Method for manufacturing a positive electrode, a positive electrode, and a lithium secondary battery including the positive electrode

Abstract

The present invention includes the steps of: preparing a positive electrode slurry containing a positive electrode active material comprising lithium iron phosphate particles in a secondary particle form in which a plurality of primary particles are aggregated; applying the positive electrode slurry onto a positive electrode current collector; and rolling the applied positive electrode slurry to form a positive electrode active material layer, wherein the particle cracking strength of the lithium iron phosphate particles in the secondary particle form is 4 kgf / mm². 2 ~20kgf / mm 2 The present invention provides a method for manufacturing a positive electrode, wherein the ratio of the surface roughness Rz1 of the positive electrode current collector before rolling to the surface roughness Rz2 of the positive electrode current collector after rolling is 0.5 or less.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a positive electrode, a positive electrode, and a lithium secondary battery including the positive electrode. [Background technology]

[0002] With the development of the information society, personal IT devices and computer networks have advanced, and consequently, society as a whole has become more dependent on electrical energy. Therefore, there is a need to develop technologies for efficiently storing and utilizing electrical energy.

[0003] Rechargeable batteries are the most suitable technology for a wide range of applications among the technologies currently under development. Among these rechargeable batteries, there is growing interest in lithium-ion batteries, which not only can be miniaturized to a degree suitable for personal IT devices, but also have the highest energy density.

[0004] Typically, lithium-ion batteries are manufactured by injecting or impregnating an electrode assembly, consisting of a positive electrode, a negative electrode, and a porous separator, with a non-aqueous electrolyte.

[0005] For the negative electrode active material of such lithium secondary batteries, carbon-based active materials and silicon-based active materials are being considered. On the other hand, for the positive electrode active material, the use of lithium-containing cobalt oxide, layered crystalline LiMnO2, spinel crystalline LiMn2O4, and lithium-containing nickel oxide (LiNiO2) is being considered. In recent years, the use of lithium iron phosphate (e.g., LiFePO4)-based compounds, which have excellent thermal stability and are relatively inexpensive, has been considered as a positive electrode active material.

[0006] On the other hand, electrodes such as the positive electrode and negative electrode include a current collector and an active material layer, and the active material layer contains an active material (positive electrode active material or negative electrode active material). In this case, the current collector plays the role of a passage that transmits electrons from the outside or receives electrons from the active material and transmits them to the outside so that an electrochemical reaction occurs in the active material layer.

[0007] At this time, in order to improve the high-rate discharge characteristics of the electrode, it is an important issue to increase the contact area between the current collector and the active material and reduce the contact resistance. From this point of view, methods such as roughening the surface of the current collector have been studied. However, for such roughening, an additional process and cost may occur because it requires applying another inorganic material or an etching process. Also, even if the current collector is roughened, if the sizes of the active material and the roughened current collector do not match, voids will occur between the active material and the roughened current collector, which will act as a disadvantageous factor in high-rate discharge characteristics.

[0008] On the other hand, in order to increase the contact area between the current collector and the active material, a primer layer having a specific surface roughness may be introduced between the current collector and the active material layer. In this case, an additional process of introducing the primer layer is required, which causes cost problems and is disadvantageous in achieving the target capacity due to an unnecessary increase in volume.

Summary of the Invention

Problems to be Solved by the Invention

[0009] One problem of the present invention is to solve the above problems, and to provide a method for manufacturing a positive electrode that can increase the contact area between the positive electrode current collector and the positive electrode active material to a remarkable level and realize a positive electrode having excellent high-rate discharge characteristics and capacity characteristics.

[0010] Another problem of the present invention is to reduce the contact resistance between the positive electrode current collector and the positive electrode active material and provide a positive electrode having excellent high-rate discharge characteristics and capacity characteristics.

[0011] Still another problem of the present invention is to provide a lithium secondary battery including the aforementioned positive electrode.

Means for Solving the Problems

[0012] [1] The present invention comprises the steps of preparing a positive electrode slurry containing a positive electrode active material comprising lithium iron phosphate particles in a secondary particle form in which a plurality of primary particles are aggregated; applying the positive electrode slurry onto a positive electrode current collector; and rolling the applied positive electrode slurry to form a positive electrode active material layer, wherein the particle cracking strength of the lithium iron phosphate particles in the secondary particle form is 4 kgf / mm 2 ~20kgf / mm 2 The present invention provides a method for manufacturing a positive electrode, wherein the ratio of the surface roughness Rz1 of the positive electrode current collector before rolling to the surface roughness Rz2 of the positive electrode current collector after rolling is 0.5 or less.

[0013] [2] The present invention provides a method for manufacturing a positive electrode as described in [1], wherein the rolling process causes the positive electrode active material contained in the coated positive electrode slurry to roughen the positive electrode current collector.

[0014] [3] The present invention relates to the average particle size (D) of lithium iron phosphate particles in the secondary particle form. 50 The present invention provides a method for manufacturing a positive electrode according to [1] or [2], wherein the diameter of the electrode is 7 μm to 30 μm.

[0015] [4] The present invention relates to the average particle size (D) of the primary particles. 50 The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [3] above, wherein the diameter is 0.2 μm to 3 μm.

[0016] [5] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [4] above, wherein the lithium iron phosphate particles in secondary particle form are broken into primary particles by rolling the coated positive electrode slurry.

[0017] [6] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [5] above, wherein the surface roughness Rz2 of the positive electrode current collector after rolling is 3 μm or more.

[0018] [7] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [6] above, wherein the surface roughness Rz1 of the positive electrode current collector before rolling is 1.5 μm or less.

[0019] [8] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [7] above, wherein the rolling ratio of the positive electrode active material layer is 30% or more.

[0020] [9] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [8] above, wherein the rolling is performed by a roll press and the linear pressure during the roll press is 30 kN / cm to 80 kN / cm.

[0021]

[10] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to [9] above, wherein the current collector comprises aluminum.

[0022]

[11] The present invention provides a method for manufacturing a positive electrode according to any one of [1] to

[10] , further comprising the step of drying the coated positive electrode slurry between the application of the positive electrode slurry and the rolling of the applied positive electrode slurry.

