Positive electrode for lithium secondary battery and lithium secondary battery

Abstract

The present technology relates to a positive electrode for a lithium secondary battery and a lithium secondary battery, specifically, to a positive electrode and a lithium secondary battery containing the positive electrode, the positive electrode comprising: a safety functional layer arranged on a positive electrode current collector; and a positive electrode mixture layer arranged on the safety functional layer, wherein the safety functional layer is formed by a multilayer structure of more than two layers, including a first safety functional layer in contact with the positive electrode current collector, and a second safety functional layer arranged on the first safety functional layer, and the composition of the first safety functional layer and the composition of the positive electrode mixture layer are mixed in the second safety functional layer.

Description

Technical Field

[0001] This application claims priority based on Korean Patent Application No. 10-2020-0152863 filed on November 16, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery, and more particularly, to a positive electrode for a lithium secondary battery having improved safety without deteriorating the life characteristics of the battery. Background Art

[0003] With the development of technology and the increase in demand for mobile devices, the demand for secondary batteries as energy sources has rapidly increased, and among these secondary batteries, lithium secondary batteries with high energy density and working potential, long cycle life and low self-discharge rate have been commercialized and widely used.

[0004] Recently, as lithium secondary batteries are used as power sources for medium or large devices (such as electric vehicles), high capacity, high energy density and low cost of lithium secondary batteries are required. Therefore, research has been actively conducted to use low-cost Ni, Mn, Fe, etc. that can replace expensive Co.

[0005] A major research task of lithium secondary batteries is to realize high-capacity and high-output electrode active materials and improve the safety of batteries using the materials. Lithium transition metal composite oxides are used as positive electrode active materials, among which lithium cobalt composite metal oxides with excellent capacity characteristics and high operating voltage are mainly used. In addition, since LiCoO2 has very poor thermal characteristics due to the unstable crystal structure caused by delithiation and is expensive, it is difficult to use a large amount of LiCoO2 as a power source for electric vehicles, etc.

[0006] Lithium manganese composite metal oxides (LiMnO2 or LiMn2O4, etc.), lithium iron phosphate compounds (LiFePO4, etc.) or lithium nickel composite metal oxides (LiNiO2, etc.) have been developed. Among these materials, lithium nickel composite metal oxides are currently being actively studied and developed, which can be easily implemented as large-capacity batteries due to their large reversible capacity of about 200 mAh / g. However, compared with LiCoO2, LiNiO2 has poor thermal stability, and when an internal short circuit occurs due to pressure from the outside in a charged state, the positive electrode active material itself decomposes, resulting in battery rupture and fire.

[0007] Therefore, a method of replacing part of nickel (Ni) with cobalt (Co) or manganese (Mn) has been proposed as a method of improving the low thermal stability while maintaining the excellent reversible capacity of LiNiO2. 1-α Co αIn the case of O2 (α = 0.1 to 0.3), the charge-discharge characteristics and life characteristics are excellent, but the thermal stability is low. In addition, in the case of a nickel-manganese-based lithium composite metal oxide obtained by replacing part of Ni with Mn having excellent thermal stability and a nickel-cobalt-manganese-based lithium composite metal oxide obtained by replacing part of Ni with Mn and Co (hereinafter referred to as "NCM-based lithium oxide"), the cycle characteristics and thermal stability are relatively excellent, but due to low penetration resistance, when a metal body (such as a nail) penetrates, an internal short circuit does not occur, so a fire or explosion may occur due to overcurrent.

[0008] Korean Patent Publication No. 2019-0047203 discloses a technology for ensuring battery safety by providing an overcharge prevention layer between a positive electrode current collector and a positive electrode active material layer to increase resistance during overcharging to block charging current.

[0009] However, in this prior art document, the safety of the electrode having the overcharge prevention layer is improved, but interlayer cracks may be generated due to the different compositions of the overcharge prevention layer and the positive electrode active material layer, and therefore, the life characteristics of the battery may be deteriorated. In addition, in the above-mentioned prior art document, the overcharge prevention layer has low penetration resistance. Thus, when penetrated by a needle body, there may be a problem from a safety point of view.

[0010] Therefore, there is a need for a technology for a positive electrode for a secondary battery that can increase penetration resistance in a case where a metal body such as a nail penetrates the electrode without deteriorating life characteristics while improving safety. Summary of the invention

[0011] [Technical issues]

[0012] An object of the present invention is to provide a positive electrode for a secondary battery and a lithium secondary battery including the positive electrode, wherein the positive electrode can increase the penetration resistance when a metal body such as a nail penetrates the electrode from the outside, while having high capacity and high output performance, excellent cycle characteristics and thermal stability.

[0013] [Technical solution]

[0014] The positive electrode for lithium secondary battery of the present invention comprises: a safety function layer disposed on a positive electrode current collector; and a positive electrode mixture layer disposed on the safety function layer.

[0015] The safety functional layer is formed of a multilayer structure of two or more layers, the multilayer structure comprising a first safety functional layer in contact with the positive electrode current collector, and a second safety functional layer disposed on the first safety functional layer, and

[0016] The second safety functional layer is obtained by mixing the composition of the first safety functional layer with the composition of the positive electrode mixture layer.

[0017] In one embodiment of the present invention, the first safety functional layer includes a first positive electrode active material, and the positive electrode mixture layer includes a second positive electrode active material different from the first positive electrode active material.

[0018] In one embodiment of the present invention, the first positive electrode active material is lithium iron phosphate having an olivine structure represented by the following Chemical Formula 1:

[0019] [Chemical formula 1]

[0020] Li 1+a Fe 1-x M x (PO 4-b )X b

[0021] wherein M is at least one selected from the group consisting of Al, Mg and Ti, X is at least one selected from the group consisting of F, S and N, -0.5≤a≤+0.5, 0≤x≤0.5, and 0≤b≤0.1.

