short-circuit current in a battery can occur either when a needle-shaped object penetrates the battery, causing a short circuit inside the secondary battery, or when a short circuit occurs in electronic equipment connected to the secondary battery.

[0075] Therefore, the present invention includes a lithium composite metal oxide represented by chemical formula 1 throughout the entire multilayer positive electrode composite material layer as the first positive electrode active material, and further includes an iron phosphate compound represented by chemical formula 2 as the second positive electrode active material in the second positive electrode composite material layer or the mth positive electrode composite material layer separated from the positive electrode current collector. This allows the first positive electrode active material, which generates heat during battery charging and discharging, to be distributed in a position adjacent to the positive electrode current collector where heat transfer to the outside is easy. As a result, the present invention can improve the heat resistance of the positive electrode. Furthermore, the second positive electrode active material will detach its internal lithium and shrink in volume at an overcharge voltage of approximately 4.5V or higher. This rapidly blocks the conductive path within the positive electrode composite material layer containing the second positive electrode active material, thereby realizing an insulating effect. In addition, the second positive electrode active material has relatively lower electrical conductivity compared to the first positive electrode active material, which can prevent an increase in the short-circuit current on the surface of the positive electrode composite material layer during internal short circuits, thus suppressing the generation of short-circuit heat. Furthermore, the second positive electrode active material can increase the rigidity of the positive electrode surface, thereby reducing the risk of internal short circuits caused by external forces or penetration by needle-like objects.

[0076] In this case, the first positive electrode active material containing the lithium composite metal oxide represented by the above chemical formula 1 is a metal oxide containing lithium along with nickel (Ni), cobalt (Co), and manganese (Mn), and optionally other transition metals (M 1 ) may have a doped form. More specifically, in a concrete example, the lithium composite metal oxide may be Li(Ni 0.6 Co 0.2 Mn 0.2 )O2, Li(Ni 0.7 Co 0.15 Mn 0.15 )O2, Li(Ni 0.8 Co 0.1 Mn 0.1 )O2, Li(Ni 0.9 Co 0.05 Mn 0.05 )O2, Li(Ni 0.6 Co 0.2 Mn 0.1 Zr 0.1 )O2, Li(Ni 0.6 Co 0.2 Mn 0.15 Zr 0.05)O2 and Li(Ni 0.7 Co 0.1 Mn 0.1 Zr 0.1 ) May include one or more selected from the group consisting of O2.

[0077] The particle size of the first positive electrode active material described above is not particularly limited, but it may have an average particle size of 0.5 to 5 μm, and more specifically, it may have an average particle size of 0.8 to 1.5 μm, 1.0 to 3.0 μm, 1.2 to 1.8 μm, or 1.5 to 2.5 μm.

[0078] Furthermore, the iron phosphate compound represented by the above chemical formula 2 is a lithium phosphate oxide containing iron, and in some cases, other transition metals (M 2 ) may have a doped form. For example, the iron phosphate compound may be LiFePO4, LiFe 0.8 Mn 0.2 PO4, LiFe 0.5 Mn 0.5 This may include PO4, etc.

[0079] The second positive electrode active material containing the above iron phosphate compound may have an average particle size of 0.5 to 5 μm, specifically an average particle size of 0.5 to 1.0 μm, 0.8 to 1.2 μm, 1.0 to 2.0 μm, 1.5 to 3.0 μm, 2.0 to 3.0 μm, or 2.5 to 4.0 μm.

[0080] Furthermore, the above-mentioned second positive electrode active material may show a tendency for the average particle size of the second positive electrode active material contained in each positive electrode active material layer to increase as the position of the individual positive electrode active material layers changes from the second positive electrode active material layer to the mth positive electrode active material layer.

[0081] Specifically, the second positive electrode active material contained in the second positive electrode composite layer may have an average particle size of 0.5 to 1.2 μm, and the second positive electrode active material contained in the m-th positive electrode composite layer (where m≧2) may have an average particle size of 1.3 to 3.0 μm.

[0082] For example, the second positive electrode active material contained in the second positive electrode composite layer may have an average particle size of 0.8 to 1.0 μm, and the second positive electrode active material contained in the third positive electrode composite layer may have an average particle size of 1.2 to 1.5 μm.

