Improved safety of lithium secondary batteries

Abstract

The present invention relates to a positive electrode for a lithium secondary battery. The positive electrode contains a ternary compound containing nickel (Ni), cobalt (Co), manganese (Mn) and the like as a positive electrode active material, and not only has excellent energy density but also has the advantage of improving safety against internal short circuits of the secondary battery by containing an iron phosphate compound at the outermost corner based on the positive electrode current collector.

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. 10-2022-0045874, filed on April 13, 2022, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference. [Background technology]

[0003] BACKGROUND ART In recent years, secondary batteries have been widely used not only in small devices such as portable electronic devices but also in medium to large devices such as battery packs for hybrid cars and electric cars, and power storage devices.

[0004] To apply such secondary batteries to medium- to large-scale devices, high energy density is required. Therefore, ternary compounds containing nickel (Ni), cobalt (Co), manganese (Mn), etc., specifically LiNi, which has a nickel (Ni) content of 60% or more, are being developed. a Co b Mn c O2 (0.6≦a≦0.9, a+b+c=1), a layered lithium nickel metal oxide is used as the positive electrode active material, achieving high capacity.

[0005] However, although the capacity of lithium nickel metal oxide increases as the nickel (Ni) content increases, it exhibits low chemical and structural stability, making it prone to exothermic reactions. The exothermic reaction of the positive electrode active material can be induced when a short-circuit current flows inside the battery, i.e., when an internal short circuit occurs. In the case of lithium nickel metal oxide, the onset point of heat generation is low, and once the exothermic reaction begins, the temperature inside the battery rises rapidly, which can lead to fire, resulting in low safety.

[0006] Therefore, there is a need to develop a battery that exhibits high energy density while improving safety issues due to internal short circuits. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 10-2020-0024980 [Patent Document 2] Korean Patent Publication No. 10-2017-0004253 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to provide a positive electrode containing a ternary compound including nickel (Ni), which has a high energy density and is improved in terms of safety issues due to internal short circuits, and a lithium secondary battery including the positive electrode. [Means for solving the problem]

[0009] To solve the above-described problems, there is provided a positive electrode for a lithium secondary battery, in which n (n≧2) positive electrode composite layers are positioned on a positive electrode current collector, a first positive electrode composite layer in contact with a surface of the positive electrode current collector includes a first positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 1 below, and a second positive electrode composite layer or an nth positive electrode composite layer disposed on the first positive electrode composite layer includes the first positive electrode active material including the lithium composite metal oxide represented by Chemical Formula 1 and a second positive electrode active material including an iron phosphate compound represented by Chemical Formula 2 below.

[0010] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0011] [Chemical formula 2] Life a M2 1-a XO4

[0012] In the above chemical formula 1 and chemical formula 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, and x, y, z, w, and v are 1.0≦x≦1.30, 0.1≦y<1, 0≦z≦1, 0≦w≦1, 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; and a is 0≦a≦0.5.

[0013] In this case, the concentration of the second positive electrode active material in each positive electrode composite layer may increase as the position of the individual positive electrode composite layer changes from the second positive electrode composite layer to the nth positive electrode composite layer.

[0014] The second positive electrode active material may be contained in an amount of less than 10 wt % of the total weight of the positive electrode mixture layer, and the second positive electrode active material may be contained in an amount of 0.5 to 20 wt % of the weight of each individual positive electrode mixture layer.

[0015] The second positive electrode active material may have an average particle size of 0.5 μm to 5 μm, and the average particle size may increase as the position of the individual positive electrode mixture layer changes from the second positive electrode mixture layer to the nth positive electrode mixture layer.

[0016] The n positive electrode mixture layers may have a total thickness of 50 μm to 200 μm, of which the thickness of the first positive electrode mixture layer may be 10% to 60% of the total thickness of the positive electrode mixture layers.

