Positive electrode and lithium secondary battery containing the same

Abstract

The present invention relates to a positive electrode and a lithium secondary battery including the same. The positive electrode includes a positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. The first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant. The ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and the porosity calculated by the following formula 1 is 25% to 28%. [Formula 1] Porosity = {1 - (measured density of the positive electrode active material layer / true density of the positive electrode active material)} × 100

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0177463, filed on December 16, 2022.

[0002] The present invention relates to a positive electrode and a lithium secondary battery manufactured using the same. [Background technology]

[0003] As technological development and demand for electric vehicles and energy storage systems (ESS) increases, the demand for batteries as energy sources is rapidly increasing, and various research projects are being conducted on batteries that can meet various requirements. In particular, research on lithium secondary batteries, which have high energy density and excellent life and cycle characteristics as power sources for such devices, is being actively conducted.

[0004] Lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (LNCMO), lithium iron phosphate (LFP), etc. are used as positive electrode active materials for lithium secondary batteries.

[0005] Lithium iron phosphate is inexpensive because it contains iron, a resource-rich and inexpensive material. Furthermore, its low toxicity means that its use can reduce environmental pollution. Furthermore, because lithium iron phosphate has an olivine structure, its active material structure can be more stably maintained at high temperatures than lithium transition metal oxides, which have a layered structure. This provides the battery with excellent high-temperature stability and high-temperature life characteristics.

[0006] However, lithium iron phosphate has a drawback in that it has lower lithium mobility and lower electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, in the past, lithium iron phosphate with a small average particle size was used to shorten the lithium migration path, and the surface of the lithium iron phosphate was coated with carbon to improve electrical conductivity, and an excessive amount of conductive material was used.

[0007] However, as the size of lithium iron phosphate particles decreases, their specific surface area increases, and the lithium iron phosphate with a carbon-coated surface exhibits reduced wettability with solvents. As a result, the lithium iron phosphate particles aggregate severely, reducing the stability and coating processability of the positive electrode slurry. Furthermore, the lithium iron phosphate and binder are not effectively mixed, resulting in a decrease in the adhesive strength between the positive electrode current collector and the positive electrode active material layer (hereinafter referred to as "positive electrode adhesive strength"). This phenomenon can be exacerbated in high-loading positive electrodes.

[0008] If the adhesive strength of the positive electrode is reduced, the positive electrode active material layer may be detached during the electrode manufacturing process or during charge and discharge, which may increase the battery resistance and reduce the capacity of the secondary battery.

[0009] As the demand for high energy density batteries increases, cathodes containing lithium iron phosphate have a density of 550 mg / 25 cm 2 There is a need for technology to improve the adhesive strength and flexibility of the cathode while maintaining the above loading amount. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention aims to provide a positive electrode and a lithium secondary battery that improve the adhesive strength between a positive electrode current collector and a positive electrode active material layer in a high-loading positive electrode, improve the flexibility of the positive electrode, thereby preventing electrode detachment and improving the resistance characteristics of the battery. [Means for solving the problem]

[0011] According to one embodiment of the present invention, there is provided a positive electrode comprising: a positive electrode current collector; a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector; and a second positive electrode active material layer formed on the first positive electrode active material layer, wherein the first positive electrode active material layer and the second positive electrode active material layer each comprise lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant, and wherein a ratio (P2 / P1) of a weight of fluorine contained in the second positive electrode active material layer (P2) to a weight of fluorine contained in the first positive electrode active material layer (P1) is 1 or less, and the positive electrode has a porosity of 25% to 28%, preferably 25.5% to 27.5%, calculated according to the following Equation 1:

[0012] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0013] In an exemplary embodiment, the rubber binder can be hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) of 130,000 g / mol or greater.

[0014] In an exemplary embodiment, the rubber dispersant may be a hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) ranging from 10,000 g / mol to 100,000 g / mol.

[0015] In an exemplary embodiment, the fluorine-based binder may be contained in the first positive electrode active material layer in a range of 1.5% to 3.0% by weight.