[0023]

[12] The present invention relates to a positive electrode comprising a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein the surface roughness Rz2 of the positive electrode current collector is 3 μm or more, the positive electrode active material layer contains a positive electrode active material, the positive electrode active material contains lithium iron phosphate particles, and the MP resistance measured with a multi probe resistivity measuring device on a sample of the positive electrode punched out in a sheet shape of 5 cm horizontally and 5 cm vertically is 0.10 Ω·cm 2 The following positive electrode is provided.

[0024]

[13] The present invention provides the positive electrode described in

[12] , wherein the porosity of the positive electrode active material layer is 25% to 45%.

[0025]

[14] The present invention provides the positive electrode described in

[12] or

[13] , wherein the lithium iron phosphate particles exist in the form of primary particles or as a mixture of primary and secondary particles.

[0026]

[15] The present invention provides a positive electrode according to any one of

[12] to

[14] , wherein, when the lithium iron phosphate particles are a mixture of primary particles and secondary particles, the content of the primary particles is 90% by weight or more and less than 100% by weight in the lithium iron phosphate particles.

[0027]

[16] The present invention provides a positive electrode according to any one of

[12] to

[15] , wherein the thickness of the positive electrode active material layer is 50 μm to 500 μm.

[0028]

[17] The present invention provides a positive electrode according to any one of

[12] to

[16] , wherein the positive electrode current collector and the positive electrode active material layer are in direct contact.

[0029]

[18] The present invention provides a lithium secondary battery comprising a positive electrode according to any one of

[12] to

[17] above, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. [Effects of the Invention]

[0030] According to the method for manufacturing a positive electrode according to the present invention, a positive electrode active material containing lithium iron phosphate particles in a secondary particle form having a specific particle cracking strength is added to a positive electrode slurry, and then the positive electrode slurry is rolled onto a positive electrode current collector to form a positive electrode active material layer, and the rolling process ensures that the ratio of the surface roughness of the current collector before and after rolling satisfies a specific range. The lithium iron phosphate particles in a secondary particle form are roughened on the positive electrode current collector by the rolling process and then converted into primary particles, thereby increasing the contact area between the positive electrode active material and the positive electrode current collector and minimizing the generation of voids between the positive electrode active material and the positive electrode current collector. Therefore, a positive electrode realized by the method for manufacturing a positive electrode according to the present invention can have excellent high-rate discharge characteristics and high capacity.

[0031] Furthermore, the positive electrode according to the present invention is characterized in that the surface roughness of the positive electrode current collector meets a specific range, and the MP resistance of the positive electrode measured under specific conditions meets a specific range. Meeting the MP resistance range in this way means that the contact area between the positive electrode active material containing lithium iron phosphate particles and the positive electrode current collector is large, and the contact resistance is small. The positive electrode according to the present invention and the lithium secondary battery containing it can have excellent high-rate discharge characteristics and high capacity. [Brief explanation of the drawing]

[0032] [Figure 1] This is an SEM image of the cross-section of the positive electrode in Example 1. [Figure 2] This is an SEM image of the cross-section of the positive electrode of Comparative Example 1. [Figure 3] This is an SEM image of the cross-section of the positive electrode of Comparative Example 2. [Figure 4] This is an SEM image of the cross-section of the positive electrode of Comparative Example 3. [Figure 5] This shows the results of a three-dimensional microscope observation of the positive electrode current collector according to Example 1. [Figure 6] This shows the results of a three-dimensional microscope observation of the positive electrode current collector related to Comparative Example 1. [Modes for carrying out the invention]

[0033] First, before describing the present invention, the terms and words used in this specification and the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings. Rather, they should be interpreted in a manner consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may appropriately define the concepts of terms in order to best describe their invention.

[0034] On the other hand, the terms used herein are used solely to describe exemplary embodiments and are not intended to limit the invention. Unless the context clearly indicates otherwise, singular expressions include plural expressions.

[0035] In this specification, terms such as “includes,” “equip,” or “have” are intended to specify the presence of implemented features, figures, steps, components, or combinations thereof, and should be understood not to preemptively exclude the presence or possibility of adding one or more other features, figures, steps, components, or combinations thereof.

[0036] In this specification, "%" means weight percent unless explicitly indicated otherwise.

[0037] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 The particle size can be measured, for example, by laser diffraction. This laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can produce results with high reproducibility and high resolution.

[0038] In this specification, "primary particle" means a single particle, and "secondary particle" means an aggregate formed by the aggregation of multiple primary particles through an intentional granulation or bonding process.

[0039] The present invention will be described in more detail below.

[0040] Method for manufacturing a positive electrode The present invention provides a method for manufacturing a positive electrode. Specifically, the method for manufacturing the positive electrode may be a method for manufacturing a positive electrode for a lithium secondary battery.

[0041] Specifically, the method for manufacturing a positive electrode according to the present invention includes the steps of preparing a positive electrode slurry containing a positive electrode active material including lithium iron phosphate particles in the form of secondary particles in which a plurality of primary particles are aggregated, applying the positive electrode slurry onto a positive electrode current collector, and rolling the applied positive electrode slurry to form a positive electrode active material layer. The particle cracking strength of the lithium iron phosphate particles in the form of secondary particles is 4 kgf / mm 2 ~20 kgf / mm 2 and the ratio of the surface roughness Rz1 of the positive electrode current collector before rolling to the surface roughness Rz2 of the positive electrode current collector after rolling is 0.5 or less.

[0042] According to the method for manufacturing a positive electrode according to the present invention, after a positive electrode active material including lithium iron phosphate particles in the form of secondary particles having a specific particle cracking strength is included in a positive electrode slurry, the positive electrode slurry is rolled onto a positive electrode current collector to form a positive electrode active material layer. By the rolling process, the ratio of the surface roughness of the current collector before and after rolling satisfies a specific range. The lithium iron phosphate particles in the form of secondary particles are primary particleized after the positive electrode current collector is roughened by the rolling process, thereby increasing the contact area between the positive electrode active material and the positive electrode current collector and minimizing the generation of voids between the positive electrode active material and the positive electrode current collector. Therefore, the positive electrode realized by the method for manufacturing a positive electrode according to the present invention has excellent high-rate discharge characteristics and can have a high capacity.