[0083] As another example, the second positive electrode active material contained in the second positive electrode composite layer may have an average particle size of 0.6 to 0.8 μm, the second positive electrode active material contained in the third positive electrode composite layer may have an average particle size of 1.5 to 1.8 μm, and the second positive electrode active material contained in the fourth positive electrode composite layer may have an average particle size of 2.0 to 2.2 μm.

[0084] The positive electrode of the present invention can further increase the rigidity of the positive electrode surface by increasing the average particle size of the second positive electrode active material as the position of the individual positive electrode composite layers changes from the second positive electrode composite layer to the mth positive electrode composite layer.

[0085] Furthermore, the above-mentioned second positive electrode active material may be included in an amount of less than 10% by weight relative to the weight of the entire positive electrode composite layer, specifically in amounts of 0.1-9.9% by weight, 0.5-8.0% by weight, 0.5-6.0% by weight, 0.1-5.0% by weight, 0.1-3.0% by weight, 1.0-3.0% by weight, 2.5-5.0% by weight, 4.0-8.0% by weight, or 6.0-9.9% by weight relative to the weight of the entire positive electrode composite layer.

[0086] Furthermore, the second positive electrode active material containing the iron phosphate compound represented by the above chemical formula 2 may be included in individual positive electrode material layers in an amount of 0.5 to 20% by weight relative to the weight of each positive electrode material layer. Specifically, it may be included in amounts of 1 to 18% by weight, 1 to 15% by weight, 1 to 12% by weight, 1 to 10% by weight, 1 to 8% by weight, 1 to 5% by weight, 0.5 to 1% by weight, 0.5 to 5% by weight, 2 to 6% by weight, 0.5 to 0.9% by weight, 5 to 16% by weight, 7 to 15% by weight, or 8 to 12% by weight relative to the weight of each positive electrode material layer.

[0087] The present invention prevents insufficient rigidity on the positive electrode surface due to a small amount of second positive electrode active material, while preventing an increase in electrode resistance on the positive electrode surface and a decrease in the electrical performance of the battery due to an excessive amount of second positive electrode active material, by controlling the content of the second positive electrode active material to the above range relative to the weight of the overall positive electrode composite layer and individual positive electrode composite layers.

[0088] Furthermore, the second positive electrode active material is contained in the second positive electrode composite layer or the m-th positive electrode composite layer, and as its position changes from the second positive electrode composite layer adjacent to the first positive electrode composite layer to the m-th positive electrode composite layer furthest from the first positive electrode composite layer, the amount contained within the individual positive electrode composite layer, or the ratio of the amount contained in the individual positive electrode composite layer to the weight of the individual positive electrode composite layer, may tend to increase.

[0089] The second positive electrode active material undergoes a relatively slower oxidation-reduction reaction compared to the first positive electrode active material when the battery overheats or short-circuits. Therefore, by increasing the concentration of the second positive electrode active material closer to the outermost surface of the positive electrode composite layer, the possibility of fire or explosion during an internal short circuit in the battery is reduced.

[0090] On the other hand, the positive electrode for lithium secondary batteries according to the present invention may further contain a conductive material, a binder, other additives, etc., in the positive electrode composite layer as needed.

[0091] In this case, the first positive electrode active material and the second positive electrode active material contained in each positive electrode composite layer may be present in an amount of 85% by weight or more based on the total weight of each positive electrode composite layer, specifically in amounts of 90% by weight or more, 93% by weight or more, or 95% by weight or more.

[0092] Furthermore, the conductive material described above is used to improve the electrical performance of the positive electrode and may include materials commonly used in the industry, but specifically may include one or more selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Denka black, Ketjen black, Super P, channel black, furnace black, lamp black, summer black, graphene, and carbon nanotubes.

[0093] Furthermore, the conductive material may be included in an amount of 0.1 to 5% by weight based on the total weight of each positive electrode composite layer, specifically in amounts of 0.1 to 4% by weight, 2 to 4% by weight, 1.5 to 5% by weight, 1 to 3% by weight, 0.1 to 2% by weight, or 0.1 to 1% by weight.