[0017] In one embodiment, the present invention provides an electrode assembly for a lithium secondary battery, including the above-described positive electrode according to the present invention, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0018] Here, the negative electrode includes a negative electrode composite layer on a negative electrode current collector, and the negative electrode composite layer may include one or more carbon-based negative electrode active materials selected from natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotubes, fullerene, activated carbon, acetylene black, and ketjen black.

[0019] Furthermore, in one embodiment, the present invention provides a lithium secondary battery including the above-described electrode assembly according to the present invention, a battery case into which the electrode assembly is inserted, and an electrolyte composition injected into the battery case together with the electrode assembly.

[0020] In this case, the lithium secondary battery may be a prismatic secondary battery. [Effects of the Invention]

[0021] The positive electrode for a lithium secondary battery according to the present invention contains a ternary compound containing nickel (Ni), cobalt (Co), manganese (Mn), etc. as a positive electrode active material, and not only does it have excellent energy density, but also has the advantage of improving safety against internal short circuits in the secondary battery by containing an iron phosphate compound at the outermost corner based on the positive electrode current collector. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a cross-sectional view showing the structure of a positive electrode for a lithium secondary battery according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Because the present invention is susceptible to various modifications and can have various embodiments, specific embodiments are described in detail in the detailed description.

[0024] However, this is not intended to limit the invention to any particular embodiment, but should be understood to include all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

[0025] In the present invention, the terms "comprise" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, and should be understood as not precluding the presence or possibility of addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0026] Furthermore, in the present invention, when a layer, film, region, plate, or other portion is described as being "on" another portion, this includes not only the case where it is "directly on" the other portion, but also the case where there is another portion therebetween. Conversely, when a layer, film, region, plate, or other portion is described as being "under" the other portion, this includes not only the case where it is "directly under" the other portion, but also the case where there is another portion therebetween. Furthermore, in this application, being "located on" can include not only the case where it is located at the top, but also the case where it is located at the bottom.

[0027] The present invention will now be described in more detail.

[0028] <Positive electrodes for lithium secondary batteries> In one embodiment, the present invention provides a positive electrode for a lithium secondary battery, in which n (n≧2) positive electrode composite layers are located on a positive electrode current collector, a first positive electrode composite layer in contact with a surface of the positive electrode current collector includes a first positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 1 below, and a second positive electrode composite layer or an nth positive electrode composite layer disposed on the first positive electrode composite layer includes the first positive electrode active material including the lithium composite metal oxide represented by Chemical Formula 1 and a second positive electrode active material including an iron phosphate compound represented by Chemical Formula 2 below.

[0029] [Chemical formula 1] Li x [Ni y Coz Mn w M 1 v ]O2

[0030] [Chemical formula 2] Life a M 2 1-a XO4

[0031] In the above chemical formula 1 and chemical formula 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, and x, y, z, w, and v are 1.0≦x≦1.30, 0.1≦y<1, 0≦z≦1, 0≦w≦1, 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; and a is 0≦a≦0.5.

[0032] The positive electrode for a lithium secondary battery according to the present invention includes a positive electrode current collector and a positive electrode composite layer having a multilayer structure in which two or more individual composite layers are laminated on the positive electrode current collector.

[0033] 1, the positive electrode composite material layer has a structure in which n (n≧2) individual positive electrode composite material layers are stacked on a positive electrode current collector. In this case, the positive electrode composite material layer 20 stacked on the surface in contact with the positive electrode current collector 10 is a first positive electrode composite material layer 21a, and a second positive electrode composite material layer or an nth positive electrode composite material layer 21b is sequentially stacked on the first positive electrode composite material layer 21a, so that n individual positive electrode composite material layers are located on the positive electrode current collector.

[0034] The positive electrode composite layer is not particularly limited in the number of layers as long as it has a structure of two or more layers, and specifically may have 2 to 10 layers, 2 to 8 layers, 2 to 6 layers, or 2 to 4 layers. By adjusting the number of stacked positive electrode composite layers 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, can effectively release heat generated during charging and discharging of the battery to the outside.