[0043] According to the method for manufacturing a positive electrode according to the present invention, first, a positive electrode slurry is prepared.

[0044] The positive electrode slurry contains a positive electrode active material. The positive electrode active material includes lithium iron phosphate particles in the form of secondary particles in which a plurality of primary particles are aggregated.

[0045] The lithium iron phosphate particles may contain a compound represented by the following chemical formula A.

[0046] [Chemical formula A] Li 1+a Fe 1-s M s (PO 4-b )Xb

[0047] In the above chemical formula A, M is one or more elements selected from Co, Ni, Mn, Al, Mg, Ti, and V, and X is F, S, or N, with 0 ≤ s ≤ 0.5; -0.5 ≤ a ≤ +0.5; 0 ≤ b ≤ 0.1.

[0048] The aforementioned chemical formula A can specifically be represented as LiFePO4 (a=0, s=0, and b=0).

[0049] The positive electrode active material includes lithium iron phosphate particles in a secondary particle form in which multiple primary particles are aggregated. In this case, the positive electrode active material may refer to the positive electrode active material before coating the positive electrode slurry, or before rolling the coated positive electrode slurry.

[0050] The particle fracture strength (or compressive fracture strength) of the lithium iron phosphate particles in the aforementioned secondary particle form is 4 kgf / mm². 2 ~20kgf / mm 2 The above range is satisfied, so that when the coated positive electrode slurry is rolled, the lithium iron phosphate particles in secondary particle form can roughen the positive electrode current collector and then break to become primary particles, maximizing the contact area of ​​the roughened positive electrode current collector, and thus significantly improving the high-rate discharge characteristics of the positive electrode. Preferably, the particle cracking strength of the lithium iron phosphate particles in secondary particle form is 5 kgf / mm 2 ~15 kgf / mm² 2 Specifically, 8 kgf / mm 2 ~12 kgf / mm² 2 That's fine.

[0051] The particle fracture strength of the lithium iron phosphate particles in the secondary particle form is 4 kgf / mm². 2 If the particle cracking strength of the lithium iron phosphate particles is less than 20 kgf / mm², cracking will occur before the lithium iron phosphate particles can roughen the surface of the positive electrode current collector, and the desired effect of roughening the surface of the positive electrode current collector or increasing the contact area between the positive electrode active material and the current collector cannot be achieved. 2If the particle size exceeds a certain limit, the particles will not break even when rolled, which actually reduces the contact area between the positive electrode active material and the positive electrode current collector, leading to problems such as the creation of air gaps between the positive electrode active material and the positive electrode current collector.

[0052] The particle cracking strength (St) of the present invention may be measured using Shimadzu's microcompression testing machine MCT-W and determined by the formula of Hiramatsu et al., shown in Equation 1 below (Reference [Journal of the Mining Society of Japan, Vol. 81, No. 932, December 1965, pp. 1024-1030]).

[0053] [Formula 1] St = 0.28P / πd 2 (P: Load applied to the particle [kgf], d: Particle diameter (mm))

[0054] The specific measurement conditions are as follows: 1. Test indenter: FLAT50 2. Measurement mode: Compression test 3. Load applied to the particle: 20.00 [mN] 4. Load speed: 0.892405[mN / sec]

[0055] The average particle size (D) of the lithium iron phosphate particles in the secondary particle form. 50 The particle size (D) of the primary particles may be 7 μm to 30 μm, more specifically 10 μm to 20 μm, and more specifically 12 μm to 18 μm. 50 The particle size may be 0.2 μm to 3.0 μm, more specifically 0.2 μm to 2.0 μm, more specifically 0.3 μm to 1.5 μm, and even more specifically 0.3 μm to 1 μm.

[0056] The positive electrode active material may further include a carbon coating layer located on the surface of the lithium iron phosphate particles. The carbon coating layer can be introduced for purposes such as protecting the lithium iron phosphate particles and improving electrical conductivity.

[0057] The positive electrode active material may be included in the positive electrode slurry in an amount of 80% to 99% by weight, based on the weight of the solid content of the positive electrode slurry.

[0058] The positive electrode slurry may further contain, along with the positive electrode active material, a binder, a conductive material, and / or a solvent.

[0059] The binder is a component that assists in the bonding of the active material to the conductive material and to the current collector, and specifically may contain at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0060] The binder may be included in the positive electrode slurry in an amount of 1% to 20% by weight, preferably 1.2% to 10% by weight, based on the weight of the solid content of the positive electrode slurry, in order to ensure sufficient bonding strength between components such as the positive electrode active material.

[0061] The conductive material is used to assist and improve the conductivity of a secondary battery and is not particularly limited as long as it does not cause chemical changes and is conductive. Specifically, the conductive material may include at least one selected from the group consisting of graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjenblack (registered trademark), channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0062] The conductive material may be included in the positive electrode slurry in an amount of 1% to 20% by weight, preferably 1.2% to 10% by weight, based on the weight of the solid content of the positive electrode slurry, in order to ensure sufficient electrical conductivity.

[0063] The aforementioned solvent may include organic solvents such as NMP (N-methyl-2-pyrrolidone).

[0064] The solid content of the positive electrode slurry may be 40% to 90% by weight, specifically 50% to 80% by weight.

[0065] Next, the positive electrode slurry is applied onto the positive electrode current collector.

[0066] The positive electrode slurry may be applied to at least one side of the positive electrode current collector, specifically, to one or both sides of the positive electrode current collector.

[0067] The thickness of the positive electrode current collector may typically be 3 μm to 500 μm, more specifically 5 μm to 30 μm, and more specifically 10 μm to 25 μm.

[0068] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the positive electrode current collector can be made of aluminum, stainless steel, copper, nickel, titanium, calcined carbon, copper or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy. More specifically, the positive electrode current collector may contain aluminum.

[0069] For coating the positive electrode slurry, any method known in the art, such as spray coating, spin coating, or doctor blade coating, can be used without limitation.

[0070] Next, the coated positive electrode slurry is rolled to form a positive electrode active material layer.