[0094] Furthermore, the binder plays a role in binding the positive electrode active material, positive electrode additive, and conductive material to each other, and any binder having such a function can be used without particular limitations. Specifically, the binder may include one or more resins selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. As one example, the binder may include polyvinylidene fluoride.

[0095] Furthermore, the above-mentioned binder may be included in an amount of 1 to 10% by weight based on the total weight of each cathode composite layer, specifically in an amount of 2 to 8% by weight, or 1 to 5% by weight.

[0096] Furthermore, the total thickness of the positive electrode composite layer is not particularly limited, but it can be 50 μm to 300 μm, and more specifically, it can be 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0097] Furthermore, the thickness of the first positive electrode composite layer, which is in contact with the positive electrode current collector among the individual positive electrode composite layers constituting the positive electrode composite layer, can be adjusted to a certain range. Specifically, the thickness of the first positive electrode composite layer may be 10% to 60% of the total thickness of the positive electrode composite layer, and more specifically, it may be 10% to 40%, 30% to 50%, 10% to 20%, or 40% to 60% of the total thickness of the positive electrode composite layer.

[0098] The present invention not only prevents a reduction in the energy density of the electrode by adjusting the total thickness and individual thicknesses of the positive electrode composite layer to the above range, but also enables high adhesion between the positive electrode current collector and the positive electrode composite layer.

[0099] Furthermore, the positive electrode current collector provided in the positive electrode may be made of a material that has high conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, or calcined carbon may be used, and in the case of aluminum or stainless steel, it may also be made of a material that has been surface-treated with carbon, nickel, titanium, silver, etc.

[0100] Furthermore, the average thickness of the current collector can be preferably set to 5 to 500 μm, taking into consideration the conductivity and total thickness of the manufactured positive electrode.

[0101] On the other hand, the separation membrane is interposed between the positive and negative electrodes, and a thin insulating film with high ion permeability and mechanical strength is used. The separation membrane is not particularly limited as long as it is commonly used in this industry, but specifically, sheets or nonwoven fabrics made of chemically resistant and hydrophobic polypropylene, glass fiber, or polyethylene may be used, and in some cases, a composite separation membrane may be used in which inorganic / organic particles are coated with an organic binder polymer on a porous polymer substrate such as the above sheets or nonwoven fabrics. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte can also serve as the separation membrane. The pore diameter of the above separation membrane may average 0.01 to 10 μm, and the thickness may average 5 to 300 μm.

[0102] Furthermore, the above-mentioned electrolyte composition includes, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0103] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0104] The above organic solvent can be used without particular limitations, as long as it can act as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, the above organic solvents include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (propylene Carbonate solvents such as carbonate (PC), alcohol solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group of C2-C20, and may include a double-bonded aromatic ring or ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, or sulfolanes can be used. Among these, carbonate solvents are preferred, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery, and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferred. In this case, mixing the cyclic carbonate and linear carbonate in a volume ratio of about 1:1 to 9 may result in superior electrolyte performance.

[0105] Furthermore, the lithium salts mentioned above can be used without particular limitation as long as they are compounds capable of providing lithium ions for use in lithium secondary batteries. Specifically, the lithium salts that can be used include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2.

[0106] Furthermore, the concentration of the lithium salt can be used within the range of 0.1 M to 2.0 M. When the lithium salt concentration falls within this range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance and allowing lithium ions to move effectively.

[0107] In addition to the components of the electrolyte, the electrolyte may also contain one or more additives for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, or pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride. In this case, the additive may be present in an amount of 0.1% to 5% by weight relative to the total weight of the electrolyte.

[0108] As described above, lithium secondary batteries containing the positive electrode active material composition according to the present invention or a positive electrode manufactured using it exhibit excellent discharge capacity, output characteristics, and capacity retention rate in a stable manner, making them useful in portable devices such as mobile phones, laptop computers, and digital cameras, as well as in the electric vehicle field, such as hybrid electric vehicles (HEVs).

[0109] Furthermore, the lithium secondary battery according to the present invention is not limited in external shape depending on the battery's application, and its form may be adopted according to cases commonly used in the industry. For example, the lithium secondary battery may include a cylindrical, rectangular, pouch-type, or coin-type battery case using a can.

[0110] As one example, the lithium secondary battery described above may be a rectangular secondary battery that includes a rectangular can as its battery case.