[0016] In an exemplary embodiment, the fluorine-based binder may be included in the second positive electrode active material layer in a range of 0.8% to 2.4% by weight.

[0017] In an exemplary embodiment, the fluorine-based binder may be a polyvinylidene fluoride (PVDF)-based polymeric binder.

[0018] In an exemplary embodiment, the rubber binders contained in the first and second positive electrode active material layers may satisfy the following condition 1.

[0019] [Condition 1] 1≦R2 / R1≦3

[0020] R1 represents the content (wt %) of the rubber binder contained in the first positive electrode active material layer, and R2 represents the content (wt %) of the rubber binder contained in the second positive electrode active material layer.

[0021] In an exemplary embodiment, the rubber binder may be contained in the first positive electrode active material layer in a range of 0.2 wt % to 0.9 wt % and in the second positive electrode active material layer in a range of 0.3 wt % to 1.0 wt %.

[0022] In an exemplary embodiment, the rubber binder can have a weight average molecular weight (Mw) ranging from 150,000 g / mol to 1,000,000 g / mol.

[0023] In an exemplary embodiment, the conductive material may be carbon nanotubes.

[0024] In an exemplary embodiment, the conductive material may be contained in each of the first and second positive electrode active material layers in a range of 0.3% to 2.0% by weight.

[0025] In an exemplary embodiment, the lithium iron phosphate may be a compound represented by the following Chemical Formula 1:

[0026] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0027] (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

[0028] In an exemplary embodiment, the average particle size D of the lithium iron phosphate 50 can be 0.6 μm to 3.0 μm.

[0029] In an exemplary embodiment, the fluorine-based binder may be a polyvinylidene fluoride (PVDF)-based polymer binder.

[0030] The present invention also provides a lithium secondary battery including the above-described positive electrode. [Effects of the Invention]

[0031] The positive electrode according to the present invention includes a first positive electrode active material layer that maximizes adhesion and a second positive electrode active material layer that maximizes flexibility, and thus may have excellent positive electrode adhesion and flexibility even with a low binder content compared to conventional single-layer positive electrodes.

[0032] In addition, the positive electrode according to the present invention has the effect of controlling the porosity of the positive electrode active material layer within an optimum range, thereby increasing the electrical conductivity of the positive electrode within a range that does not impair ionic conductivity, thereby improving the resistance characteristics of the battery.

[0033] As a result, the positive electrode according to the present invention can provide a battery that is excellent in terms of capacity and electrical resistance while realizing a high loading positive electrode. DETAILED DESCRIPTION OF THE INVENTION

[0034] The advantages and features of the present invention, as well as methods for achieving them, will become apparent from the following detailed description of the embodiments. However, the present invention is not limited to the embodiments disclosed below, and may be embodied in various different forms. The present embodiments are provided solely to ensure that the disclosure of the present invention is complete and to fully convey the scope of the invention to those skilled in the art. The present invention is defined solely by the scope of the claims. The same reference numerals refer to the same elements throughout the specification.

[0035] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the sense commonly understood by those skilled in the art to which the present invention pertains. Furthermore, terms defined in commonly used dictionaries are not to be interpreted ideally or excessively unless otherwise clearly defined.

[0036] The terms used in this specification are for the purpose of describing the embodiments and are not intended to limit the present invention. In this specification, the singular includes the plural unless otherwise stated in the phrase. When used in this specification, "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements other than the elements mentioned.

[0037] In this specification, when a part is said to include a certain component, this does not mean that it excludes other components, and that it may further include other components, unless otherwise specified.

[0038] In this specification, the expression "A and / or B" means A or B, or A and B.

[0039] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0040] In this specification, D 50The above D means the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0041] In this specification, the "specific surface area" is measured by the BET method, and specifically, can be calculated from the amount of nitrogen gas adsorbed at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan.

[0042] In this specification, "weight average molecular weight (Mw)" refers to a value measured by gel permeation chromatography (GPC) and converted to standard polystyrene. Specifically, the weight average molecular weight is a value measured by GPC under the following conditions and converted. Standard polystyrene from an Agilent system was used to prepare the calibration curve.

