Electrode for secondary battery having particle size gradient and composition gradient, and lithium secondary battery comprising same

Abstract

This electrode for a secondary battery comprises: a current collector; and an active material layer disposed on the current collector and including active material particles. The active material layer includes a surface layer and an intermediate layer disposed between the surface layer and the current collector and including lithium manganese phosphate iron. The intermediate layer includes at least a first intermediate layer adjacent to the current collector and a second intermediate layer disposed between the first intermediate layer and the surface layer and having a different particle diameter and a different composition from the first intermediate layer. The particle diameter of the active material particles of the first intermediate layer is 500 nm to 2,000 nm, and the manganese / iron ratio is 1 to 20. The particle diameter of the active material particles of the second intermediate layer is 100 nm to 500 nm, and the manganese / iron ratio is 0.05 to 1. The particle diameter of the active material particles of the surface layer is 10 nm to 200 nm, is smaller than the particle diameter of the active material particles of the second intermediate layer, and includes lithium iron phosphate.

Description

Electrode for secondary battery having particle size gradient and composition gradient and lithium secondary battery including the same

[0001] The present invention relates to an electrode for a secondary battery, and more particularly, to an electrode for a secondary battery having a particle size gradient and a composition gradient, and a lithium secondary battery including the same.

[0002] The cathode materials of lithium-ion secondary batteries are one of the most important materials that determine the capacity and performance of the battery.

[0003] Currently, layered structures based on lithium transition metal oxides, referred to as ternary systems, are widely applied. Ternary systems refer to nickel (Ni), cobalt (Co), and manganese (Mn) that can be included in transition metals.

[0004] However, transition metals have limited reserves and mining costs, which limit the reduction in manufacturing costs, and it is difficult to completely resolve stability issues such as thermal runaway in batteries using ternary materials.

[0005] Accordingly, olivine-based materials, which offer lower energy densities than conventional materials but can be implemented at lower costs, are attracting attention. Olivine-based materials can enhance price competitiveness by utilizing iron (Fe) instead of conventional nickel, cobalt, and manganese. Furthermore, by configuring the material based on polyanion bonding-based phosphates instead of oxides, thermal stability can be improved.

[0006] A representative olivine-structured material is lithium iron phosphate (LiFePO4, LFP), which is already being applied to major sales models by many electric vehicle manufacturers or is considering applying to future production models due to its price competitiveness (30-40% cheaper than ternary materials) and thermal stability. However, the low energy density compared to existing ternary materials needs to be improved.

[0007] The technical problem of the present invention is conceived from this point of view, and is to provide an electrode for a secondary battery with improved energy density without using a ternary material.

[0008] Another technical object of the present invention is to provide a lithium secondary battery including the above secondary battery electrode.

[0009] However, the problem to be solved by the present invention is not limited to the problem mentioned above, and may be expanded in various ways without departing from the spirit and scope of the present invention.

[0010] According to one embodiment of the present invention for realizing the above-described object, a secondary battery electrode includes a current collector and an active material layer disposed on the current collector and including active material particles. The active material layer includes a surface layer and an intermediate layer disposed between the surface layer and the current collector and including lithium manganese iron phosphate. The intermediate layer includes at least a first intermediate layer adjacent to the current collector and a second intermediate layer disposed between the first intermediate layer and the surface layer and having a different particle size and a different composition from those of the first intermediate layer. The particle size of the active material particles of the first intermediate layer is 500 nm to 2,000 nm, and the manganese / iron ratio is 1 to 20. The particle size of the active material particles of the second intermediate layer is 100 nm to 500 nm, and the manganese / iron ratio is 0.05 to 1. The particle size of the active material particles of the surface layer is 10 nm to 200 nm, smaller than the particle size of the active material particles of the second intermediate layer, and includes lithium iron phosphate.

[0011] According to one embodiment, the thicknesses of the first intermediate layer and the second intermediate layer are each 10 μm to 50 μm.

[0012] In one embodiment, the thickness of the entire intermediate layer is 100 μm to 150 μm.

[0013] According to one embodiment, the active material layer further includes one or more conductive materials 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.

[0014] According to one embodiment, the active material layer further includes one or more binders selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof.

[0015] According to one embodiment, the intermediate layer further comprises a third intermediate layer having a particle size smaller than that of the first intermediate layer and larger than that of the second intermediate layer, and having a manganese / iron ratio smaller than that of the first intermediate layer and larger than that of the second intermediate layer.