[0071] The rolling process allows the positive electrode active material (lithium iron phosphate particles in secondary particle form) contained in the coated positive electrode slurry to roughen the positive electrode current collector. The rolling process allows the lithium iron phosphate particles in secondary particle form to press against the positive electrode current collector, roughening its surface and forming an uneven structure. Furthermore, the rolling process allows the lithium iron phosphate particles in secondary particle form to be converted into primary particles after roughening the positive electrode current collector. In particular, since the lithium iron phosphate particles in secondary particle form have the aforementioned level of particle cracking strength, they crack after sufficiently roughening the positive electrode current collector, thereby minimizing the void between the positive electrode active material and the current collector, and significantly increasing the contact area between the positive electrode active material and the current collector.

[0072] In the present invention, the process may further include a step of drying the coated positive electrode slurry between the application of the positive electrode slurry and the rolling of the applied positive electrode slurry.

[0073] The drying may be carried out at 80°C to 150°C, but is not limited to this range.

[0074] During the rolling process, the rolling ratio of the positive electrode active material layer may be 30% or more, specifically 30% to 50%, and more specifically 35% to 48%. When the rolling ratio is within this range, sufficient surface roughening occurs due to the positive electrode active material pressing against the positive electrode current collector, and sufficient primary particle formation of lithium iron phosphate particles occurs, thereby minimizing the void between the active material and the current collector.

[0075] The rolling ratio can be calculated using the following formula 2.

[0076] [Formula 2] Rolling ratio (%) = {(Thickness of positive electrode active material layer before rolling - Thickness of positive electrode active material layer after rolling) / Thickness of positive electrode active material layer before rolling} × 100

[0077] In formula 2, the thickness of the positive electrode active material layer before rolling may mean the thickness of the coated positive electrode slurry or the thickness of the dried positive electrode slurry.

[0078] The rolling may be carried out by a roll press. Specifically, the roll press process is performed by passing a positive electrode between two rolls, and the rolling rate can be adjusted by adjusting the gap between the two rolls.

[0079] The linear pressure during the roll press may be 30kN / cm to 80kN / cm, specifically 40kN / cm to 60kN / cm.

[0080] In the present invention, the ratio of the surface roughness Rz1 of the positive electrode current collector before rolling to the surface roughness Rz2 of the positive electrode current collector after rolling is 0.5 or less. If the ratio of the surface roughness Rz1 of the positive electrode current collector before rolling to the surface roughness Rz2 of the positive electrode current collector after rolling exceeds 0.5, it means that the surface roughening of the positive electrode current collector by the secondary particle form lithium iron phosphate particles pressing against the positive electrode current collector has not occurred sufficiently, making it difficult to improve the contact area between the positive electrode active material and the positive electrode current collector and to improve the high-rate discharge performance. In particular, the change in surface roughness within the above ratio range is difficult to achieve by methods such as etching of the positive electrode current collector, but can be achieved by pressing the positive electrode current collector against the secondary particle form lithium iron phosphate particles having the aforementioned specific particle cracking strength, particle cracking, etc. The ratio of the surface roughness Rz1 of the positive electrode current collector before rolling to the surface roughness Rz2 of the positive electrode current collector after rolling may be 0.4 or less, more specifically 0.35 or less, and even more specifically 0.38 or less. On the other hand, when lithium iron phosphate particles in secondary particle form having excessively high particle cracking strength are used, the above level of surface roughness change can be achieved to some extent, but this may lead to problems such as a decrease in the contact area between the positive electrode active material and the positive electrode current collector, and the creation of air gaps between the positive electrode active material and the positive electrode current collector, which may result in increased resistance.

[0081] In this specification, surface roughness (Rz) can be measured using an optical profiler from NanoSystems Inc. Specifically, using the optical profiler, surface roughness is measured from the Topo image obtained by selecting five measurement areas for each sample at a measurement magnification (e.g., 500x).

[0082] On the other hand, the surface roughness Rz2 of the rolled positive electrode current collector can be measured after immersing the manufactured positive electrode in a solvent (e.g., NMP), heating it at 80°C to 150°C (specifically 100°C) for 1 to 10 hours (specifically 2 hours), and peeling and removing the positive electrode active material layer to obtain the positive electrode current collector.

[0083] The surface roughness Rz2 of the rolled positive electrode current collector may be 3 μm or more, specifically 4 μm to 1,000 μm, more specifically 4 μm to 500 μm, and even more specifically 4 μm to 20 μm. The surface roughness Rz1 of the rolled positive electrode current collector may be 1.5 μm or less, more specifically 0.5 μm to 1.5 μm, and even more specifically 1 μm to 1.5 μm.

[0084] The thickness of the positive electrode active material layer may be 50 μm to 500 μm, specifically 100 μm to 300 μm, and more specifically 100 μm to 200 μm.

[0085] positive electrode Furthermore, the present invention provides a positive electrode, specifically a positive electrode for a lithium secondary battery. Specifically, the positive electrode may be a positive electrode manufactured by the positive electrode manufacturing method described above.

[0086] Specifically, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, wherein the surface roughness Rz2 of the positive electrode current collector is 3 μm or more, the positive electrode active material layer contains a positive electrode active material, the positive electrode active material contains lithium iron phosphate particles, and the MP resistance measured with a multi probe resistivity measuring device in a sample punched out of the positive electrode in a sheet shape measuring 5 cm horizontally and 5 cm vertically is 0.1 Ω·cm. 2The following characteristics apply:

[0087] The positive electrode according to the present invention has a surface roughness Rz2 of 3 μm or more for the positive electrode current collector, and its MP resistance satisfies a specific range. A positive electrode having such characteristics means that the contact area between the positive electrode active material and the positive electrode current collector is large, and the gap formed between them is minimized. This makes it possible to realize a positive electrode and lithium secondary battery with excellent high-rate discharge characteristics.

[0088] The surface roughness Rz2 of the positive electrode current collector is 3 μm or more, specifically 4 μm to 1,000 μm, more specifically 4 μm to 500 μm, and even more specifically 4 μm to 20 μm. If the surface roughness Rz2 of the positive electrode current collector is less than 3 μm, the surface roughening of the positive electrode current collector does not occur sufficiently, making it difficult to improve the contact area between the positive electrode active material and the positive electrode current collector and to improve the high-rate discharge performance.

[0089] The explanation of the positive electrode current collector is as described above.

[0090] The positive electrode active material layer is disposed on the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on at least one side of the positive electrode current collector, or more specifically, on one or both sides of the positive electrode current collector.