[0043] <Measurement conditions> Measuring instrument: Agilent GPC (Agilent 1200 series, USA) Column: PL Mixed B 2 columns connected Column temperature: 40℃ Eluent: tetrahydrofuran Flow rate: 1.0mL / min Concentration: ~1mg / mL (100μL injection)

[0044] The positive electrode adhesive strength herein can be measured by the following method. A positive electrode cut to a length of 150 mm and a width of 20 mm was prepared. The positive electrode active material layer was placed facing a 75 mm long and 25 mm wide glass slide, and the positive electrode was attached to the glass slide lengthwise using double-sided tape. That is, the glass slide was attached to an area corresponding to half of the positive electrode lengthwise. An evaluation sample was then prepared by rolling the double-sided tape 10 times with a roller to ensure uniform adhesion. Next, the glass slide portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM) (LS5, AMETEK), and the half of the positive electrode not attached to the glass slide was connected to the load cell of the UTM. The load cell was applied at a 90° angle at a speed of 100 mm / min and moved up to 50 mm, measuring the load applied to the load cell. The average load measured over a 20 mm to 40 mm section of the travel section was calculated. This is repeated a total of five times, and the average value is evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0045] In this specification, the flexibility of the positive electrode can be measured by the following method. A measuring rod is prepared by phi (φ) and 600 mg / 25 cm 2 A positive electrode with a loading of 10 ...

[0046] In this specification, "phi (φ)" refers to the diameter of a measuring rod in millimeters (mm).

[0047] In this specification, the porosity of a positive electrode active material layer may be defined by the following Equation 1. In Equation 1, the "measured density of the positive electrode active material layer" is a value calculated from the density by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of a rolled positive electrode, and the "true density of the positive electrode active material" is the density relative to the volume of only the positive electrode active material particles themselves excluding pores, and may be measured using a Pycnometer (AccuPycII 1340).

[0048] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0049] The present invention provides a positive electrode and a lithium secondary battery manufactured using the same.

[0050] Lithium iron phosphate, which is commonly used as a positive electrode active material, has lower lithium mobility and electrical conductivity compared to lithium transition metal oxides such as lithium nickel cobalt manganese oxide. Therefore, lithium iron phosphate, which has a small average particle size, is commonly used as a positive electrode active material. However, when the lithium iron phosphate particles are small, their specific surface area increases, which can lead to severe particle aggregation and ineffective mixing of the lithium iron phosphate and the binder, resulting in reduced positive electrode adhesion. Furthermore, fluorine-based binders used as positive electrode binders tend to adhere better to the lithium iron phosphate particles than to the positive electrode current collector. Therefore, when the positive electrode active material is lithium iron phosphate, the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer can be reduced compared to when the positive electrode active material is lithium nickel cobalt manganese oxide. Therefore, a positive electrode containing lithium iron phosphate as a positive electrode active material can suffer from detachment of the positive electrode active material layer during electrode fabrication or charge / discharge, resulting in increased battery resistance and reduced secondary battery capacity. In addition, a positive electrode containing lithium iron phosphate as a positive electrode active material has poor brittleness due to the characteristics of the positive electrode active material, compared to a positive electrode containing lithium nickel cobalt manganese oxide, and is vulnerable to detachment of the positive electrode active material layer due to external physical forces.

[0051] As a result of extensive research into solving these problems, the inventors of the present invention discovered that when the positive electrode active material layer is composed of multiple layers rather than a single layer, and the first and second positive electrode active material layers each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant, the ratio (P2 / P1) of the weight of fluorine contained in the second positive electrode active material layer to the weight of fluorine contained in the first positive electrode active material layer (P1) is 1 or less, and the porosity calculated according to the following Equation 1 is 25% to 28%, the interfacial adhesion between the current collector and the first positive electrode active material layer is excellent, and detachment of the positive electrode active material layer is suppressed due to improved flexibility, so that the ionic conductivity of the positive electrode active material layer is not inhibited and electrical conductivity is improved, thereby improving the resistance characteristics of the battery, which led to the present invention.