[0016] A lithium secondary battery according to one embodiment of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator disposed between the positive electrode and the negative electrode, and an electrolyte in contact with the positive electrode and the negative electrode.

[0017] According to embodiments of the present invention, a secondary battery having high energy / power density and improved lifespan characteristics can be manufactured without using a ternary transition metal compound through a particle size gradient and composition gradient of active material particles.

[0018] FIG. 1 is a cross-sectional view schematically illustrating an electrode for a secondary battery according to one embodiment of the present invention.

[0019] FIG. 2 is a drawing schematically illustrating an increase in voltage and capacity of an electrode for a secondary battery according to one embodiment of the present invention.

[0020] Figure 3 is a graph showing the results of the charge retention test of Example 1 (bilayer) and Comparative Example 1 (single layer).

[0021] Figure 4 is a graph showing the results of a charge retention test of Example 1 (thick film, bilayer), Example 2 (thin film, bilayer), Comparative Example 1 (thick film, single layer), and Comparative Example 2 (thick film, single layer).

[0022] Figure 5 is a graph showing voltage vs. Li / Li+ of Example 3 (h-LMFP), Comparative Example 3-1 (LFP), and Comparative Example 3-2 (LMFP64).

[0023] Figure 6 is a graph showing the energy density and nominal voltage of Example 3 (h-LMFP), Comparative Example 3-1 (LFP), and Comparative Example 3-2 (LMFP64).

[0024] Figure 7 is a graph showing the specific capacity and capacity retention rate for the C-rate of Example 3 (h-LMFP), Comparative Example 3-1 (LFP), and Comparative Example 3-2 (LMFP64).

[0025] Hereinafter, with reference to the attached drawings, a secondary battery electrode and a lithium secondary battery according to embodiments of the present invention will be described in detail. The present invention can be modified in various ways and can take various forms, and thus specific embodiments will be illustrated and described in detail in the text. However, this is not intended to limit the present invention to a specific disclosed form, but should be understood to include all modifications, equivalents, and substitutes included in the spirit and technical scope of the present invention. In the attached drawings, the dimensions of structures are illustrated larger than actual dimensions to ensure clarity of the present invention.

[0026] The terminology used in this application is only used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this application, it should be understood that the terms "comprise" or "have" indicate the presence of a feature, number, step, operation, component, or combination thereof described in the specification, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, or combinations thereof.

[0027] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and shall not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0028] FIG. 1 is a cross-sectional view schematically illustrating an electrode for a secondary battery according to one embodiment of the present invention.

[0029] Referring to Fig. 1, an electrode for a secondary battery includes a current collector and an active material layer. The electrode for the secondary battery may be a positive electrode (cathode). The active material layer includes active material particles. The active material layer is disposed between a surface layer (10) including lithium iron phosphate (LiFePO4) and the current collector, and lithium manganese iron phosphate (LiMn x Fe 1-x PO4(0 <x<1))을 포함하는 중간층(20)을 포함한다.

[0030] The current collector may include a material having high conductivity without causing chemical changes in the battery. For example, the current collector may include stainless steel, aluminum, nickel, titanium, calcined carbon, etc., and a material surface-treated with carbon, nickel, titanium, silver, etc. may also be used. For example, the thickness of the current collector may be 5 μm to 500 μm.

[0031] Since lithium manganese phosphate has a higher voltage band (about 4.1 V) than lithium iron phosphate, it can improve energy density compared to lithium iron phosphate, but lithium manganese phosphate has Mn 3+ ↔Mn 4+ The material's electrochemical properties are poor due to issues such as low oxidation / reduction reaction efficiency and manganese dissolution caused by the Jahn-Teller effect. Therefore, increasing the content of lithium manganese phosphate in the electrode may deteriorate output characteristics or cycle life.

[0032] In one embodiment, the active material layer has an overall particle size gradient and composition gradient. Within the active material layer, particles with a high manganese content are positioned in a region close to the current collector and have relatively large particle sizes. Accordingly, the specific surface area of ​​the layer including the active material particles with a high manganese content can be reduced, thereby reducing contact with the electrolyte. Consequently, by reducing manganese dissolution, the lifespan characteristics can be improved.

[0033] Additionally, within the active material layer, active material particles containing lithium iron phosphate or lithium manganese iron phosphate with a high iron content are arranged in a region close to the electrode surface and have a relatively small particle size. Accordingly, by increasing the contact area with the electrolyte, output characteristics can be increased and lithium ion kinetics can be activated.