[0022] In one embodiment of the present invention, the second positive electrode active material is a lithium transition metal oxide represented by the following Chemical Formula 2:

[0023] [Chemical formula 2]

[0024] Li a Ni 1-x-y Co x Mn y M z O2

[0025] wherein M is at least one selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, and wherein 0.9≤a≤1.5, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.1, and 0≤x+y≤1.

[0026] In one embodiment of the present invention, the second safety functional layer comprises a first positive electrode active material, a second positive electrode active material and a binder.

[0027] In one embodiment of the present invention, the second safety functional layer includes the first positive electrode active material and the second positive electrode active material in a weight ratio of 85:15 to 25:75.

[0028] In one embodiment of the present invention, the adhesion force A between the positive electrode current collector and the first safety functional layer is greater than the adhesion force B between the first safety functional layer and the second safety functional layer.

[0029] In one embodiment of the present invention, the adhesion force B is equal to or greater than the adhesion force C between the second safety functional layer and the positive electrode mixture layer.

[0030] In one embodiment of the present invention, the content of the binder contained in the first safety functional layer is 5 wt % to 30 wt % of the total weight of the first safety functional layer.

[0031] In one embodiment of the present invention, the weight ratio of the binder contained in each layer gradually decreases as the distance from the current collector increases.

[0032] In one embodiment of the present invention, the content of the binder contained in the second safety functional layer is 0.5 wt % to 10 wt % based on the total weight of the second safety functional layer.

[0033] In one embodiment of the present invention, the total thickness of the safety functional layer is 1 to 20 μm.

[0034] In one embodiment of the present invention, a thickness of a safety functional layer is equal to or less than 7 μm.

[0035] In one embodiment of the present invention, the average particle size (D 50 ) is equal to or less than 4 μm and is smaller than the average particle size (D 50 ).

[0036] In one embodiment of the present invention, the average particle size (D 50 ) is 0.1 to 3 μm.

[0037] The lithium secondary battery of the present invention comprises: the above-mentioned positive electrode; a separator; and a negative electrode.

[0038] [Beneficial Effects]

[0039] According to the positive electrode for lithium secondary batteries and the lithium secondary batteries including the positive electrode of the present invention, the safety functional layer is formed by multiple layers, a second safety functional layer is arranged between the positive electrode mixture layer and the first safety functional layer closest to the current collector, and since the composition of the first safety functional layer is mixed with the composition of the positive electrode mixture layer, the composition of the second safety functional layer gradually changes from the first safety functional layer to the positive electrode mixture layer, which can reduce interface cracks between the safety functional layer and the positive electrode mixture layer and improve the life characteristics of the battery.

[0040] In addition, by reducing the elongation of the safety functional layer adjacent to the positive electrode current collector, the penetration resistance in the case where a metal body such as a nail penetrates the electrode can be improved. In this way, a positive electrode for a secondary battery and a secondary battery including the positive electrode can be provided, which can prevent the battery from catching fire or exploding due to overcurrent and improve safety by suppressing overcurrent. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 is a cross-sectional view of a positive electrode of conventional technology.

[0042] Figure 2 This is a cross-sectional view of a positive electrode according to one embodiment of the present invention.

[0043] Figure 3 It is a cross-sectional view of a positive electrode according to another embodiment of the present invention. DETAILED DESCRIPTION

[0044] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. The terms and words used in this specification and claims should not be interpreted as limited to common or dictionary terms, and the inventors can appropriately define the concepts of the terms to best describe their inventions. The terms and words should be interpreted as meanings and concepts consistent with the technical ideas of the present invention.

[0045] In the present application, it should be understood that terms such as "including" or "having" are intended to indicate the presence of features, quantities, steps, operations, components, parts or combinations thereof described in the specification, and they do not preclude the possibility of the presence or addition of one or more other features or quantities, steps, operations, components, parts or combinations thereof. When a part such as a layer, a film, a region, a plate, etc. is referred to as being "on" another part, this includes not only the case where the part is "directly" on the other part, but also the case where another part is placed therebetween. On the other hand, when a part such as a layer, a film, a region, a plate, etc. is referred to as being "under" another part, this includes not only the case where the part is "directly" under the other part, but also the case where another part is placed therebetween. In addition, in the present application, being set "on..." may include the case where it is set at the bottom and the top.

[0046] Figure 1 is a cross-sectional view of a positive electrode including a conventional safety functional layer. Figure 1 In the positive electrode 10 including the conventional safety functional layer 12, the safety functional layer 12 is placed between the positive electrode current collector 11 and the positive electrode mixture layer 13. When the positive electrode 10 is penetrated by the needle-like conductor, the safety functional layer 12 can improve the safety of the battery by preventing the needle-like conductor from directly contacting the current collector, or reduce the short-circuit current by reducing the contact area between the needle-like conductor and the current collector. However, the volume difference between the safety functional layer and the positive electrode mixture layer occurs with charging and discharging in the positive electrode, which causes cracks to be generated at the interface, thereby deteriorating the life characteristics of the battery.

[0047] Therefore, an object of the present invention is to provide a positive electrode for improving the safety of a battery without deteriorating life characteristics due to interface cracks between a safety function layer and a positive electrode mixture layer.

[0048] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0049] Figure 2 : is a cross-sectional view of a positive electrode according to one embodiment of the present invention. Figure 2 , the positive electrode 100 for lithium secondary battery of the present invention includes: a safety function layer 120 disposed on a positive electrode current collector 110; and a positive electrode mixture layer 130 disposed on the safety function layer 120. Among them, the safety function layer 120 is formed by a multilayer structure of more than two layers, including a first safety function layer 121 contacting the positive electrode current collector and a second safety function layer 122 disposed on the first safety function layer. In addition, the second safety function layer is obtained by mixing the composition of the first safety function layer with the composition of the positive electrode mixture layer.