[0052] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0053] Hereinafter, the positive electrode according to the present invention and the lithium secondary battery manufactured using the same will be described in detail.

[0054] <Positive electrode> A positive electrode according to one embodiment of the present invention includes a positive electrode current collector, a first positive electrode active material layer formed on one or both surfaces of the positive electrode current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer, wherein the first positive electrode active material layer and the second positive electrode active material layer each include lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant, and wherein a ratio (P2 / P1) of a weight P2 of fluorine contained in the second positive electrode active material layer to a weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and the porosity calculated by the following Equation 1 may be 25% to 28%, preferably 25.5% to 27.5%.

[0055] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0056] If the porosity is less than 25%, the flexibility of the positive electrode may decrease, which is undesirable, and if the porosity is more than 28%, the resistance of the battery may increase, which is undesirable.

[0057] The porosity refers to the porosity of the positive electrode active material layer including both the first and second positive electrode active material layers. In Equation 1, the "measured density of the positive electrode active material layer" is a value calculated by measuring the weight and volume of the positive electrode active material layer separated from the positive electrode current collector of a rolled positive electrode, and the "true density of the positive electrode active material" is the density relative to the volume of only the positive electrode active material particles themselves, excluding pores, and can be measured using a Pycnometer (AccuPycII 1340).

[0058] The porosity can be adjusted to the above range by appropriately adjusting the thickness change rate of the positive electrode active material layer before and after rolling during the rolling process during the positive electrode manufacturing process. Specifically, the porosity can be reduced as the thickness change rate before and after rolling increases, and conversely, the porosity can be increased as the thickness change rate before and after rolling decreases. The thickness change rate can be calculated as follows:

[0059] Thickness change rate = (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

[0060] The thickness change rate can be adjusted by the linear pressure applied to the positive electrode during rolling. That is, the greater the linear pressure, the greater the thickness change rate before / after rolling, and the smaller the linear pressure, the smaller the thickness change rate before / after rolling.

[0061] The positive electrode according to the present invention has a porosity that satisfies the above range, which is slightly lower than the conventional porosity range of a positive electrode containing lithium iron phosphate as a positive electrode active material. Resistance is affected by ionic conductivity and electrical conductivity, but the positive electrode according to the present invention is rolled by applying a high linear pressure to improve interfacial adhesion during rolling, and is rolled to a degree that does not reduce ionic conductivity, thereby achieving improved resistance characteristics by rolling the positive electrode to a degree that satisfies the above range.

[0062] The positive electrode according to an exemplary embodiment of the present invention has a structure in which the positive electrode active material layer is in direct contact with the positive electrode current collector, and due to excellent interfacial adhesion between the positive electrode active material layer and the positive electrode current collector, a separate layer for improving adhesion between the positive electrode active material layer and the positive electrode current collector may not be included. That is, the positive electrode according to an exemplary embodiment of the present invention may exhibit excellent interfacial adhesion without including a separate layer, such as a binder layer, adhesive layer, bonding layer, or primer coating layer, that may be interposed between the positive electrode current collector and the positive electrode active material layer for improving adhesion.

[0063] The positive electrode of the present invention will now be described in detail.

[0064] A positive electrode according to one embodiment of the present invention may include a positive electrode active material layer. Specifically, the positive electrode according to the present invention may include a positive electrode current collector, a first positive electrode active material layer formed on the current collector, and a second positive electrode active material layer formed on the first positive electrode active material layer. In this case, the first positive electrode active material layer and the second positive electrode active material layer may be formed on one or both sides of the current collector.

[0065] In the positive electrode according to the present invention, the first positive electrode active material layer, which is the lower layer region, has a positive electrode composition for increasing the interfacial adhesion between the positive electrode current collector and the positive electrode active material layer, and the second positive electrode active material layer, which is the upper layer region, has a positive electrode composition for improving the flexibility of the electrode. As a result, the positive electrode according to the present invention has improved both adhesion and flexibility compared to a single-layer positive electrode.