[0034] According to one embodiment, the intermediate layer (20) may include at least two layers having different compositions and different particle sizes. For example, the intermediate layer (20) may include a first intermediate layer (22), a second intermediate layer (24), and a third intermediate layer (26). The first intermediate layer (22), the second intermediate layer (24), and the third intermediate layer (26) may have different particle sizes and compositions.

[0035] The first intermediate layer (22) may be the layer closest to the current collector within the intermediate layer (20). The second intermediate layer (24) may be disposed on the first intermediate layer (22), and the third intermediate layer (26) may be disposed on the second intermediate layer (24). For example, the first intermediate layer (22) may contact the current collector, and the third intermediate layer (26) may contact the surface layer (10). In the present application, “first”, “second”, etc. are not intended to refer to a specific configuration, and thus, the third intermediate layer may be described as being disposed between the first intermediate layer and the second intermediate layer.

[0036] The diameter (particle size) of the active material particles constituting the first intermediate layer (22) may be larger than the particle size of the second intermediate layer (24), the particle size of the third intermediate layer (26), and the particle size of the surface layer (10). In addition, the manganese / iron ratio of the active material particles constituting the first intermediate layer (22) is larger than the manganese / iron ratio of the active material particles constituting the second intermediate layer (24) and the active material particles constituting the third intermediate layer (26).

[0037] According to one embodiment, the particle size of the first intermediate layer (22) may be 500 nm to 2,000 nm. If the particle size of the first intermediate layer (22) is too small, manganese dissolution may increase, which may deteriorate the life characteristics of the battery. In addition, the manganese / iron ratio of the first intermediate layer (22) may be 3 to 20. If the manganese / iron ratio of the first intermediate layer (22) is excessive, the kinetic activity of lithium ions may be deteriorated. For example, the composition of the active material particles of the first intermediate layer (22) may be LiMn 0.8 Fe 0.2 It can be expressed as PO4.

[0038] The particle size of the second intermediate layer (24) may be larger than the particle size of the third intermediate layer (26) and the particle size of the surface layer (10). In addition, the manganese / iron ratio of the active material particles constituting the second intermediate layer (24) is larger than the manganese / iron ratio of the active material particles constituting the third intermediate layer (26).

[0039] According to one embodiment, the particle size of the second intermediate layer (24) may be 300 nm to 600 nm or 350 nm to 500 nm. In addition, the manganese / iron ratio of the second intermediate layer (24) may be 1 to 3. For example, the composition of the active material particles of the second intermediate layer (24) may be LiMn 0.6 Fe 0.4 It can be expressed as PO4.

[0040] The particle size of the third intermediate layer (26) may be larger than the particle size of the surface layer (10). According to one embodiment, the particle size of the third intermediate layer (26) may be 100 nm to 500 nm or 200 nm to 350 nm. In addition, the manganese / iron ratio of the third intermediate layer (26) may be 0.05 to 1. If the particle size of the third intermediate layer (26) is excessive or the iron content is insufficient, the output characteristics of the battery may be deteriorated. For example, the composition of the active material particles of the third intermediate layer (26) may be LiMn 0.4 Fe0.6 It can be expressed as PO4.

[0041] According to one embodiment, the active material particles of the surface layer (10) include LiFePO4, and the particle size may be 10 nm to 200 nm.

[0042] The thicknesses of the intermediate layers (10, 20, 30) may be the same or different. For example, the thickness of each of the intermediate layers may be 10 μm to 100 μm or 10 μm to 50 μm. If the thickness of the intermediate layers is too small or the number of intermediate layers having a composition gradient is small, a sufficient gradient may not be formed, resulting in a decrease in energy density.

[0043] According to one embodiment, lithium manganese iron phosphate (LiMn x Fe 1-x PO4(0 <x<1))을 포함하는 중간층(20)의 전체 두께는 50㎛ 내지 200㎛ , 바람직하게는 100㎛ 내지 150㎛ 일 수 있다. 상기 중간층의 두께가 과소할 경우, 에너지 밀도가 저하될 수 있으며, 상기 중간층의 두께가 과다할 경우, 출력 특성 또는 수명 특성이 저하될 수 있다.