[0091] Specifically, the positive electrode current collector and the positive electrode active material layer may be in direct contact with each other. More specifically, the positive electrode current collector and the positive electrode active material layer may be in direct contact without any other substrate such as a primer layer interposed between them.

[0092] The positive electrode active material contains lithium iron phosphate particles.

[0093] The lithium iron phosphate particles may exist in the form of primary particles or as a mixture of primary and secondary particles. When the lithium iron phosphate particles are a mixture of primary and secondary particles, the content of primary particles may be 90% by weight or more and less than 100% by weight, more specifically 95% by weight or more and less than 100% by weight in the lithium iron phosphate particles.

[0094] In the lithium iron phosphate particles, the average particle size (D) of the secondary particles 50 The particle size (D) of the primary particles may be 7 μm to 30 μm, more specifically 10 μm to 20 μm, and more specifically 12 μm to 18 μm. 50 The particle size may be 0.2 μm to 3.0 μm, more specifically 0.2 μm to 2.0 μm, more specifically 0.3 μm to 1.5 μm, and even more specifically 0.3 μm to 1 μm.

[0095] The other positive electrode active materials are described above.

[0096] The positive electrode active material layer may further include a binder and / or a conductive material together with the positive electrode active material.

[0097] The binder is a component that assists in the bonding of the active material to the conductive material and to the current collector, and specifically may contain at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0098] The binder may be present in an amount of 1% to 20% by weight, preferably 1.2% to 10% by weight, based on the weight of the positive electrode active material layer, in order to ensure sufficient bonding strength between components such as the positive electrode active material.

[0099] The conductive material is used to assist and improve the conductivity of a secondary battery and is not particularly limited as long as it does not cause chemical changes and is conductive. Specifically, the conductive material may include at least one selected from the group consisting of graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjenblack (registered trademark), channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0100] The conductive material may be included in an amount of 1% to 20% by weight, preferably 1.2% to 10% by weight, based on the weight of the positive electrode active material layer, in order to ensure sufficient electrical conductivity.

[0101] The explanation of the other positive electrode active material layers is as described above.

[0102] A sample of the aforementioned positive electrode punched out into a sheet measuring 5 cm horizontally and 5 cm vertically yielded an MP resistance of 0.1 Ω·cm when measured with a multi-probe resistivity measuring device. 2 The following is true: MP resistance is 0.1Ω·cm 2 If the value exceeds this, it can be concluded that the contact area between the positive electrode current collector and the positive electrode active material is not sufficiently secured, and in this case, it will be difficult to improve the desired high-rate discharge performance.

[0103] The aforementioned MP resistance is 0.1Ω·cm 2 Specifically, the following is 0.01 Ω·cm 2 ~0.1Ω·cm 2 More specifically, 0.01 Ω·cm 2 ~0.07Ω·cm 2 That's fine.

[0104] The method for achieving the above MP resistance range is not particularly limited, and for example, the surface roughness of the positive electrode current collector can be adjusted to the above level, or the average particle size (D) of the lithium iron phosphate particles used can be adjusted. 50 Various methods may be used, such as adjusting the ). Specifically, the MP resistance range may be achieved by the positive electrode manufacturing method described above.

[0105] The porosity of the positive electrode may be 25% to 45%, specifically 30% to 40%. When it is within this range, sufficient movement paths for lithium ions in the positive electrode active material are secured, and the contact area between the positive electrode active material and the positive electrode active material, or between the positive electrode active material and the positive electrode current collector, is increased, which is advantageous in improving high-rate discharge performance.

[0106] The porosity of the positive electrode can be calculated using the following formula 3.

[0107] [Formula 3] Porosity (%) = {1 - (electrode density of positive electrode / true density of positive electrode)} × 100

[0108] In equation 3 above, the true density of the positive electrode is the density of the positive electrode active material layer measured when the positive electrode is taken in a fixed size and pressed in a press device until the thickness of the positive electrode no longer changes, and the electrode density of the positive electrode is the density of the positive electrode active material layer measured when the positive electrode is taken in a fixed size.

[0109] Lithium-ion battery Furthermore, the present invention provides a lithium secondary battery. Specifically, the lithium secondary battery may include the positive electrode described above.

[0110] More specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte.

[0111] The lithium secondary battery can be manufactured by a method comprising the steps of: manufacturing an electrode assembly including a positive electrode, a negative electrode, and a separator; housing the electrode assembly in a battery case; manufacturing a non-aqueous electrolyte including a lithium salt, an organic solvent, and an additive; and injecting or impregnating the battery case with the non-aqueous electrolyte.

[0112] The positive electrode is as described above, and the other components will be explained below.

[0113] (1) Negative electrode The negative electrode faces the positive electrode.

[0114] The aforementioned negative electrode includes a negative electrode active material.

[0115] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. In this case, the negative electrode active material may be contained in the negative electrode active material layer.

[0116] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the negative electrode current collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with surface treatment using carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy.

[0117] The negative electrode current collector typically has a thickness of 3 μm to 500 μm.

[0118] The negative electrode current collector may have its surface textured to enhance the bonding force of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0119] The negative electrode active material layer is disposed on at least one side of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both sides of the negative electrode current collector.

[0120] The aforementioned negative electrode active material layer may contain a negative electrode active material.

[0121] The negative electrode active material is a material capable of reversibly intercalating / deintercalating lithium ions and may include at least one selected from the group consisting of carbon-based active materials, metalloid-based active materials, and lithium metal. Specifically, it may include at least one selected from carbon-based active materials and metalloid-based active materials.

[0122] The carbon-based active material may include at least one selected from the group consisting of graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and preferably includes graphite. The graphite may include at least one selected from the group consisting of artificial graphite and natural graphite.

[0123] The average particle size (D) of the carbon-based active material 50 The thickness of the ) may be 5 μm to 30 μm, preferably 7 μm to 15 μm, in order to ensure structural stability during charging and discharging and to reduce side reactions with the electrolyte.

[0124] Specifically, the metalloid active material may include: at least one metalloid selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an alloy of lithium with at least one metalloid selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an oxide of at least one metalloid selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; lithium titanium oxide (LTO); lithium vanadium oxide; and the like.