[0066] The positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the positive electrode current collector may be made of stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like.

[0067] The positive electrode current collector may have a thickness of 3 μm to 500 μm, and may have fine irregularities on its surface to enhance adhesion to the positive electrode active material layer. It may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0068] The positive electrode active material layer may include a positive electrode active material. In addition, the positive electrode active material layer may further include a conductive material, a binder, and a dispersant, if necessary, in addition to the positive electrode active material.

[0069] Hereinafter, each component contained in the positive electrode active material layer will be specifically described.

[0070] (1) Positive electrode active material The present invention includes lithium iron phosphate as a positive electrode active material. Because lithium iron phosphate has an olivine structure, its active material structure is more stable at high temperatures than lithium transition metal oxides, which have a layered structure. As a result, when lithium iron phosphate is used as a positive electrode active material, the high-temperature stability and high-temperature life characteristics of the positive electrode are significantly improved, thereby reducing the risk of fire in a lithium secondary battery including the positive electrode.

[0071] The lithium iron phosphate may be a compound represented by the following formula 1:

[0072] [Chemical formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b

[0073] In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.

[0074] For example, the lithium iron phosphate may be LiFePO4.

[0075] In the present invention, the term "monolith structure" refers to a structure in which particles are morphologically present in independent phases and are not aggregated with each other. A particle structure that contrasts with this monolith structure is a structure in which small particles ("primary particles") are physically and / or chemically aggregated to form relatively large particles ("secondary particles").

[0076] When lithium iron phosphate has a monolithic structure consisting of primary particles, it is less likely to crack during the rolling process compared to secondary particles, which is preferable because capacity loss due to detachment of broken particles is reduced.Furthermore, when lithium iron phosphate is a monolithic primary particle, it can mitigate binder migration during the drying process of the positive electrode slurry, which is also preferable in terms of interfacial adhesion between the positive electrode current collector and the positive electrode active material layer.

[0077] The lithium iron phosphate may include a carbon coating layer on the surface thereof. When a carbon coating layer is formed on the surface of the lithium iron phosphate, electrical conductivity is improved, and the resistance characteristics of the positive electrode may be improved.

[0078] The carbon coating layer may be formed using at least one raw material selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, furfuryl alcohol polymers, ethylene-ethylene oxide block copolymers, vinyl resins, cellulose resins, phenolic resins, pitch resins, and tar resins. Specifically, the carbon coating layer may be formed by mixing the raw material with the lithium iron phosphate and then heat-treating the mixture.

[0079] Average particle size D of lithium iron phosphate 50The average particle size D of the positive electrode active material may be 0.5 μm to 20.0 μm, preferably 0.5 μm to 10.0 μm, more preferably 0.6 μm to 3 μm, still more preferably 0.6 μm to 2.5 μm, and most preferably 0.7 μm to 1.5 μm. 50 When the above range is satisfied, the mobility of lithium in the lithium iron phosphate is improved, and the charge / discharge characteristics of the battery can be improved.

[0080] The BET specific surface area of ​​lithium iron phosphate is 5m 2 / g~20m 2 / g, specifically 7m 2 / g~18m 2 / g, more specifically 9m 2 / g~16m 2 / g. This range corresponds to a lower value compared to typical lithium iron phosphate. When this range is satisfied, aggregation of the lithium iron phosphate can be effectively suppressed even in a positive electrode slurry composition with a relatively low dispersant content.

[0081] The lithium iron phosphate of the present invention may be contained in the first positive electrode active material layer and the second positive electrode active material layer at the same or different contents. In a specific example, the lithium iron phosphate may be contained in the first positive electrode active material layer at 93.5 wt% to 98 wt%, specifically 94 wt% to 97.5 wt%, specifically 94.5 wt% to 97 wt%, based on the total weight of the first positive electrode active material layer. The lithium iron phosphate may be contained in the second positive electrode active material layer at 94 wt% to 99 wt%, specifically 94.5 wt% to 98.5 wt%, specifically 95 wt% to 98 wt%, based on the total weight of the second positive electrode active material layer. When the lithium iron phosphate content satisfies the above range, sufficient positive electrode energy density can be ensured, thereby improving the battery capacity of the positive electrode.