[0044] Although the above has described an embodiment in which the intermediate layer is composed of three layers, the embodiments of the present invention are not limited thereto, and the intermediate layer may be composed of two layers or four or more layers. For example, the intermediate layer may include a first intermediate layer and a second intermediate layer. The first intermediate layer may be adjacent to the current collector. The second intermediate layer may be disposed between the first intermediate layer and the surface layer, and may have a smaller particle size than the first intermediate layer and may include a higher iron content than the first intermediate layer. For example, the particle size of the first intermediate layer may be 500 nm to 2,000 nm, and the manganese / iron ratio in the active material particles may be 1 to 20. The particle size of the second intermediate layer may be 100 nm to 500 nm, and the manganese / iron ratio in the active material particles may be 0.05 to 1.

[0045] The above active material layer may further include a conductive material, a binder, other additives, etc., as needed.

[0046] For example, in each of the surface layer (10) and the intermediate layer (20), the content of active material particles may be 80 wt% or more or 90 wt% or more, for example, 90 wt% to 98 wt%.

[0047] The above-mentioned conductive material is used to improve the electrical performance of the anode, and can be applied as a material commonly used in the art. Specifically, it can 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.

[0048] For example, in each of the surface layer (10) and the intermediate layer (20), the content of the conductive material may be 0.1 to 5 wt%.

[0049] For example, the binder may include at least one resin selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof.

[0050] For example, in each of the surface layer (10) and the intermediate layer (20), the content of the binder may be 1 to 10 wt%.

[0051] The electrode can be manufactured through various electrode manufacturing methods known in the art. For example, a composition including active material particles of a first intermediate layer may be applied and dried on the current collector to form the first intermediate layer, a composition including active material particles of the second intermediate layer may be applied and dried on the first intermediate layer to form the second intermediate layer, a composition including active material particles of the third intermediate layer may be applied and dried on the second intermediate layer to form the third intermediate layer, and a composition including active material particles of the surface layer may be applied and dried on the third intermediate layer to form the surface layer.

[0052] According to one embodiment of the present invention, a lithium secondary battery includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0053] The above positive electrode may be the same as the electrode for the secondary battery described above.

[0054] The above negative electrode may include a negative electrode active material. For example, the negative electrode active material may include one or more 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.

[0055] In addition to the negative electrode active material, the negative electrode may further include a conductive material, a binder, other additives, etc., as needed. The conductive material and the binder may be the same as or similar to those usable in the positive electrode.

[0056] As the above separator, an insulating thin film having high ion permeability and mechanical strength is used. The separator is not particularly limited as long as it is commonly used in the art, and specifically, sheets or non-woven fabrics made of chemically resistant and hydrophobic polypropylene; glass fiber; or polyethylene, etc. may be used. In some cases, a composite separator may be used in which inorganic particles / organic particles are coated with an organic binder polymer on a porous polymer substrate such as the sheet or non-woven fabric. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also function as the separator. In addition, the pore diameter of the separator may be on average 0.01 ㎛ to 10 ㎛, and the thickness may be 5 ㎛ to 300 ㎛.

[0057] The electrolyte contacts the anode and cathode and transfers ions between the anode and cathode.

[0058] As the above electrolyte, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, etc. that can be used in the manufacture of a lithium secondary battery can be used.

[0059] For example, the electrolyte may include an organic solvent and a lithium salt.

[0060] The organic solvent may be used without any particular limitation as long as it can serve as a medium through which ions involved in the electrochemical reaction of the battery can move. For example, the organic solvent may include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; Carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethyl alcohol and isopropyl alcohol; nitriles such as R-CN (wherein R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, and may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used.

[0061] In addition, the lithium salt may be used without any special limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. For example, the lithium salt may be LiPF 6, LiClO4, LiAsF 6,LiBF4, LiSbF6, LiAl04, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2 can be used.

[0062] According to embodiments of the present invention, a secondary battery having high energy / power density and improved lifespan characteristics can be manufactured without using a ternary transition metal compound through a particle size gradient and composition gradient of active material particles.

[0063] Below, the production and effects of embodiments of the present invention will be described with reference to specific experimental examples.

[0064] Example 1

[0065] On an aluminum foil (current collector) having a thickness of about 20 ㎛, a lower layer containing LiFePO4 particles having an average particle size of about 400 nm and having a thickness of about 50 ㎛ was formed, and then an upper layer containing LiFePO4 particles having an average particle size of about 100 nm and having a thickness of about 50 ㎛ was formed on the lower layer.