[0035] The positive electrode composite layer is manufactured by applying, drying, and pressing a slurry containing a positive electrode active material that is capable of reversibly intercalating and deintercalating lithium ions during charging and discharging of the battery, and different types of the positive electrode active material may be contained in each layer.

[0036] Specifically, the positive electrode according to the present invention includes a first positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 1 in a positive electrode mixture layer, and a second positive electrode mixture layer or an n-th positive electrode mixture layer separated from the positive electrode current collector, i.e., a second positive electrode mixture layer or an n-th positive electrode mixture layer disposed on the first positive electrode mixture layer, further includes a second positive electrode active material including an iron phosphate compound represented by Chemical Formula 2:

[0037] [Chemical formula 1] Li x [Ni y Co z Mn w M 1 v ]O2

[0038] [Chemical formula 2] Life a M 2 1-a XO4

[0039] In the above chemical formula 1 and chemical formula 2, M 1is 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, and x, y, z, w, and v are 1.0≦x≦1.30, 0.1≦y<1, 0≦z≦1, 0≦w≦1, 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; and a is 0≦a≦0.5.

[0040] The lithium composite metal oxide represented by Chemical Formula 1 is a ternary lithium oxide composed primarily of nickel (Ni), cobalt (Co), and manganese (Mn), and has the advantage of being suitable for use in medium- to large-sized secondary batteries for power storage in the transportation sector, such as electric vehicles (EVs), and energy storage systems (ESSs), due to its high energy density and performance, such as high output. However, although the capacity of the lithium composite metal oxide increases as the nickel (Ni) content increases, it also has the problem of being prone to exothermic reactions, which can lead to a high risk of fire, due to its low chemical and structural stability.

[0041] The exothermic reaction can be induced when a short-circuit current flows inside the battery, i.e., when an internal short circuit occurs. Generally, the short-circuit current in a battery occurs when a short circuit occurs inside the secondary battery due to penetration by a needle-like object, or when a short circuit occurs in an electronic device connected to the secondary battery.

[0042] Therefore, in the present invention, a lithium composite metal oxide represented by Chemical Formula 1 is included as a first positive electrode active material in all of the multilayered positive electrode composite layers, and an iron phosphate compound represented by Chemical Formula 2 is further included as a second positive electrode active material in the second positive electrode composite layer or the nth positive electrode composite layer separated from the positive electrode current collector. This allows the first positive electrode active material, which generates heat during charge and discharge of the battery, to be distributed adjacent to the positive electrode current collector, where heat can be easily transferred to the outside, thereby improving the heat resistance of the positive electrode. At the same time, the second positive electrode active material can increase the rigidity of the positive electrode surface, thereby reducing the risk of internal short-circuiting due to external force or penetration by a needle-like object.

[0043] In this case, the first positive electrode active material including the lithium composite metal oxide represented by Chemical Formula 1 is a metal oxide including nickel (Ni), cobalt (Co) and manganese (Mn) together with lithium, and may optionally contain other transition metals (M 1 ) may be doped. In a specific example, more specifically, the lithium composite metal oxide may have a doped form of 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 )O2.

[0044] The particle size of the first positive electrode active material is not particularly limited, but may specifically have an average particle size of 0.5 to 5 μm, more specifically 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.

[0045] In addition, 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 It may contain PO4 etc.

[0046] The second positive electrode active material containing the iron phosphate compound may have an average particle size of 0.5 to 5 μm, specifically 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.

[0047] In addition, the second positive electrode active material may exhibit a tendency that the average particle size of the second positive electrode active material included in each positive electrode composite layer increases as the position of the individual positive electrode composite layer changes from the second positive electrode composite layer to the nth positive electrode composite layer.

[0048] 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 nth positive electrode composite layer (where n≧2) may have an average particle size of 1.3 to 3.0 μm.

[0049] As one example, the second positive electrode active material contained in the second positive electrode mixture 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 mixture layer may have an average particle size of 1.2 to 1.5 μm.

[0050] 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.