[0050] In this article, the mixture in which the composition of the first safety functional layer is mixed with the composition of the positive electrode mixture layer may include the positive electrode active material contained in the first safety functional layer and the positive electrode active material contained in the positive electrode mixture layer, or may include the positive electrode active material and binder contained in the first safety functional layer and the positive electrode active material and binder contained in the positive electrode mixture layer, or in the case where the first safety functional layer or the positive electrode mixture layer additionally contains a conductive material or additive, may include all of these conductive materials and other additives.

[0051] The safety function layer 120 of the positive electrode of the embodiment of the present invention may be formed of 2 layers, the second safety function layer 122 is provided between the positive electrode mixture layer 130 and the first safety function layer 121 contacting the positive electrode current collector 110, and the composition of the second safety function layer is configured by mixing the composition of the first safety function layer 121 with the composition of the positive electrode mixture layer 130. In this way, the second safety function layer buffers the difference between the first safety function layer and the positive electrode mixture layer, and therefore, it is possible to suppress the generation of cracks between the layers as charging and discharging are repeated, and prevent the degradation of life characteristics.

[0052] In an embodiment of the present invention, the first safety functional layer includes a first positive electrode active material, and the positive electrode mixture layer includes a second positive electrode active material different from the first positive electrode active material. Therefore, the second safety functional layer that performs the function of buffering the volume difference between the two includes both the first positive electrode active material and the second positive electrode active material as positive electrode active materials.

[0053] In one embodiment of the present invention, the first positive electrode active material is lithium iron phosphate having an olivine structure represented by the following Chemical Formula 1:

[0054] [Chemical formula 1]

[0055] Li 1+a Fe 1-x Mx (PO 4-b )X b

[0056] wherein M is at least one selected from the group consisting of Al, Mg and Ti, X is at least one selected from the group consisting of F, S and N, -0.5≤a≤+0.5, 0≤x≤0.5, and 0≤b≤0.1.

[0057] The positive electrode active material having an olivine structure is characterized in that when the lithium in the safety functional layer overflows at an overcharge voltage of about 4.5V or more, the volume of the positive electrode active material decreases. In this way, by selecting lithium iron phosphate as the first positive electrode active material contained in the safety functional layer, the conductive path of the safety functional layer is quickly blocked, and the safety functional layer acts as an insulating layer, and the resistance increases, which increases the resistance and blocks the charging current, thereby reaching the overcharge termination voltage. Therefore, in the present invention, by selecting a positive electrode active material having an olivine structure as the first positive electrode active material, a synergistic effect can be exhibited in terms of safety improvement.

[0058] Similarly, in the case of selecting a positive electrode active material having an olivine structure as the first positive electrode active material, when the battery operates normally, the safety functional layer of the present invention operates like a normal positive electrode active material layer, and when penetrated by an external needle-like conductor or in an overcharged state, prevents contact with the needle-like conductor or prevents overcharging due to increased resistance, thereby ultimately improving safety.

[0059] In one embodiment of the present invention, the content of the first positive electrode active material contained in the first safety functional layer is 50 wt % to 99 wt % of the total weight of the first safety functional layer.

[0060] In addition, the second positive electrode active material included in the positive electrode mixture layer may include a lithium nickel composite metal oxide, which can easily realize a large capacity battery due to its high reversible capacity. A lithium transition metal oxide represented by the following Chemical Formula 2 is a specific example of the second positive electrode active material.

[0061] [Chemical formula 2]

[0062] Li a Ni 1-x-y Co x Mn y M z O2

[0063] wherein M is at least one selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, and wherein 0.9≤a≤1.5, 0≤x≤1, 0≤y≤1, 0≤z≤0.1 and 0≤x+y≤1.

[0064] However, the second positive electrode active material is not necessarily limited to the lithium transition metal oxide represented by Chemical Formula 2, and the second positive electrode active material may include: a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium manganese oxide such as Li 1+x1 Mn 2-x1 O4 (where x1 is 0 to 33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5 and Cu2V2O7; LiNi 1-x2 M 1 x2 Lithium nickel oxide represented by O2 (herein, M 1 =Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x2 = 0.01 to 0.3); LiMn 2-x3 M 2 x3 O2 (here, M 2 =Co, Ni, Fe, Cr, Zn or Ta, x3 = 0.01 to 0.1) or Li2Mn3M 3 O8 (here, M 3 =Fe, Co, Ni, Cu or Zn) represented by lithium manganese composite oxide; LiNi x4 Mn 2-x4 A spinel lithium manganese composite oxide represented by O4 (here, x4=0.01 to 1); LiMn2O4 in which a part of Li is replaced by an alkaline earth metal ion; a disulfide; Fe2(MoO4)3, etc.

[0065] The second safety functional layer comprises the first positive electrode active material, the second positive electrode active material and the binder. As described above, the second safety functional layer of the present invention reduces the generation of cracks at the interface between the first safety functional layer and the positive electrode mixture layer. Therefore, the second safety functional layer comprises the positive electrode active material of the first safety functional layer and the positive electrode active material of the positive electrode mixture layer as the positive electrode active material.

[0066] At this time, the weight ratio of the first positive electrode active material to the second positive electrode active material contained in the second safety functional layer is 85:15 to 25:75, preferably 80:20 to 30:70. In terms of reducing the generation of interface cracks and preventing overcharging, the mixing ratio of the first positive electrode active material to the second positive electrode active material contained in the second safety functional layer is preferably within the above range.