[0125] More specifically, the metalloid-based active material may include a silicon-based active material.

[0126] The silicon-based active material is SiO x The compound may contain compounds represented by (0 ≤ x < 2). SiO2 does not react with lithium ions and therefore cannot store lithium. For this reason, x is preferably within the above range, and more preferably the silicon-based active material is SiO2.

[0127] The average particle size (D) of the silicon-based active material 50 The thickness of the ) may be 1 μm to 30 μm, preferably 2 μm to 15 μm, in order to ensure structural stability during charging and discharging and to reduce side reactions with the electrolyte.

[0128] The negative electrode active material may be included in the negative electrode active material layer in an amount of 60% to 99% by weight, preferably 75% to 95% by weight.

[0129] The negative electrode active material layer may further contain, together with the negative electrode active material, a binder, a conductive material, and / or a thickening agent.

[0130] The binder is used to improve the performance of the battery by improving the adhesion between the negative electrode active material layer and the negative electrode current collector, and may contain, for example, at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and substances in which the hydrogen of these substances is substituted with Li, Na, or Ca, or may contain various copolymers thereof.

[0131] The binder may be included in the negative electrode active material layer in an amount of 0.5% to 10% by weight, preferably 1% to 5% by weight.

[0132] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. Examples of such materials include graphite such as natural graphite or artificial graphite; carbon black such as carbon black, acetylene black, Ketjenblack (registered trademark), channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0133] The conductive material may be included in the negative electrode active material layer in an amount of 0.5% to 10% by weight, preferably 1% to 5% by weight.

[0134] The thickness of the negative electrode active material layer may be 50 μm to 300 μm, preferably 100 μm to 200 μm.

[0135] The negative electrode can be manufactured by coating at least one surface of a negative electrode current collector with a negative electrode slurry containing a negative electrode active material, a binder, a conductive material, and / or a solvent for forming the negative electrode slurry, followed by drying and rolling.

[0136] The solvent for forming the negative electrode slurry may include, for example, at least one selected from the group consisting of distilled water, NMP (N-methyl-2-pyrrolidone), ethanol, methanol, and isopropyl alcohol, specifically distilled water, in order to facilitate the dispersion of the negative electrode active material, binder, and / or conductive material. The solid content of the negative electrode slurry may be 30% to 80% by weight, specifically 40% to 70% by weight.

[0137] (2) Separator The separator can be interposed between the positive electrode and the negative electrode.

[0138] The separator may be a conventional porous polymer film used as a separator, such as a porous polymer film made from polyolefin polymers such as ethylene monopolymer, propylene monopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, used alone or in a laminated form. Alternatively, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used, but is not limited to these. Furthermore, a coated separator containing ceramic components or polymeric substances may be used to ensure heat resistance or mechanical strength, and may be selectively used as a single-layer or multi-layer structure.

[0139] (3) Non-aqueous electrolytes Furthermore, the electrolytes used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0140] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0141] The lithium salt can be any compound capable of providing lithium ions for use in lithium secondary batteries, and is not particularly limited. 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 within the range of 0.1M to 2.0M. When the concentration of the lithium salt falls within the above range, the electrolyte has appropriate conductivity and viscosity, exhibiting excellent electrolyte performance and allowing lithium ions to move effectively.

[0142] The organic solvent may contain at least one selected from linear carbonates, cyclic carbonates, linear esters, cyclic esters, ethers, glymes, and nitriles.

[0143] The linear carbonate may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0144] The cyclic carbonate may include at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, and 2,3-pentylene carbonate.

[0145] Specific examples of the linear esters mentioned above include, but are not limited to, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0146] Specific examples of the aforementioned cyclic esters include, but are not limited to, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0147] Specific examples of the aforementioned ethers include, but are not limited to, dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL), and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL).

[0148] Specific examples of the aforementioned glyme include, but are not limited to, dimethoxyethane (glyme, DME), diethoxyethane, diglyme, triglyme, and tetraglyme (TEGDME).

[0149] Specific examples of the aforementioned nitriles include, but are not limited to, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonile, cyclohexanecarbonile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0150] The electrolyte may further contain additives along with the lithium salt and organic solvent.

[0151] The aforementioned additive may include at least one selected from the group consisting of vinylethylene carbonate, propanesultone, LiBF4 (Lithium tetrafluoroborate), LiODFB (Lithium difluoro(oxalato)borate), 1,3,6-HTCN (Hexane Tri-Cyanide), and NaO2 (Sodium superoxide), specifically, fluoroethylene carbonate, difluoroethylene carbonate, vinylethylene carbonate, propanesultone, LiBF4 (Lithium tetrafluoroborate), LiODFB (Lithium difluoro(oxalato)borate), 1,3,6-HTCN (Hexane Tri-Cyanide), succinonitrile, 1,4-dicyano-2-butyne, adiponitrile, lithium difluorophosphate (LiPO2F2), and NaO2 (Sodium superoxide).

[0152] The lithium secondary battery according to the present invention, as described above, can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras, as well as in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0153] Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

[0154] The aforementioned battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0155] The external shape of the lithium secondary battery of the present invention is not particularly limited, but cylindrical, rectangular, pouch-type, or coin-type batteries using a can are possible.

[0156] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also suitably as a unit battery in medium- and large-sized battery modules containing a large number of battery cells.

[0157] The present invention will be specifically described below with reference to examples.

[0158] In this regard, the embodiments of the present invention can be modified into various other forms, and the scope of the invention should not be construed as being limited to the embodiments detailed below. The embodiments of the present invention are provided to give a more complete explanation of the invention to a person of average skill in the art.

[0159] The present invention will be specifically described below with reference to concrete examples.

[0160] Examples Example 1 (Manufacturing of positive electrodes) A cathode slurry was prepared by adding lithium iron phosphate (LiFePO4) particles as the cathode active material, carbon nanotubes as the conductive material, and polyvinylidene fluoride (PVdF) as the binder in a weight ratio of 96.5:1.0:2.5 to the solvent N-methyl-2-pyrrolidone (NMP). In this process, the lithium iron phosphate particles were in secondary particle form, and the average particle size (D 50 The diameter of the primary particles is 16 μm, and the average particle size (D 50 The particle size was 0.4 μm. Furthermore, the particle fracture strength of the lithium iron phosphate particles in the secondary particle form was 10 kgf / mm². 2 That was the case.