[0051] In the positive electrode of the present invention, the average particle size of the second positive electrode active material increases as the position of the individual positive electrode composite layer changes from the second positive electrode composite layer to the nth positive electrode composite layer, thereby making it possible to further increase the rigidity of the positive electrode surface.

[0052] The second positive electrode active material may be included in an amount of less than 10 wt % relative to the weight of the entire positive electrode composite layer, specifically, 0.1 to 9.9 wt %, 0.5 to 8.0 wt %, 0.5 to 6.0 wt %, 0.1 to 5.0 wt %, 0.1 to 3.0 wt %, 1.0 to 3.0 wt %, 2.5 to 5.0 wt %, 4.0 to 8.0 wt %, or 6.0 to 9.9 wt % relative to the weight of the entire positive electrode composite layer.

[0053] The second positive electrode active material containing the lithium composite metal oxide represented by Chemical Formula 2 may be contained in an individual positive electrode composite layer at 0.5 to 20 wt % relative to the weight of each positive electrode composite layer, and more specifically, may be contained at 1 to 18 wt %, 1 to 15 wt %, 1 to 12 wt %, 1 to 10 wt %, 1 to 8 wt %, 1 to 5 wt %, 0.5 to 1 wt %, 0.5 to 5 wt %, 2 to 6 wt %, 0.5 to 0.9 wt %, 5 to 16 wt %, 7 to 15 wt %, or 8 to 12 wt % relative to the weight of each positive electrode composite layer.

[0054] By controlling the content of the second positive electrode active material within the above range relative to the weight of the entire positive electrode composite layer and the individual positive electrode composite layer, the present invention can prevent a situation in which a small content of the second positive electrode active material results in insufficient rigidity on the positive electrode surface, while preventing a deterioration in the electrical performance of the battery due to an increase in electrode resistance on the positive electrode surface caused by an excessive amount of the second positive electrode active material.

[0055] Furthermore, the second positive electrode active material may be included in the second positive electrode composite layer or the nth positive electrode composite layer, and its concentration may tend to increase with position from the second positive electrode composite layer adjacent to the first positive electrode composite layer to the nth positive electrode composite layer farthest from the first positive electrode composite layer. Here, "increasing concentration" may also mean that the content or content ratio of the second positive electrode active material increases based on the total weight of the individual positive electrode composite layers. Since the second positive electrode active material undergoes a relatively slower oxidation-reduction reaction than the first positive electrode active material when the battery overheats or a short circuit occurs, increasing the concentration closer to the outermost surface of the positive electrode composite layer has the advantage of reducing the possibility of fire or explosion in the event of an internal short circuit in the battery.

[0056] Meanwhile, the positive electrode for a lithium secondary battery according to the present invention may further contain a conductive material, a binder, other additives, and the like in the positive electrode mixture layer, if necessary.

[0057] 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 contained in an amount of 85 parts by weight or more, specifically 90 parts by weight or more, 93 parts by weight or more, or 95 parts by weight or more, based on the weight of each positive electrode composite layer.

[0058] The conductive material is used to improve the electrical performance of the positive electrode and may be any conductive material commonly used in the art. Specifically, the conductive material may include at least one 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.

[0059] The conductive material may be contained in an amount of 0.1 to 5 parts by weight, specifically 0.1 to 4 parts by weight, 2 to 4 parts by weight, 1.5 to 5 parts by weight, 1 to 3 parts by weight, 0.1 to 2 parts by weight, or 0.1 to 1 part by weight, based on the weight of each positive electrode composite layer.

[0060] The binder functions to bind the positive electrode active material, the positive electrode additive, and the conductive material together, and any material having this function may be used without particular limitation. 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, polymethylmethacrylate, and copolymers thereof. For example, the binder may include polyvinylidene fluoride.

[0061] The binder may be contained in an amount of 1 to 10 parts by weight, specifically 2 to 8 parts by weight, or the conductive material may be contained in an amount of 1 to 5 parts by weight, based on the weight of each positive electrode mixture layer.

[0062] Furthermore, the total thickness of the positive electrode composite layer is not particularly limited, but may specifically be 50 μm to 300 μm, more specifically 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.