[0067] also, Figure 2An example in which the security function layer is composed of two layers is shown, but the present invention is not limited to this example, and Figure 3 As shown in the example of FIG. 1 , the safety function layer may be formed of three or more layers.

[0068] In this case, the composition of the remaining safety functional layers except the first safety functional layer tends to gradually become similar to the composition of the positive electrode mixture layer toward the positive electrode mixture layer, and tends to gradually become similar to the composition of the first safety functional layer toward the positive electrode current collector. Figure 3 As shown, the positive electrode is formed by 3 layers and has a structure obtained by stacking a positive electrode collector 210, a first safety functional layer 221, a second safety functional layer 222, a third safety functional layer 223 and a positive electrode mixture layer 230 in this order. The weight percent of the first positive electrode active material contained in the safety functional layer gradually decreases toward the positive electrode mixture layer, and on the contrary, the weight percent of the second positive electrode active material gradually increases toward the positive electrode mixture layer.

[0069] In addition, in the positive electrode of the present invention, the adhesion force A between the positive electrode collector and the first safety functional layer is greater than the adhesion force B between the first safety functional layer and the second safety functional layer. Therefore, since the first safety functional layer adheres to the positive electrode collector when penetrated by the needle-like conductor and does not peel off from the positive electrode collector, it shows the effect of improving battery safety by preventing or inhibiting direct contact with the positive electrode collector.

[0070] In an embodiment of the present invention, the adhesion force B between the first safety functional layer and the second safety functional layer is equal to or greater than the adhesion force C between the second safety functional layer and the positive electrode mixture layer.

[0071] This is to minimize the contact area between the positive electrode collector and the metal body when the metal body (such as a nail) penetrates the positive electrode. That is, when the metal body penetrates the positive electrode, an external force is applied to the positive electrode, and a gap can be generated in each of the space between the positive electrode collector and the first safety functional layer, the space between the first safety functional layer and the second safety functional layer, and the space between the second safety functional layer and the positive electrode mixture layer. At this time, when the adhesion force A is greater than the adhesion force B and the adhesion force C, respectively, even if the first safety functional layer is separated from the second safety functional layer, since the first safety functional layer is still attached to the positive electrode collector, it is difficult for the metal body to directly contact the positive electrode collector. In addition, when the adhesion force B is relatively greater than the adhesion force C, even if the second safety functional layer is separated from the positive electrode mixture layer, the second safety functional layer can still be attached to the first safety functional layer and protect the first safety functional layer from directly bearing the external force applied by the metal body. In this way, the tendency of the first safety functional layer to separate from the positive electrode collector due to the external force of the metal body can be further suppressed.

[0072] Furthermore, by adjusting the content of each binder contained in each of the first safety functional layer, the second safety functional layer, and the positive electrode mixture layer, the magnitude of the adhesion between the layers can be controlled to be A>B≥C.

[0073] Therefore, in the positive electrode of one embodiment of the present invention, the weight % of the binder contained in each layer decreases as it moves away from the current collector. Herein, "weight %" refers to the proportion of the weight of the binder to the total weight of a layer. Assuming that "a" is defined as the weight % of the binder contained in the first safety functional layer based on the total weight of the first safety functional layer, "b" is defined as the weight % of the binder contained in the second safety functional layer based on the total weight of the second safety functional layer, and "c" is defined as the weight % of the binder contained in the positive electrode mixture layer based on the total weight of the positive electrode mixture layer, the relationship between a, b and c is a>b>c.

[0074] In addition, the content of the binder contained in the first safety functional layer is 5 wt % to 30 wt %, preferably 7 wt % to 25 wt %, and more preferably 8 wt % to 20 wt %, based on the total weight of the first safety functional layer. In the case where the content of the binder contained in the first safety functional layer is less than 5 wt %, when penetrated by the needle-like conductor, the effect of preventing direct contact between the needle-like conductor and the current collector is weak, so a short circuit may occur. On the contrary, if the content of the binder contained in the first safety functional layer exceeds 30 wt %, the balance between the second safety functional layer and the positive electrode mixture layer is lost.

[0075] In addition, the content of the binder contained in the second safety functional layer can be 0.5 wt% to 10 wt% of the total weight of the second safety functional layer. Since the second safety functional layer buffers the difference between the first safety functional layer and the positive electrode mixture layer, the weight % of the binder thereof can be similar to or slightly greater than the weight % of the binder contained in the positive electrode mixture layer.

[0076] In an embodiment of the present invention, the adhesion force A between the current collector and the first safety functional layer may be 100 N / m to 500 N / m, preferably 150 N / m to 300 N / m, and more preferably 200 N / m to 300 N / m.

[0077] The adhesion force B between the first safety functional layer and the second safety functional layer may be 20 N / m to 150 N / m, preferably 20 N / m to 100 N / m, more preferably 40 N / m to 100 N / m.

[0078] The adhesion force C between the second safety functional layer and the positive electrode mixture layer may be 10 to 40 N / m, preferably 15 to 35 N / m, and more preferably 20 to 35 N / m.

[0079] The safety function layer and the positive electrode mixture layer of the present invention include a binder, which adheres to the positive electrode active material particles and improves the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber or its various copolymers, and one or a mixture of two or more thereof can be used.

[0080] In an embodiment of the present invention, the binder contained in the safety function layer and the binder contained in the positive electrode mixture layer may be binders having the same physical properties. For example, if the binder contained in the safety function layer is a hydrophilic binder, the binder contained in the positive electrode mixture layer may also be a hydrophilic binder. On the contrary, if the binder contained in the safety function layer is a lipophilic binder, the binder contained in the positive electrode mixture layer may also be a lipophilic binder.