[0161] The aforementioned positive electrode slurry was added to a positive electrode current collector (Al thin film) with a thickness of 20 μm and a surface roughness Rz1 of 1.5 μm at a rate of 330 mg / 25 cm². 2 It was applied and dried with the specified loading amount.

[0162] Subsequently, the positive electrode was manufactured by roll pressing with a linear pressure of 50 kN / cm (thickness of positive electrode active material: 120 μm). At this time, the rolling ratio of the positive electrode was 46%, and the porosity was 33%.

[0163] Comparative Example 1 The positive electrode was manufactured in the same manner as in Example 1, except that primary particle form lithium iron phosphate (LiFePO4) particles were used as the positive electrode active material. The average particle size (D) of the primary particle form lithium iron phosphate (LiFePO4) particles used in Comparative Example 1 50 The particle size is 0.9 μm, and the particle fracture strength is 50 kgf / mm². 2 In this case, the rolling ratio of the positive electrode was 50%, and the porosity was 33%.

[0164] Comparative Example 2 Lithium iron phosphate particles in secondary particle form, wherein the average particle size of the secondary particles (D 50 The diameter of the primary particles is 16 μm, and the average particle size (D 50 The particle size is 0.4 μm, and the particle fracture strength of lithium iron phosphate particles in secondary particle form is 1.5 kgf / mm². 2The positive electrode was manufactured in the same manner as in Example 1, except that a material was used as the positive electrode active material. In this case, the rolling ratio of the positive electrode was 46%, and the porosity was 33%.

[0165] Comparative Example 3 Lithium iron phosphate particles in secondary particle form, wherein the average particle size of the secondary particles (D 50 The diameter of the primary particles is 13 μm, and the average particle size (D 50 The particle size is 0.2 μm, and the particle fracture strength of lithium iron phosphate particles in secondary particle form is 25 kgf / mm². 2 The positive electrode was manufactured in the same manner as in Example 1, except that a material was used as the positive electrode active material. In this case, the rolling ratio of the positive electrode was 46%, and the porosity was 33%.

[0166] Experimental example Experimental Example 1: Measurement of surface roughness Rz2 of the positive electrode current collector after rolling. The positive electrodes of Example 1 and Comparative Examples 1-3 were immersed in NMP solvent and then heated at 100°C for 2 hours to peel and remove the positive electrode active material layer. Using NanoSystems' Optical Profiler, the surface roughness Rz2 was measured from the Topo images obtained by selecting five measurement areas for each sample at a measurement magnification (e.g., 500x).

[0167] The results are shown in Table 1 below.

[0168] [Table 1]

[0169] Referring to Table 1, it can be seen that Example 1, which uses lithium iron phosphate particles in secondary particle form as the positive electrode active material and whose particle cracking strength meets a specific range, differs from the comparative example in that the surface roughness Rz2 and Rz1 / Rz2 of the rolled positive electrode current collector meet the range of the present invention. From this, it can be confirmed that the positive electrode current collector was roughened to an excellent level by rolling using lithium iron phosphate particles in secondary particle form. Furthermore, it is expected that this will increase the contact area between the positive electrode active material and the positive electrode current collector, resulting in excellent high-rate discharge characteristics.

[0170] Experiment Example 2: SEM Photo Observation The cross-sections of the positive electrodes of Example 1 and Comparative Examples 1 to 3 were observed using a scanning electron microscope (SEM).

[0171] Figure 1 shows an SEM image of the cross-section of the positive electrode of Example 1, Figure 2 shows an SEM image of the cross-section of the positive electrode of Comparative Example 1, Figure 3 shows an SEM image of the cross-section of the positive electrode of Comparative Example 2, and Figure 4 shows an SEM image of the cross-section of the positive electrode of Comparative Example 3.

[0172] Referring to Figures 1 to 4, in the positive electrode of Example 1, it can be seen that all of the lithium iron phosphate particles in secondary particle form are broken and converted into primary particles, and these primary particles are in contact with the current collector. The positive electrode of Comparative Example 1 uses lithium iron phosphate particles in primary particle form, and during rolling, these particles press against the current collector at a uniform level, so the surface roughening of the current collector is not sufficient. In the positive electrode of Comparative Example 2, the current collector breaks before it can be roughened during rolling for manufacturing the positive electrode, so the surface roughening of the current collector is not sufficient. In the positive electrode of Comparative Example 3, some surface roughening of the positive electrode current collector is observed, but because the lithium iron phosphate particles in secondary particle form did not break, a gap is observed between the positive electrode active material and the positive electrode current collector.

[0173] If the current collector is not sufficiently roughened, as in Comparative Examples 1 and 2, or if a gap is created between the positive electrode active material and the current collector, as in Comparative Example 3, the contact area between the positive electrode active material and the current collector may not be sufficient, which could lead to a decrease in battery performance, such as resistance characteristics.

[0174] Experiment Example 3: Three-Dimensional Microscope Observation The positive electrodes of Example 1 and Comparative Example 1 were immersed in NMP solvent, then heated at 100°C for 2 hours to peel off and remove the positive electrode active material layer, thereby obtaining the positive electrode current collector.

[0175] Subsequently, a three-dimensional microscope was used to observe the surface of the rolled positive electrode current collector.

[0176] Figure 5 shows the surface observation results of the positive electrode current collector after rolling in Example 1, and Figure 6 shows the surface observation results of the positive electrode current collector in Comparative Example 1.

[0177] Referring to Figures 5 and 6, it can be observed that the positive electrode current collector according to Example 1 has a roughened surface, while the positive electrode current collector according to Comparative Example 1 is hardly roughened at all.

[0178] Experiment Example 4: Measurement of MP Resistance The positive electrodes manufactured in Example 1 and Comparative Examples 1-3 were punched out into sheets measuring 5 cm horizontally and 5 cm vertically, and their MP resistance was measured using a multi-probe resistivity measuring device.