[0063] Furthermore, among the individual positive electrode composite layers constituting the positive electrode composite layer, the first positive electrode composite layer in contact with the positive electrode current collector may have a thickness 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, more specifically, 10% to 40%, 30% to 50%, 10% to 20%, or 40% to 60% of the total thickness of the positive electrode composite layer.

[0064] By adjusting the total thickness and individual thicknesses of the positive electrode composite layers within the above ranges, the present invention can not only prevent a reduction in the energy density of the electrode, but also realize high adhesive strength between the positive electrode current collector and the positive electrode composite layer.

[0065] 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, such as stainless steel, aluminum, nickel, titanium, or calcined carbon. In the case of aluminum or stainless steel, it may be surface-treated with carbon, nickel, titanium, silver, or the like.

[0066] The average thickness of the current collector is preferably 5 to 500 μm, taking into consideration the conductivity and total thickness of the positive electrode to be produced.

[0067] <Electrode assembly for lithium secondary battery> In one embodiment, the present invention provides an electrode assembly for a lithium secondary battery, including the above-described positive electrode according to the present invention, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

[0068] The electrode assembly for a lithium secondary battery according to the present invention includes the above-described positive electrode of the present invention, and has excellent output performance due to the high energy density of the battery. The first positive electrode active material, which generates heat during charging and discharging of the battery, can be disposed adjacent to the positive electrode current collector, which facilitates heat transfer to the outside, thereby improving the heat resistance of the positive electrode. Meanwhile, the second positive electrode active material can increase the rigidity of the positive electrode surface, thereby reducing the risk of internal short-circuiting due to external force or penetration by a needle-like object.

[0069] Here, the positive electrode has the same structure as the positive electrode for the lithium secondary battery of the present invention described above, and therefore, detailed description of the structure will be omitted.

[0070] In addition, the negative electrode includes a negative electrode composite layer prepared by applying a negative electrode active material to a negative electrode current collector, drying, and pressing the negative electrode active material in the same manner as the positive electrode, and may optionally further include a conductive material, a binder, other electrolyte additives, and the like, as needed.

[0071] In this case, the negative electrode active material may be one commonly used in the art, and specifically, may include one or more carbon-based negative electrode active materials selected from natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotubes, fullerene, activated carbon, acetylene black, and ketjen black.

[0072] The negative electrode mixture layer may also contain a binder to bond the negative electrode active material, conductive material, and other additives while providing adhesion to the negative electrode current collector. Examples of such binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These binders may be used alone or in combination. The binder may be present in an amount of 1 to 10 parts by weight, specifically 2 to 8 parts by weight, or 1 to 5 parts by weight of the conductive material, based on the weight of the negative electrode mixture layer.

[0073] The negative electrode composite layer may have an average thickness of 100 μm to 200 μm, specifically 100 μm to 180 μm, 100 μm to 150 μm, 120 μm to 200 μm, 140 μm to 200 μm, or 140 μm to 160 μm.

[0074] The negative electrode may also include a negative electrode current collector that has high conductivity without inducing chemical changes in the battery. For example, the negative electrode current collector may be made of copper, stainless steel, nickel, titanium, calcined carbon, or the like. In the case of copper or stainless steel, it may also be surface-treated with carbon, nickel, titanium, silver, or the like. The negative electrode current collector, like the positive electrode current collector, may have a finely textured surface to strengthen its bonding with the negative electrode active material, and may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. The average thickness of the negative electrode current collector may be preferably 3 to 500 μm, taking into account the conductivity and total thickness of the negative electrode to be manufactured.

[0075] The separator is a thin insulating membrane interposed between the positive and negative electrodes, exhibiting high ion permeability and mechanical strength. The separator may be any commonly used material in the art, including sheets or nonwoven fabrics made of chemically resistant and hydrophobic materials such as polypropylene, glass fiber, or polyethylene. In some cases, composite separators may be used, in which inorganic particles or organic particles are coated with an organic binder polymer on a porous polymer substrate such as a sheet or nonwoven fabric. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also function as the separator. The separator may have an average pore diameter of 0.01 to 10 μm and an average thickness of 5 to 300 μm.