[0081] In an embodiment of the present invention, the total thickness of the safety functional layer may be 1 to 20 μm, preferably 2 to 15 μm, and more preferably 3 to 15 μm. Herein, the safety functional layer refers to a multilayer safety functional layer disposed between the positive electrode collector and the positive electrode mixture layer.

[0082] In an embodiment of the present invention, the thickness of each layer constituting the safety functional layer may be equal to or less than 7 μm, preferably equal to or less than 5 μm, and more preferably between 1 μm and 5 μm. If the thickness of one safety functional layer is too large, the total thickness of the entire installation functional layer will become large, which is disadvantageous.

[0083] In an embodiment of the present invention, the average particle size (D 50 ) is equal to or less than 4 μm and is smaller than the average particle size (D 50 Specifically, the average particle size (D 50 ) may correspond to the average particle size (D 50 ) from 10% to 80%.

[0084] That is, according to an embodiment of the present invention, the first safety functional layer is configured to include particles having a relatively small average particle size (D 50) of the positive electrode active material, thereby reducing the elongation of the first safety functional layer, and due to the reduction in the elongation of the first safety functional layer, the positive electrode current collector does not elongate like the needle-like conductor and breaks when the needle-like conductor penetrates, thereby improving the penetration safety.

[0085] In the present invention, the average particle size D 50 It can be defined as the particle size corresponding to the volume accumulation of 50% in the particle size distribution curve. Average particle size D 50 The average particle size (D 50 ), after dispersing the particles of the positive electrode active material in a dispersion medium, by introducing a commercially available laser diffraction particle size measuring device (e.g., Microtrac MT 3000) to radiate ultrasonic waves of about 28 kHz at an output of 60 W, the average particle size (D) corresponding to 50% of the volume accumulation in the measuring device can be calculated. 50 ).

[0086] Specifically, the average particle size of the first positive electrode active material may be equal to or less than 4 μm. More preferably, the average particle size (D 50 ) may be 0.1 μm to 3 μm, and preferably 0.1 μm to 2 μm. When the average particle size (D 50 ) is less than 0.1 μm, electrode side reactions may occur during electrode manufacturing or dispersibility may be reduced. 50 ) exceeds 4 μm, the adhesion to the positive electrode collector may be reduced, and as the elongation of the first safety functional layer increases, the safety improvement effect may be reduced.

[0087] The second positive electrode active material may have an average particle size (D 50 ) particles having a relatively larger average particle size than the first positive electrode active material.

[0088] Specifically, the average particle size (D 50 ) may be equal to or greater than 3 μm. More preferably, the average particle size (D 50 ) may be 3 μm to 30 μm, and preferably 3 μm to 20 μm. When the average particle size (D 50 ) is less than 3 μm, there may be difficulties in the rolling process when manufacturing the electrode.

[0089] In addition, the specific surface area of ​​the first positive electrode active material may be equal to or greater than 3 m 2 / g, preferably 5m 2 / g to 25m 2 / g, and more preferably 7m2 / g to 20m 2 / g. If the specific surface area is less than 2m 2 / g, the elongation of the first safety functional layer and the second safety functional layer may increase, which is disadvantageous.

[0090] In the present invention, the specific surface area is measured by the BET method, and can be specifically calculated using the nitrogen adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II of BEL Japan.

[0091] The porosity of the first safety functional layer of the embodiment of the present invention may be 20% to 40%, and the porosity of the positive electrode mixture layer may be less than the porosity of the first safety functional layer and may be 15% to 35%. By controlling the porosity of the first safety functional layer to be larger, the amount of gas oxidation on the surface of small particles can be increased at a high voltage, thereby preventing overcurrent generation by increasing the resistance of the first safety functional layer, thereby improving safety.

[0092] Porosity can be measured by SEM analysis. As a method for quantifying the porosity distribution in the thickness direction of the electrode, the electrode before analysis is filled with epoxy resin, dried under vacuum, thereby preparing a sample for analysis, which is then divided into 9 parts at regular intervals, and the electrode active material layer sample is cut along the straight line divided into 9 parts in the thickness direction by an ion milling scheme. Thereafter, an SEM (10 kV) image of the cross section is taken, the pore area ratio in the entire cross-sectional area is calculated, and the average value of the 9 pore area ratios is used as the porosity value of the electrode active material layer.

[0093] In the positive electrode of the embodiment of the present invention, the difference in elongation between the first safety functional layer and the positive electrode mixture layer may be 0.1% to 1.0%, and more preferably 0.2% to 0.7%. In the present invention, the elongation is a value measured using a UTM device, and the elongation is measured by the change in length when the positive electrode mixture layer is stretched to the maximum extent compared to the length of the existing positive electrode mixture layer when the first safety functional layer or the positive electrode mixture layer is installed at a rate of about 5 mm / min.

[0094] Similarly, by using a positive electrode active material formed of large-sized particles with a small specific surface area to form a positive electrode mixture layer, the difference in elongation between the lower and upper parts of the electrode is maximized, while the reduction in elongation of the first safety functional layer is maximized, thereby improving the penetration resistance and improving the battery performance, such as life characteristics.

[0095] Specifically, the elongation of the first safety functional layer can be 0.2% to 1.2%, and more preferably 0.2% to 0.5%. When the elongation of the positive electrode current collector and the first safety functional layer meets the above range, in the case where the metal body penetrates the electrode, the penetration resistance can be significantly increased, and the safety can be improved by preventing the generation of overcurrent by increasing the penetration resistance.

[0096] In addition, the elongation of the positive electrode mixture layer may be 0.6% to 2.0%, and more preferably 0.6% to 0.9%. When the elongation of the positive electrode mixture layer located at the upper portion of the electrode satisfies the above range, the elongation of the entire positive electrode can be maintained at or above a certain level, and the problem of disconnection during rolling during the electrode manufacturing process can be prevented.