[0179] [Table 2]

[0180] Referring to Table 2 above, it can be seen that Example 1, which uses lithium iron phosphate particles in secondary particle form as the positive electrode active material and whose particle cracking strength meets a specific range, exhibits a significantly lower MP resistance compared to the comparative example. This confirms that rolling using lithium iron phosphate particles in secondary particle form roughens the positive electrode current collector to an excellent degree. Furthermore, this increases the contact area between the positive electrode active material and the positive electrode current collector, which is expected to result in excellent high-rate discharge characteristics.

[0181] Experiment Example 5: Resistance Evaluation (Manufacturing of secondary batteries) A negative electrode slurry with a solid content of 50% was prepared by dispersing a negative electrode active material (graphite), a conductive material (carbon black), a binder (SBR), and a thickener (CMC) in water in a weight ratio of 93.5:2.0:1.0:3.5. The negative electrode slurry was then applied to an 8 μm thick copper foil at a concentration of 160 mg / cm³. 2 The negative electrode was coated to achieve the desired loading amount, and a thickness of 145 μm was manufactured.

[0182] An electrode assembly was manufactured by sequentially stacking the positive electrode, separator, and negative electrode manufactured in Example 1. This assembly was then placed in a pouch-type battery case, and an electrolyte was injected to produce a pouch-type cell of Example 1 with a capacity of 5 Ah.

[0183] The pouch-type cells of Comparative Examples 1 to 3 were manufactured in the same manner as in Example 1, except that the respective positive electrodes of Comparative Examples 1 to 3 were used.

[0184] (Measurement of discharge resistance at 0.1 seconds) The manufactured pouch-type cells are activated, the activated secondary batteries are fully charged to 100% SOC, a 2.5C discharge pulse is applied to discharge them, the voltage change is measured after 0.1 seconds, and the discharge resistance (R) is measured. 0.1s The discharge resistance (R) was measured at 0.1 seconds. 0.1s From this, the resistance between the current collector and the positive electrode active material layer can be confirmed. The results are shown in Table 3 below.

[0185] (Measurement of Ohmic resistance) After setting the SOC to 50%, the relative ohmic resistance of the pouch-type cells manufactured in Example 1 to Comparative Examples 1-3 was measured using EIS (Electrochemical Impedance Spectroscopy). The results are shown in Table 3 below.

[0186] [Table 3]

[0187] Referring to Table 3 above, it can be seen that the positive electrode of Example 1 has significantly lower 0.1-second discharge resistance and Ohmic resistance compared to the comparative example. From this, it can be confirmed that the positive electrode of Example 1 has reduced resistance between the positive electrode current collector and the positive electrode active material layer, enabling it to exhibit high-level high-temperature discharge characteristics.

Claims

1. A step of preparing a cathode slurry containing a cathode active material that includes lithium iron phosphate particles in a secondary particle form in which multiple primary particles are aggregated, The steps include applying the positive electrode slurry onto the positive electrode current collector, The step includes rolling the coated positive electrode slurry to form a positive electrode active material layer, The particle fracture strength of the lithium iron phosphate particles in the secondary particle form is 4 kgf / mm². ２ ~20 kgf / mm ２ And, The surface roughness Rz of the positive electrode current collector after rolling ２ The surface roughness Rz of the positive electrode current collector before rolling. １ A method for manufacturing a positive electrode, wherein the ratio of [the specified part] is 0.5 or less.

2. The method for manufacturing a positive electrode according to claim 1, wherein the rolling process causes the positive electrode active material contained in the coated positive electrode slurry to roughen the positive electrode current collector.

3. The average particle size (D) of the lithium iron phosphate particles in the secondary particle form. ５０ A method for manufacturing a positive electrode according to claim 1, wherein the thickness of the electrode is 7 μm to 30 μm.

4. The average particle size (D) of the primary particles ５０ A method for manufacturing a positive electrode according to claim 1, wherein the thickness of the ) is 0.2 μm to 3 μm.

5. The method for manufacturing a positive electrode according to claim 1, wherein the lithium iron phosphate particles in secondary particle form are broken into primary particles by rolling the coated positive electrode slurry.

6. The surface roughness Rz of the positive electrode current collector after rolling ２ A method for manufacturing a positive electrode according to claim 1, wherein the diameter is 3 μm or more.

7. The surface roughness Rz of the positive electrode current collector before rolling １ A method for manufacturing a positive electrode according to claim 1, wherein the particle size is 1.5 μm or less.

8. The method for manufacturing a positive electrode according to claim 1, wherein the rolling ratio of the positive electrode active material layer is 30% or more.

9. The rolling is performed by a roll press. The method for manufacturing a positive electrode according to claim 1, wherein the linear pressure during the roll press is 30 kN / cm to 80 kN / cm.

10. The method for manufacturing a positive electrode according to claim 1, wherein the positive electrode current collector contains aluminum.

11. A method for manufacturing a positive electrode according to claim 1, further comprising the step of drying the coated positive electrode slurry between the application of the positive electrode slurry and the rolling of the applied positive electrode slurry.

12. A positive electrode comprising a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector, the surface roughness Rz of the positive current collector ２ is 3 μm or more, The positive electrode active material layer contains a positive electrode active material. The positive electrode active material contains lithium iron phosphate particles, A sample of the aforementioned positive electrode punched out into a sheet measuring 5 cm horizontally and 5 cm vertically showed an MP resistance of 0.10 Ω·cm when measured with a Multi Probe Resistivity measuring device. ２ The positive electrode is as follows.

13. The positive electrode according to claim 12, wherein the porosity of the positive electrode active material layer is 25% to 45%.

14. The positive electrode according to claim 12, wherein the lithium iron phosphate particles exist in the form of primary particles or as a mixture of primary and secondary particles.

15. The positive electrode according to claim 12, wherein, if the lithium iron phosphate particles are a mixture of primary and secondary particles, the content of the primary particles is 90% by weight or more and less than 100% by weight in the lithium iron phosphate particles.

16. The positive electrode according to claim 12, wherein the thickness of the positive electrode active material layer is 50 μm to 500 μm.

17. The positive electrode according to claim 12, wherein the positive electrode current collector and the positive electrode active material layer are in direct contact.

18. The positive electrode according to claim 12, A negative electrode opposite the positive electrode, A separator interposed between the positive electrode and the negative electrode, A lithium secondary battery containing a non-aqueous electrolyte.

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