[0076] <Lithium secondary battery> Furthermore, in one embodiment, the present invention provides a lithium secondary battery including the above-described electrode assembly according to the present invention, a battery case into which the electrode assembly is inserted, and an electrolyte composition injected into the battery case together with the electrode assembly.

[0077] The lithium secondary battery according to the present invention includes an electrode assembly including the above-described positive electrode of the present invention, and the electrode assembly may have a structure in which it is inserted into a battery case together with an electrolyte composition.

[0078] At this time, the electrode assembly has the same structure as the electrode assembly of the present invention described above, and therefore, a detailed description of the structure will be omitted.

[0079] In addition, examples of the electrolyte composition include 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 producing lithium secondary batteries, but are not limited to these.

[0080] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0081] The organic solvent may be any solvent capable of serving as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone, ether solvents such as dibutyl ether and tetrahydrofuran, ketone solvents such as cyclohexanone, aromatic hydrocarbon solvents such as benzene and fluorobenzene, dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of suitable solvents include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a C2-C20 linear, branched, or cyclic hydrocarbon group that may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can enhance the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred. In this case, the cyclic carbonate and the chain carbonate are mixed in a volume ratio of approximately 1:1 to 9 to achieve excellent electrolyte performance.

[0082] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries, without particular limitation. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCFSO, LiC4F9SO, LiN(C2F5SO3), LiN(C2F5SO2), LiN(CF3SO2), LiCl, LiI, or LiB(C2O4)2.

[0083] The lithium salt can be used at a concentration in the range of 0.1 M to 2.0 M. When the lithium salt concentration is within the above range, the electrolyte has suitable conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0084] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0085] As described above, the lithium secondary battery including the positive electrode according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0086] Furthermore, the lithium secondary battery according to the present invention is not limited in shape depending on the intended use of the battery, and may be shaped according to cases commonly used in the art. For example, the lithium secondary battery may be a battery including a cylindrical, prismatic, pouch, or coin-shaped battery case using a can.

[0087] As one example, the lithium secondary battery may be a prismatic secondary battery including a prismatic can as a battery case.

[0088] The present invention will be described in more detail below with reference to examples and experimental examples.

[0089] However, the following examples and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following examples and experimental examples.

[0090] <Examples 1 to 7 and Comparative Examples 1 and 2. Production of Positive Electrodes for Lithium Secondary Batteries> N-methylpyrrolidone solvent was poured into a homomixer, and LiNi was added as the first positive electrode active material to form the first to third positive electrode composite layers. 0.8 Co 0.1 Mn 0.1 The first positive electrode active material was LiFePO4 (hereinafter referred to as "LFP"), with an average particle size of approximately 2 μm, and the second positive electrode active material was LiFePO4 (hereinafter referred to as "LFP"), with a conductive material of carbon black, and the binder of polyvinylidene fluoride (PVDF). The mixture was then mixed at 3,000 rpm for 60 minutes to prepare a first positive electrode composite layer slurry, a second positive electrode composite layer slurry, and a third positive electrode composite layer slurry.

[0091] The slurries prepared for forming each positive electrode composite layer contained 48.5 wt % of the positive electrode active material, 1 wt % of the conductive material, and 0.5 wt % of the binder based on the solid content, and the content ratios (unit: parts by weight) of the first positive electrode active material and the second positive electrode active material contained in each slurry and the average particle size (unit: μm) of the second positive electrode active material were adjusted as shown in Table 1.

[0092] An aluminum thin plate (average thickness: 12 μm) was prepared as a positive electrode current collector, and the previously prepared first to third positive electrode composite layer slurries were sequentially cast onto the prepared aluminum thin plate, dried in a vacuum oven at 130°C, and rolled to prepare a positive electrode. The total thickness of the rolled positive electrode composite layer was 150 μm, and the thickness of the individual positive electrode composite layer was 50 μm for the three-layer structure and 75 μm for the two-layer structure.