[0111] <Secondary battery module> Furthermore, in one embodiment, the present invention provides a secondary battery module including the lithium secondary battery according to the present invention as described above.

[0112] The secondary battery module according to the present invention includes the lithium secondary battery of the present invention as a unit battery and not only has excellent electrical performance but also excellent safety against internal short circuits, so it can be used as a power source for medium to large devices that require high temperature stability, long cycle characteristics and high rate characteristics.

[0113] Specific examples of such medium- and large-sized devices include power tools powered by battery-powered motors, electric vehicles (EVs), electric vehicles including hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), electric motorcycles including electric bicycles (E-bikes) and electric scooters (E-scooters), electric golf carts, and power storage systems. More specifically, hybrid electric vehicles (HEVs) are a prime example, but the examples are not limited to them.

[0114] The present invention will be described in more detail below with reference to examples and experimental examples.

[0115] However, the following examples and experimental examples are illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0116] <Examples 1-10 and Comparative Examples 1-4. Manufacturing of Lithium Secondary Batteries> i) Manufacturing of negative electrodes Water is injected into a homo mixer, and a carbon-based material consisting of natural graphite and artificial graphite mixed in a 1:1 weight ratio is used as the first negative electrode active material, and SiO2 is used as the second negative electrode active material. q (where 0.9 ≤ q ≤ 2.2), styrene-butadiene rubber (SBR) was added. The mixture was then mixed at 2500 rpm for 80 minutes to prepare the slurry for forming the first negative electrode composite layer, the slurry for forming the second negative electrode composite layer, and the slurry for forming the third negative electrode composite layer, respectively.

[0117] At this time, the slurry prepared to form each negative electrode composite layer was prepared to contain 98.5% by weight of negative electrode active material and 1.5% by weight of binder, based on the solid content. Also, (1) second negative electrode active material (SiO q The degree of spheroidization of (2) the content ratio of the first and second anode active materials in each slurry relative to 150 parts by weight of the total anode active material (unit: parts by weight) was adjusted as shown in Table 1.

[0118] A thin copper sheet (average thickness: 12 μm) was prepared as the negative electrode current collector, and the slurries for forming the first to third negative electrode composite layers, which had been manufactured earlier, were sequentially cast onto the prepared copper sheet. The copper sheet with the cast slurry was dried in a vacuum oven at 130°C and then rolled to manufacture the negative electrode. At this time, the total thickness of the rolled negative electrode composite layer was 140 μm, and the thickness of each individual negative electrode composite layer was adjusted to be the same.

[0119] [Table 1]

[0120] (b) Manufacturing of positive electrodes N-methylpyrrolidone solvent is injected into a homo mixer, and LiNi is used as the first positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2 (hereinafter referred to as "NCM," average particle size: approximately 2 μm), LiFePO4 (hereinafter referred to as "LFP") as the second cathode active material, carbon black as the conductive material, and polyvinylidene fluoride (PVDF) as the binder were added. Then, the mixture was mixed at 3,000 rpm for 60 minutes to prepare the slurry for forming the first cathode composite layer, the slurry for forming the second cathode composite layer, and the slurry for forming the third cathode composite layer, respectively.

[0121] At this time, the slurry prepared to form each positive electrode composite layer was prepared to contain 97% by weight of positive electrode active material, 2% by weight of conductive material, and 1% by weight of binder, based on the solid content. In addition, (1) the average particle size of the second positive electrode active material (unit: μm) and (2) the ratio of the content of the first to third positive electrode active materials in each slurry to 150 parts by weight of the total positive electrode active material (unit: parts by weight) were adjusted as shown in Table 2.

[0122] A thin aluminum sheet (average thickness: 14 μm) was prepared as the positive electrode current collector. The previously manufactured slurry for forming the first to third positive electrode composite layers was sequentially cast onto the prepared aluminum sheet, dried in a vacuum oven at 130°C, and then rolled to produce the positive electrode. At this time, the total thickness of the rolled positive electrode composite layer was 150 μm, and the thickness of each individual positive electrode composite layer was adjusted to be the same.