[0066] Comparative Example 1

[0067] An active material layer having a thickness of about 100 ㎛ and containing LiFePO4 particles having an average particle size of about 100 nm was formed on an aluminum foil having a thickness of about 20 ㎛.

[0068] Example 2

[0069] On an aluminum foil (current collector) having a thickness of about 20 ㎛, a lower layer containing LiFePO4 particles having an average particle size of about 400 nm and having a thickness of about 15 ㎛ was formed, and then an upper layer containing LiFePO4 particles having an average particle size of about 100 nm and having a thickness of about 15 ㎛ was formed on the lower layer.

[0070] Comparative Example 2

[0071] An active material layer having a thickness of about 30 μm and containing LiFePO4 particles having an average particle size of about 100 nm was formed on an aluminum foil having a thickness of about 20 μm.

[0072] Example 3

[0073] LiMn with an average particle size of about 100 nm on aluminum foil with a thickness of about 20 μm 0.7 Fe 0.3 After forming a lower layer containing PO4 particles and having a thickness of about 50 μm, LiMn having an average particle size of about 100 nm is formed on the lower layer. 0.6 Fe 0.4 An upper layer containing PO4 particles and having a thickness of approximately 50 μm was formed.

[0074] Comparative Example 3-1

[0075] An active material layer having a thickness of about 100 ㎛ and containing LiFePO4 particles having an average particle size of about 100 nm was formed on an aluminum foil having a thickness of about 20 ㎛.

[0076] Comparative Example 3-2

[0077] LiMn having an average particle size of about 100 nm on an aluminum foil having a thickness of about 20 μm 0.6 Fe 0.4 An active material layer containing PO4 particles and having a thickness of approximately 100 μm was formed.

[0078] After manufacturing a battery cell using the electrode (positive electrode) obtained in the above examples and comparative examples, a lithium negative electrode (about 700 μm thick), and an electrolyte (LiPF 1.2 M in ethylene carbonate (EC): ethyl methyl carbonate (EMC) (3:7 weight ratio) + fluoroethylene carbonate (FEC) 10 wt%), the battery characteristics were measured.

[0079] FIG. 3 is a graph showing the results of the charge retention test of Example 1 (bilayer) and Comparative Example 1 (single layer). FIG. 4 is a graph showing the results of the charge retention test of Example 1 (thick film, bilayer), Example 2 (thin film, bilayer), Comparative Example 1 (thick film, single layer), and Comparative Example 2 (thick film, single layer).

[0080] Referring to FIGS. 3 and 4, in a double-layer structure in which the particle size of the lower layer close to the current collector in the LFP layer is larger than that of the upper layer, the charge retention performance is increased compared to a single-layer structure, and this trend is more evident when a thick film having a thickness of 100 μm or more is formed.

[0081] Fig. 5 is a graph showing voltage vs. Li / Li+ of Example 3 (h-LMFP), Comparative Example 3-1 (LFP), and Comparative Example 3-2 (LMFP64). Fig. 6 is a graph showing energy density and nominal voltage of Example 3 (h-LMFP), Comparative Example 3-1 (LFP), and Comparative Example 3-2 (LMFP64). Fig. 7 is a graph showing specific capacity and capacity retention rate versus C-rate of Example 3 (h-LMFP), Comparative Example 3-1 (LFP), and Comparative Example 3-2 (LMFP64).

[0082] Referring to FIGS. 5 to 7, it was confirmed that in a double-layer structure having a composition gradient, energy density, nominal voltage, and capacity retention performance increased.

[0083] Although the present invention has been described above with reference to embodiments, it will be understood by those skilled in the art that various modifications and changes can be made to the present invention without departing from the spirit and scope of the present invention as set forth in the claims below.

[0084] The present invention can be used in lithium secondary batteries and their manufacture, and can be used in portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

Claims

1. The entire house; and It comprises an active material layer disposed on the above-mentioned collector and including active material particles, The above active material layer comprises a surface layer and an intermediate layer disposed between the surface layer and the current collector and containing lithium manganese iron phosphate, The intermediate layer includes at least a first intermediate layer adjacent to the collector and a second intermediate layer disposed between the first intermediate layer and the surface layer and having a different particle size and composition from those of the first intermediate layer. The particle size of the active material particles of the first intermediate layer is 500 nm to 2,000 nm, and the manganese / iron ratio is 1 to 20. The particle size of the active material particles of the second intermediate layer is 100 nm to 500 nm, and the manganese / iron ratio is 0.05 to 1. An electrode for a secondary battery, wherein the particle size of the active material particles of the surface layer is 10 nm to 200 nm and is smaller than the particle size of the active material particles of the second intermediate layer, and includes lithium iron phosphate.