[0093] [Table 1]

[0094] <Examples 8 to 14 and Comparative Examples 3 and 4. Production of electrode assemblies and lithium secondary batteries> A negative electrode active material was prepared by mixing natural graphite and artificial graphite in a 1:1 weight ratio. 97% by weight of the negative electrode active material and 3% by weight of styrene-butadiene rubber (SBR) were mixed with water to form a negative electrode slurry, which was then cast onto a copper sheet as a negative electrode current collector. The copper sheet onto which the negative electrode slurry was cast was dried in a vacuum oven at 130°C and rolled to produce a negative electrode. The thickness of the negative electrode composite layer was 130 μm.

[0095] The fabricated negative electrode was placed opposite a positive electrode prepared in each of the examples and comparative examples, as shown in Table 2 below, with an 18 μm polypropylene separator interposed between them to fabricate an electrode assembly. Each fabricated electrode assembly was inserted into a prismatic battery case, and an electrolyte composition was injected into the battery case, after which the case was sealed to fabricate a prismatic lithium secondary battery. 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).

[0096] [Table 2]

[0097] <Experimental Example> In order to evaluate the effect of the positive electrode for a lithium secondary battery according to the present invention, the following experiment was carried out.

[0098] a) Evaluation of secondary battery output The lithium secondary batteries prepared in each of the examples and comparative examples were fully charged at 0.1 C-rate at room temperature (22°C). The fully charged lithium secondary batteries were then discharged at 0.1 C-rate to measure their initial discharge capacities. The lithium secondary batteries were then fully charged at 0.1 C-rate and discharged at 1.0 C, 2.0 C, 5.0 C, and 9.0 C-rates, respectively, to measure the relative discharge capacities based on the initial discharge capacities for each discharge rate. The results are shown in Table 3 below.

[0099] B) Evaluation of battery heat generation during charging and discharging The secondary batteries fabricated in the examples and comparative examples were overcharged, and the surface and internal temperatures of the overcharged secondary batteries were measured. Specifically, a thermal sensor was attached to the inside of each secondary battery case, and the battery was charged to 4.2 V. The charged battery was then overcharged to 10 V at a constant current of 1 A. The constant voltage of 10 V was then maintained for six hours. After six hours, the internal temperature of each battery was measured using the thermal sensor attached to the secondary battery, and the surface of each secondary battery was photographed with a thermal imaging camera to measure the temperature of the secondary battery three times. The average value was calculated and used as the surface temperature of the battery during overcharge. The measurement results are shown in Table 3 below.

[0100] c) Nail penetration test evaluation The lithium secondary batteries manufactured in each of the examples and comparative examples were subjected to two charge-discharge cycles at a current value of 0.5 C in a voltage range of 4.2 V to 2.0 V in an environment of 25° C. After that, each lithium secondary battery was charged to 4.2 V, and then a metal object with a diameter of 3 mm was lowered at a speed of 80 mm / sec to penetrate the cell, in the same manner as the PV8450 certification conditions, and the presence or absence of ignition was evaluated. The results are shown in Table 4.

[0101] d) Impact test evaluation The lithium secondary batteries manufactured in each of the examples and comparative examples were fully charged at room temperature (22°C) at a 0.1 C-rate. The fully charged lithium secondary batteries were then subjected to a secondary battery impact test in accordance with the UN1642DL impact certification standard. A 9 kg weight was used, and the test was conducted by dropping it onto a 16 mm diameter round bar placed on the secondary battery cell. The results are shown in Table 4 below.

[0102] [Table 3]

[0103] [Table 4]

[0104] As shown in Tables 3 and 4, it is clear that the positive electrode for a lithium secondary battery according to the present invention not only has a high energy density but also has an excellent effect of improving the safety of the battery.