[0123] [Table 2]

[0124] H) Assembly of secondary batteries As shown in Table 3 below, electrode assemblies were fabricated by placing the previously prepared positive and negative electrodes opposite each other and interposing a separator made of 18 μm polypropylene between them. Each manufactured electrode assembly was inserted into a rectangular battery case, and after injecting the electrolyte composition into the battery case, the case was sealed to manufacture a rectangular lithium secondary battery. In this case, the electrolyte composition used was a solution prepared by mixing lithium hexafluorophosphate (LiPF6, 1.0 M) and vinyl carbonate (VC, 2 wt%) in a mixture of ethylene carbonate (EC):dimethyl carbonate (DMC):diethyl carbonate (DEC) = 1:1:1 (volume ratio).

[0125] [Table 3]

[0126] <Example of experiment> To evaluate the performance and safety of the lithium secondary battery according to the present invention, the following experiments were conducted.

[0127] i) Evaluation of secondary battery output The lithium secondary batteries manufactured in the examples and comparative examples were fully charged at room temperature (22°C) at a rate of 0.1C. The initial discharge capacity was then measured while discharging the fully charged lithium secondary batteries at a rate of 0.1C. Subsequently, each lithium secondary battery was fully charged again at a rate of 0.1C, and the relative discharge capacity was measured relative to the initial discharge capacity at each discharge rate while discharging at 1.0C, 2.0C, 5.0C, and 9.0C. The results are shown in Table 4 below.

[0128] (b) Evaluation of nail penetration tests The lithium secondary batteries manufactured in the examples and comparative examples were subjected to two charge-discharge cycles at a voltage range of 4.2 to 2.0 V and a current value of 0.5 C under conditions of 25°C. After charging each lithium secondary battery to 4.2 V, the presence or absence of ignition was evaluated when a 3 mm diameter metal object was lowered at a speed of 80 mm / sec to penetrate the cells, under the same conditions as the PV8450 certification. The results are shown in Table 5.

[0129] H) Evaluation of impact tests The lithium secondary batteries manufactured in the examples and comparative examples were fully charged at room temperature (22°C) to a rate of 0.1 C-rate. Subsequently, the fully charged lithium secondary batteries were subjected to a secondary battery impact test in accordance with the UN1642DL impact certification standard. The weight used was 9 kg, and the experiment was conducted by dropping it onto a 16 mm diameter round rod placed on the secondary battery cell. The results are shown in Table 5 below.

[0130] [Table 4]

[0131] [Table 5]

[0132] As shown in Tables 4 and 5, the positive electrode for lithium secondary batteries according to the present invention not only has a high energy density but also exhibits excellent effectiveness in improving battery safety.

[0133] Specifically, the secondary battery of the embodiment according to the present invention was shown to maintain a discharge capacity ratio of 89% or more even during high-rate discharge of 5.0 C-rate or higher. This means that the lithium secondary battery including the positive electrode of the embodiment has excellent output.

[0134] Furthermore, it was confirmed that the secondary battery in the example did not ignite during nail penetration and impact tests. This indicates that the secondary battery according to the present invention is highly safe.

[0135] These results show that the lithium secondary battery according to the present invention contains a ternary compound containing nickel (Ni), cobalt (Co), manganese (Mn), etc., and simultaneously contains small amounts of silicon oxide and iron phosphate compound in the outermost shell of the composite layer adjacent to the separation membrane in the positive and negative electrodes, respectively. This not only improves the energy density of the battery but also reduces the electrical conductivity of the positive and negative electrode surfaces, thereby improving safety in the event of an internal short circuit in the secondary battery.

[0136] While preferred embodiments of the present invention have been described above with reference to those skilled in the art or those with ordinary knowledge in the art, it will be understood that the present invention can be modified and altered in various ways without departing from the spirit and technical scope of the invention as described in the claims below.