[0097] In addition, the total elongation of the manufactured positive electrode can be less than 1.4%. The average particle size (D 50 ), and by controlling the porosity of each layer, the difference in elongation between the first safety function layer and the positive electrode mixture layer can be set to 0.1% to 1.0%, preferably 0.2% to 0.7%. In addition, the total elongation of the positive electrode can be set to less than 1.4%.

[0098] In an embodiment of the present invention, at least one of the safety function layer and the positive electrode mixture layer further includes a conductive material. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and examples thereof include: graphite, such as natural graphite and artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium dioxide; and conductive materials such as polyphenylene derivatives. The content of the conductive material may be 1 to 30% by weight based on the total weight of the positive electrode mixture layer.

[0099] In the present invention, the positive electrode collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. Examples of positive electrode collectors include stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium or silver. In addition, the thickness of the positive electrode collector can generally be 3 μm to 500 μm, and the adhesion of the positive electrode active material can be increased by forming tiny concave and convex objects on the surface of the positive electrode collector. It can be used in various forms, such as films, sheets, foils, nets, porous bodies, foams and non-woven fabrics.

[0100] In addition, the present invention provides an electrochemical device comprising the positive electrode. Specifically, the electrochemical device may be a battery or a capacitor, and more specifically, may be a lithium secondary battery.

[0101] Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, and the positive electrode is as described above. Additionally, the lithium secondary battery may optionally further include a battery case for accommodating the electrode assembly including the positive electrode, the negative electrode, and the separator, and a sealing member for sealing the battery case.

[0102] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode mixture layer located on the negative electrode current collector.

[0103] The negative electrode current collector is not particularly limited as long as it has high electrical conductivity without causing chemical changes in the battery. Examples thereof include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel whose surface is treated with carbon, nickel, titanium, or silver, or an aluminum-cadmium alloy, etc. Additionally, the thickness of the negative electrode current collector can generally be 3 μm to 500 μm, and like the positive electrode current collector, minute irregularities can be formed on the surface of the negative electrode current collector to enhance the binding force of the negative electrode active material. It can be used in various forms, such as a film, sheet, foil, net, porous body, foam, and non-woven fabric.

[0104] The negative electrode mixture layer includes a negative electrode active material, a binder, and a conductive material. The negative electrode mixture layer can be manufactured as follows: A composition for forming the negative electrode mixture layer containing the negative electrode active material and optionally the binder and the conductive material is coated on the negative electrode current collector and dried, or by casting the composition for forming the negative electrode mixture layer on a separate support, and then laminating the film obtained by peeling the slurry from the support on the negative electrode current collector.

[0105] Compounds that can reversibly intercalate and deintercalate lithium can be used as the negative electrode active material. Specific examples thereof include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic substances capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy; metal oxides that can be doped and undoped with lithium such as SiO x (0 < x < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composites containing the above metallic substances and carbonaceous materials such as Si-C composites or Sn-C composites, and mixtures of any one or two or more of them. Additionally, a thin film of metallic lithium can be used as the negative electrode active material. As the carbon material, low-crystalline carbon and high-crystalline carbon can be used. Examples of low-crystalline carbon include soft carbon and hard carbon. Examples of high-crystalline carbon include amorphous, flaky, scaly, spherical, or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microbead, mesophase pitch, and coke derived from petroleum or coal tar pitch.

[0106] In addition, the binder and the conductive material may be the same as those described above in the positive electrode.

[0107] Meanwhile, in lithium secondary batteries, the diaphragm is used to separate the negative electrode from the positive electrode and provide a mobile path for lithium ions, and any diaphragm commonly used in lithium secondary batteries can be used without any special restrictions. In particular, it is preferred to have a high electrolyte wetting ability and a diaphragm with low resistance to the movement of electrolyte ions. Specifically, a porous polymer film can be used, such as a porous polymer film made of a polyolefin polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexane copolymer and ethylene / methacrylate copolymer). In addition, a nonwoven made of conventional porous nonwoven fabrics can be used, such as high melting point glass fiber, polyethylene terephthalate fiber, etc. In order to ensure heat resistance or mechanical strength, a coated diaphragm comprising a ceramic component or a polymer material can be used, and can be optionally used as a single layer or multilayer structure.

[0108] Examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used to manufacture lithium secondary batteries, but the present invention is not limited to these examples.

[0109] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0110] The organic solvent may be any organic solvent that can act as a medium through which ions participating in the electrochemical reaction of the battery can move. Specifically, examples of organic solvents include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone and γ-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC) and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (R is a linear, branched or cyclic hydrocarbon group of C2 to C20, which may contain a double bond aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane. Among them, carbonate solvents are preferred, and more preferably a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery and a linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) with low viscosity. In this case, when cyclic carbonates and chain carbonates are mixed in a volume ratio of about 1:1 to about 1:9, the performance of the electrolyte solution can be excellent.

[0111] The lithium salt can be used without any particular restriction, as long as it is a compound that can provide lithium ions used in lithium secondary batteries. Specifically, the lithium salt can be LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI or LiB(C2O4)2. The concentration of the lithium salt is preferably 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that it can show excellent electrolyte performance and lithium ions can be effectively moved.

[0112] In addition to the electrolyte components, in order to improve the life characteristics of the battery, inhibit the reduction of the battery capacity, and improve the discharge capacity of the battery, the electrolyte may contain one or more of the following substances: halogenated alkylene carbonate compounds (such as difluoroethylene carbonate), pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glyme and hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidinones, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol or aluminum chloride. Herein, the content of the additive may be 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0113] The lithium secondary battery including the positive electrode active material of the present invention can be used in electric vehicles and portable devices such as mobile phones, notebook computers and digital cameras, as well as electric vehicles such as hybrid electric vehicles, because the lithium secondary battery stably shows excellent discharge capacity, output characteristics and capacity retention rate.