[0105] Specifically, the secondary battery according to the present invention maintained a discharge capacity ratio of 88% or more even during high-rate discharge at 5.0 C-rate or higher, which means that the output of the lithium secondary battery including the positive electrode of the present invention is excellent.

[0106] Furthermore, the positive electrodes of the examples exhibited low internal and external temperatures of the battery during overcharge, less than 54°C and less than 43°C, respectively, and it was confirmed that no fire occurred during nail penetration tests and impact tests, which indicates that secondary batteries containing the positive electrodes are highly safe.

[0107] From these results, it can be seen that the positive electrode according to the present invention contains a ternary compound containing nickel (Ni), cobalt (Co), manganese (Mn), etc., and not only has excellent energy density, but also improves safety against internal short circuits in the secondary battery by containing an iron phosphate compound at the outermost corner based on the positive electrode current collector.

[0108] Although the present invention has been described above with reference to preferred embodiments, it will be understood by those skilled in the art or those with ordinary knowledge in the art that various modifications and changes can be made to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims below.

[0109] Therefore, the technical scope of the present invention should not be limited to the content described in the Summary of the Invention of the specification, but should be defined by the claims. [Explanation of symbols]

[0110] 1: Positive electrode for lithium secondary batteries 10: Positive electrode current collector 20: Multilayered positive electrode composite layer 21: Individual positive electrode composite layer 21a: First positive electrode composite layer 21b: nth positive electrode composite layer

Claims

1. n (n≧2) positive electrode composite layers are located on a positive electrode current collector, The first positive electrode composite layer in contact with the surface of the positive electrode current collector includes a first positive electrode active material including a lithium composite metal oxide represented by the following Chemical Formula 1: The second positive electrode composite layer or the nth positive electrode composite layer disposed on the first positive electrode composite layer includes a first positive electrode active material including a lithium composite metal oxide represented by Chemical Formula 1 and a second positive electrode active material including 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 above Chemical Formula 1 and Chemical Formula 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 in the ranges of 1.0≦x≦1.30, 0.1≦y<1, 0≦z, 0≦w, 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 P, a is 0≦a≦0.5, A positive electrode for a lithium secondary battery, wherein the concentration of the second positive electrode active material in each positive electrode mixture layer increases as the position of the individual positive electrode mixture layer changes from the second positive electrode mixture layer to the nth positive electrode mixture layer.

2. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the second positive electrode active material is contained in an amount of less than 10 wt% based on the weight of the entire positive electrode mixture layer.

3. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the second positive electrode active material is contained in the individual positive electrode mixture layer in an amount of 0.5 wt % to 20 wt % relative to the weight of each positive electrode mixture layer.

4. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein an average particle size of the second positive electrode active material contained in each individual positive electrode composite layer increases as the position of the individual positive electrode composite layer changes from the second positive electrode composite layer to the nth positive electrode composite layer.

5. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the second positive electrode active material has an average particle size of 0.5 μm to 5 μm.

6. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the total thickness of the positive electrode mixture layer is 50 μm to 200 μm.

7. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the thickness of the first positive electrode mixture layer is 10% to 60% of the total thickness of the positive electrode mixture layers.

8. An electrode assembly for a lithium secondary battery, comprising the positive electrode according to any one of claims 1 to 7, a negative electrode, and a separator interposed between the positive electrode and the negative electrode.

9. the negative electrode includes a negative electrode mixture layer on a negative electrode current collector, 9. The electrode assembly for a lithium secondary battery of claim 8, wherein the negative electrode mixture layer comprises at least one carbon-based negative electrode active material selected from the group consisting of natural graphite, artificial graphite, expanded graphite, hard carbon, soft carbon, carbon fiber, carbon black, carbon nanotubes, fullerene, activated carbon, acetylene black, and ketjen black.

10. The electrode assembly according to claim 8 . a battery case into which the electrode assembly is inserted; and A lithium secondary battery comprising an electrolyte composition injected into a battery case together with an electrode assembly.

11. The lithium secondary battery according to claim 10, wherein the lithium secondary battery is a prismatic secondary battery.

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