[0137] Therefore, the technical scope of the present invention is not limited to what is described in the summary of the invention in the specification, but should be defined by the claims. [Explanation of symbols]

[0138] 1: Lithium-ion rechargeable battery 10:Negative electrode 11: Negative electrode current collector 12: Multilayer negative electrode composite layer 121: Individual negative electrode composite layer 121a: First negative electrode composite layer 121n: nth negative electrode composite layer 20: Positive electrode 21: Positive electrode current collector 22: Multilayer structure of positive electrode composite layer 221: Individual positive electrode composite layer 221a: First positive electrode composite layer 221m: mth positive electrode composite layer

Claims

1. A negative electrode for a lithium secondary battery, wherein a first negative electrode composite layer to an nth negative electrode composite layer (where n≧3) are arranged in this order on a negative electrode current collector, and the first negative electrode composite layer is laminated on the surface in contact with the negative electrode current collector, The first negative electrode composite layer or the n-1th negative electrode composite layer comprises a first negative electrode active material and a second negative electrode active material. The negative electrode active material in the aforementioned nth negative electrode composite layer is only the second negative electrode active material. The first negative electrode active material is graphite, and the second negative electrode active material is silicon dioxide (SiO₂ ｑ However, 0.8 ≤ q ≤ 2.5) The second negative electrode active material is a negative electrode for a lithium secondary battery in which the degree of spheroidization decreases as the position of the individual negative electrode composite layers changes from the first negative electrode composite layer to the n negative electrode composite layer.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein the content or proportion of the second negative electrode active material increases as the position of the individual negative electrode material layers changes from the first negative electrode material layer to the n negative electrode material layer.

3. The anode for a lithium secondary battery according to claim 1, wherein each of the first to n-1 anode composite layers further comprises one or more selected from the group consisting of soft carbon, hard carbon, natural graphite, artificial graphite, expanded graphite, non-graphitizable carbon, carbon black, acetylene black, Ketjen black, carbon nanotubes, fullerene, activated carbon, graphene, and carbon fiber.

4. The anode for a lithium secondary battery according to claim 1, wherein the second anode active material is contained in an amount of 1% by weight or more and 20% by weight or less relative to the total weight of the anode active material.

5. The negative electrode for a lithium secondary battery according to claim 1, wherein the second negative electrode active material has a degree of spheroidization of 0.5 or more and 1.0 or less.

6. The negative electrode for a lithium secondary battery according to claim 1, wherein the total thickness of the negative electrode composite layer is 50 μm or more and 300 μm or less.

7. The anode for a lithium secondary battery according to claim 1, wherein the thickness of the n anode composite layer is 5% or more and 30% or less of the total thickness of the anode composite layer.

8. A lithium secondary battery comprising a negative electrode, a positive electrode, and a separator membrane located between the negative electrode and the positive electrode, as described in any one of claims 1 to 7.

9. A positive electrode is provided in which a first positive electrode composite layer to the mth positive electrode composite layer (where m≧2) are arranged in this order on a positive electrode current collector, and the first positive electrode composite layer is laminated on the surface in contact with the positive electrode current collector. The first positive electrode composite layer or the mth positive electrode composite layer comprises a first positive electrode active material containing a lithium composite metal oxide represented by chemical formula 1, and a second positive electrode active material containing an iron phosphate compound represented by the following chemical formula 2. [Chemical formula 1] Li ｘ [Ni ｙ Co ｚ Mn ｗ M １ ｖ ]O ２ [Chemical formula 2] LiFe ａ M ２ １－ａ XO ４ In the aforementioned chemical formulas 1 and 2, M １ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. x, y, z, w, and v are such that 1.0 ≤ x ≤ 1.30, 0.1 ≤ y < 1, 0 ≤ z ≤ 1, 0 ≤ w ≤ 1, and 0 ≤ v ≤ 0.1, respectively, and y + z + w + v = 1. M ２ is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, CO, Ni, Mn, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo. X is one or more elements selected from the group consisting of P, Si, S, As, and Sb. The lithium secondary battery according to claim 8, wherein a is 0 ≤ a ≤ 0.

5.

10. The lithium secondary battery according to claim 9, wherein the content or proportion of the second positive electrode active material in each positive electrode composite layer increases as the position of the individual positive electrode composite layers changes from the first positive electrode composite layer to the m positive electrode composite layer.

11. The lithium secondary battery according to claim 9, wherein the second positive electrode active material is contained in an amount of less than 10% by weight relative to the total weight of the positive electrode composite layer.

12. The lithium secondary battery according to claim 9, wherein the total thickness of the positive electrode composite layer is 50 μm or more and 300 μm or less.

13. A secondary battery module comprising the lithium secondary battery described in claim 8.

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