[0114] According to another embodiment of the present invention, there are provided a battery module including the above-mentioned lithium secondary battery as a unit cell and a battery pack including the battery module.

[0115] The battery module or battery pack may be used as a power source for one or more of the following medium or large devices: power tools; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0116] Hereinafter, the embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0117] Example 1

[0118] <Manufacturing of slurry for first safety functional layer>

[0119] The average particle size (D 50 ) is 1 μm and the BET specific surface area is 15 m 2 / g of LiFePO4 positive electrode active material, 2 wt% of carbon black as a conductive material and 10 wt% of PVdF as a binder were mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a slurry for a first safety functional layer.

[0120] <Manufacturing of positive electrode mixture layer slurry>

[0121] The average particle size (D 50 ) is 4 μm and the BET specific surface area is 0.7 m 2 / gLiNi 0.8 Co 0.1 Mn 0.1 An O2 positive electrode active material, 2 wt% of carbon black as a conductive material, and 2 wt% of PVdF as a binder were mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode mixture layer slurry.

[0122] <Manufacturing of slurry for second safety functional layer>

[0123] The average particle size (D 50 ) is 1 μm and the BET specific surface area is 15 m 2 / g of LiFePO4 positive electrode active material, 23% by weight of the average particle size (D 50 ) is 4 μm and the BET specific surface area is 0.7 m 2 / gLiNi 0.8 Co 0.1 Mn 0.1 An O2 positive electrode active material, 2 wt% of carbon black as a conductive material, and 5 wt% of PVdF as a binder were mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a slurry for a second safety functional layer.

[0124] <Preparation of positive electrode>

[0125] The first safety functional layer slurry / second safety functional layer slurry / positive electrode mixture layer slurry was coated on the aluminum foil, and the aluminum foil coated with the slurry was dried and rolled to prepare a positive electrode having an aluminum foil / first safety functional layer / second safety functional layer / positive electrode mixture layer structure. The first safety functional layer and the second safety functional layer each had a thickness of 4 μm, and the positive electrode mixture layer had a thickness of 80 μm.

[0126] Example 2, Comparative Example 1

[0127] A positive electrode was manufactured in the same manner as in Example 1, except that the composition of each layer was changed as shown in Table 1.

[0128] Comparative Example 2

[0129] <Manufacturing of slurry for safety functional layer>

[0130] The average particle size (D 50 ) is 1 μm and the BET specific surface area is 15 m 2 / g of LiFePO4 positive electrode active material, 2 wt% of carbon black as a conductive material and 10 wt% of PVdF as a binder were mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a slurry for a safety functional layer.

[0131] <Manufacturing of positive electrode mixture layer slurry>

[0132] The average particle size (D 50 ) is 4 μm and the BET specific surface area is 0.7 m 2 / gLiNi 0.8 Co 0.1 Mn 0.1 An O2 positive electrode active material, 2 wt% of carbon black as a conductive material, and 2 wt% of PVdF as a binder were mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a positive electrode mixture layer slurry.

[0133] <Preparation of positive electrode>

[0134] The safety function layer slurry / positive electrode mixture layer slurry was coated on the aluminum foil, and the aluminum foil coated with the slurry was dried and rolled to prepare a positive electrode having an aluminum foil / safety function layer / positive electrode mixture layer structure. The thickness of the safety function layer was 10 μm, and the thickness of the positive electrode mixture layer was 80 μm.

[0135] Comparative Examples 3 and 4

[0136] A positive electrode was manufactured in the same manner as in Comparative Example 2, except that the composition of each layer was changed as shown in Table 1.

[0137] Table 1

[0138]

[0139] Experimental Example 1: Evaluation of Capacity Retention Rate

[0140] A lithium secondary battery was manufactured using each of the positive electrodes manufactured in Examples 1 and 2 and the positive electrodes manufactured in Comparative Examples 1 to 4.

[0141] First, natural graphite, a carbon black conductive material, and a PVDF binder were mixed in an N-methylpyrrolidone solvent at a weight ratio of 85:10:5 to prepare a negative electrode forming slurry, which was then coated on a copper foil to prepare a negative electrode.

[0142] A porous polyethylene separator is provided between the negative electrode and each positive electrode manufactured according to Examples 1 and 2 and Comparative Examples 1 to 4 to manufacture each electrode assembly, and each electrode assembly is placed in a housing, and an electrolyte is injected into the housing to manufacture a lithium secondary battery. At this time, the electrolyte is prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.0M in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC volume ratio = 3 / 4 / 3).

[0143] At 25°C, under the conditions of charge end voltage 4.25V, discharge end voltage 2.5V and 0.5C / 0.5C, the lithium secondary batteries manufactured using the respective positive electrodes in Examples 1 and 2 and Comparative Examples 1 to 4 were subjected to 100 and 200 charge and discharge cycles, and the capacity retention rate was measured during charge and discharge. The results are shown in Table 2.

[0144] Experimental Example 2: Penetration Safety Evaluation

[0145] A lithium secondary battery was manufactured in the same manner as in Experimental Example 1 using each of the positive electrodes manufactured in Examples 1 and 2 and Comparative Examples 1 to 4.

[0146] Similarly, for the manufactured lithium secondary batteries, a metal object with a diameter of 3 mm was dropped at a speed of 80 mm / sec and penetrated the battery cell under the same conditions as the PV8450 certification conditions to evaluate whether ignition occurred. The results are shown in Table 2 below.