2. An electrode for a secondary battery, wherein the thicknesses of the first intermediate layer and the second intermediate layer are each 10 ㎛ to 50 ㎛ in the first paragraph.

3. An electrode for a secondary battery, wherein the thickness of the entire intermediate layer in the first paragraph is 100 ㎛ to 150 ㎛.

4. An electrode for a secondary battery in the first paragraph, wherein the active material layer further includes at least one conductive material 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.

5. An electrode for a secondary battery in the first paragraph, wherein the active material layer further comprises at least one binder selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof.

6. An electrode for a secondary battery in the first paragraph, wherein the intermediate layer further includes a third intermediate layer having a particle size smaller than that of the first intermediate layer and larger than that of the second intermediate layer, and a manganese / iron ratio smaller than that of the first intermediate layer and larger than that of the second intermediate layer.

7. An electrode for a secondary battery in the first paragraph, wherein the current collector comprises at least one selected from the group consisting of stainless steel, aluminum, nickel, titanium, and carbon.

8. An electrode for a secondary battery, wherein the thickness of the current collector in the first paragraph is 5 ㎛ to 500 ㎛.

9. In the first paragraph, the active material particles of the first intermediate layer are LiMn. 0.8 Fe 0.2 An electrode for a secondary battery having a composition of PO4.

10. In the 9th paragraph, the active material particles of the second intermediate layer are LiMn. 0.4 Fe 0.6 An electrode for a secondary battery having a composition of PO4.

11. A positive electrode including a positive active material, a negative electrode including a negative active material, a separator disposed between the positive electrode and the negative electrode, and an electrolyte in contact with the positive electrode and the negative electrode. The above positive electrode comprises a current collector and an active material layer disposed on the current collector and including active material particles, The above active material layer comprises a surface layer and an intermediate layer disposed between the surface layer and the current collector and containing lithium manganese iron phosphate, The intermediate layer includes at least a first intermediate layer adjacent to the collector and a second intermediate layer disposed between the first intermediate layer and the surface layer and having a different particle size and composition from those of the first intermediate layer. The particle size of the active material particles of the first intermediate layer is 500 nm to 2,000 nm, and the manganese / iron ratio is 1 to 20. The particle size of the active material particles of the second intermediate layer is 100 nm to 500 nm, and the manganese / iron ratio is 0.05 to 1. A lithium secondary battery, wherein the particle size of the active material particles of the surface layer is 10 nm to 200 nm and is smaller than the particle size of the active material particles of the second intermediate layer, and includes lithium iron phosphate.

12. A lithium secondary battery, wherein in claim 11, the thicknesses of the first intermediate layer and the second intermediate layer are each 10 ㎛ to 50 ㎛.

13. A lithium secondary battery according to claim 11, wherein the thickness of the entire intermediate layer is 100 ㎛ to 150 ㎛.

14. A lithium secondary battery in claim 11, wherein the active material layer further includes at least one conductive material 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.

15. A lithium secondary battery in claim 11, wherein the active material layer further comprises at least one binder selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinylidene fluoride (PVdF), polyacrylonitrile, polymethylmethacrylate, and copolymers thereof.

16. A lithium secondary battery in claim 11, wherein the intermediate layer further includes a third intermediate layer having a particle size smaller than that of the first intermediate layer and larger than that of the second intermediate layer, and a manganese / iron ratio smaller than that of the first intermediate layer and larger than that of the second intermediate layer.

17. A lithium secondary battery in claim 11, wherein the collector comprises at least one selected from the group consisting of stainless steel, aluminum, nickel, titanium, and carbon.

18. A lithium secondary battery according to claim 11, wherein the thickness of the entire collector is 5 ㎛ to 500 ㎛.

19. In the 11th paragraph, the active material particles of the first intermediate layer are LiMn. 0.8 Fe 0.2 A lithium secondary battery having a composition of PO4.

20. In clause 19, the active material particles of the second intermediate layer are LiMn. 0.4 Fe 0.6 A lithium secondary battery having a composition of PO4.

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