[0147] Experimental Example 3: Adhesion Measurement

[0148] Each of the positive electrodes manufactured in Examples 1 and 2 and Comparative Example 1 was cut to have a width of 25 mm and a length of 70 mm. Thereafter, it was then laminated under the conditions of 70° C. and 4 MPa, thereby manufacturing a sample.

[0149] The prepared sample was attached and fixed on a glass plate using double-sided tape, and at this time, the current collector was set to face the glass plate. The positive electrode mixture layer portion of the sample was peeled off at 90 degrees at a speed of 100 mm / min at 25°C, and the peeling strength at this time was measured in real time, and the average value was defined as the interface adhesion C between the second safety functional layer and the positive electrode mixture layer, and the results are shown in Table 2.

[0150] The adhesion force B between the second safety functional layer and the first safety functional layer and the adhesion force A between the first safety functional layer and the current collector were measured in the same manner as above, and the results are shown in Table 2.

[0151] Table 2

[0152]

[0153] Referring to the results of Table 2, the secondary battery including the positive electrode of the embodiment of the present invention has excellent penetration safety, but in the case of the batteries of Comparative Examples 1, 3 and 4, fire occurred during the penetration test. In the secondary battery including the positive electrode of Comparative Example 2, since a single-layer safety functional layer with the same composition as the first safety functional layer of Examples 1 and 2 is included, no fire occurred during the penetration test, but since the second safety functional layer is not included, the life characteristics are poor compared with the positive electrode of the embodiment. Therefore, the positive electrode of the present invention and the lithium secondary battery including the positive electrode have penetration safety and exhibit excellent life characteristics.

[0154] The above description is only an explanation of the technical idea of ​​the present invention, and those skilled in the art to which the present invention belongs may make various modifications and changes without departing from the essential features of the present invention. Therefore, the drawings disclosed in the present invention are not intended to limit the technical idea of ​​the present invention, but to illustrate the present invention, and the scope of the technical idea of ​​the present invention is not limited by these drawings. The scope of protection of the present invention shall be interpreted by the attached claims, and all technical ideas within the scope equivalent thereto shall be interpreted as included within the scope of the present invention.

Claims

1. A positive electrode for a lithium secondary battery, the positive electrode comprising: A safety functional layer disposed on the positive electrode current collector; as well as A positive electrode mixture layer provided on the safety function layer, The safety functional layer is formed by a multilayer structure of two or more layers, the multilayer structure comprising a first safety functional layer in contact with the positive electrode current collector, and a second safety functional layer arranged on the first safety functional layer, and wherein the second safety functional layer is obtained by mixing the composition of the first safety functional layer with the composition of the positive electrode mixture layer; wherein the content of the adhesive contained in the first safety functional layer is 5 wt % to 30 wt % of the total weight of the first safety functional layer; The first safety function layer includes a first positive electrode active material, and the positive electrode mixture layer includes a second positive electrode active material different from the first positive electrode active material.

2. The positive electrode according to claim 1, wherein The adhesion force A between the positive electrode current collector and the first safety functional layer is 150 N / m to 500 N / m.

3. The positive electrode according to claim 1, wherein The first positive electrode active material is lithium iron phosphate having an olivine structure represented by the following Chemical Formula 1: [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b wherein M is at least one selected from the group consisting of Al, Mg and Ti, X is at least one selected from the group consisting of F, S and N, -0.5≤a≤+0.5, 0≤x≤0.5, and 0≤b≤0.

1.

4. The positive electrode according to claim 1, wherein The second positive electrode active material is a lithium transition metal oxide represented by the following Chemical Formula 2: [Chemical formula 2] Li a Ni 1-x-y Co x Mr y M z O2 Here, M is at least one selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, and wherein 0.9≤a≤1.5, 0≤x≤1, 0≤y≤0.5, 0≤z≤0.1, and 0≤x+y≤1.

5. The positive electrode according to claim 1, wherein The second safety functional layer includes a first positive electrode active material, a second positive electrode active material, and a binder.

6. The positive electrode according to claim 5, wherein The second safety functional layer includes the first positive electrode active material and the second positive electrode active material in a weight ratio of 85:15 to 25:

75.

7. The positive electrode according to claim 1, wherein The adhesion force A between the positive electrode current collector and the first safety functional layer is greater than the adhesion force B between the first safety functional layer and the second safety functional layer.

8. The positive electrode according to claim 7, wherein The adhesion force B is equal to or greater than an adhesion force C between the second safety functional layer and the positive electrode mixture layer.

9. The positive electrode according to claim 1, wherein The binder is contained in the first safety functional layer in an amount of 7 to 25 wt % based on the total weight of the first safety functional layer.

10. The positive electrode according to claim 9, wherein The weight ratio of the binder contained in each layer gradually decreases as the distance from the current collector increases.

11. The positive electrode according to claim 10, wherein The content of the binder contained in the second safety functional layer is 0.5 wt % to 10 wt % based on the total weight of the second safety functional layer.

12. The positive electrode according to claim 1, wherein The total thickness of the safety functional layer is 1 to 20 μm.

13. The positive electrode according to claim 1, wherein The thickness of one safety functional layer is equal to or less than 7 μm.

14. The positive electrode according to claim 1, wherein The average particle size D of the first positive electrode active material 50 Equal to or less than 4 μm, and smaller than the average particle size D of the second positive electrode active material 50 .

15. The positive electrode according to claim 14, wherein The average particle size D of the first positive electrode active material 50 0.1 to 3μm.

16. The positive electrode according to claim 1, wherein The total elongation of the positive electrode is less than 1.4%.

17. A lithium secondary battery comprising: A positive electrode for a lithium secondary battery as claimed in claim 1; a separator; and a negative electrode.

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