[0082] (2) Binder The first positive electrode active material layer and the second positive electrode active material layer constituting the positive electrode according to the present invention each contain both a fluorine-based binder and a rubber-based binder as the binder.

[0083] The fluorine-based binder may include a PVDF-based polymer containing vinylidene fluoride (VDF) as a monomer. Specific examples of the PVDF-based polymer include PVDF monopolymer, PVDF-HFP (Poly(vinylidene fluoride-co-hexafluoropropylene)), PVDF-CTFE (Poly(vinylidene fluoride-co-chlorotrifluoroethylene)), PVDF-TFE (Poly(vinylidene tetrafluoroethylene)), and PVDF-TrFE (Poly(vinylidene trifluoroethylene)).

[0084] The fluorine-based binder, together with the rubber-based binder, provides adhesive strength between the positive electrode active material and the conductive material, and adhesive strength between the current collector and the positive electrode active material layer.

[0085] The weight-average molecular weight (Mw) of the fluorine-based binder may be 300,000 to 2,000,000, preferably 400,000 to 1,500,000, and most preferably 500,000 to 1,300,000. When the weight-average molecular weight of the fluorine-based binder satisfies the above numerical range, the viscosity of the positive electrode slurry can be easily adjusted to a preferred range, and the positive electrode adhesive strength can be maximized even with a small content range.

[0086] The fluorine-based binders contained in the first and second positive electrode active material layers may be the same compound and may have the same weight-average molecular weight and chemical formula.

[0087] In a positive electrode according to an exemplary embodiment of the present invention, the content (wt%) of the fluorine-based binder contained in the first lower positive electrode active material layer is equal to or greater than the content (wt%) of the fluorine-based binder contained in the second upper positive electrode active material layer, and the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, specifically 0.5 to 1, preferably 0.52 to 0.95, and more preferably 0.53 to 0.85. If the ratio (P2 / P1) of P2 to P1 exceeds 1, it is undesirable from the viewpoint of adhesive strength.

[0088] The weight P1 of fluorine contained in the first positive electrode active material layer and the weight P2 of fluorine contained in the second positive electrode active material layer satisfy the above ratio range due to the difference in the content of the fluorine-based binder contained in the first positive electrode active material layer and the second positive electrode active material layer, respectively.

[0089] That is, although the fluorine-based binder contains elemental fluorine, the lithium iron phosphate, rubber-based binder, rubber-based dispersant, and conductive material constituting the positive electrode of the present invention do not contain a fluorine component, and therefore the weight of fluorine contained in each of the first positive electrode active material layer and the second positive electrode active material layer is proportional to the content (weight) of the fluorine-based binder contained in each of the first positive electrode active material layer and the second positive electrode active material layer.

[0090] Fluorine-based binders have the advantage of exhibiting excellent adhesive strength, but due to their crystalline structure, they are less flexible than rubber-based binders. Therefore, by increasing the content of fluorine-based binders, which have excellent adhesive strength, in the first positive electrode active material layer that contacts the positive electrode current collector, and decreasing the content of fluorine-based binders in the second positive electrode active material layer, which requires flexibility, the interfacial adhesive strength of the positive electrode can be improved and the electrode can be prevented from detaching from the electrode surface upon external impact. Thus, by controlling the content of fluorine-based binders in each of the first and second positive electrode active material layers as described above, the positive electrode according to the present invention has superior adhesive strength and flexibility compared to a positive electrode composed of a single layer, even with the same binder content.

[0091] The fluorine-based binder may be contained in the first positive electrode active material layer at 1.5 wt % to 3.0 wt %, preferably 1.6 wt % to 2.7 wt %, and more preferably 1.7 wt % to 2.5 wt %. The fluorine-based binder may be contained in the second positive electrode active material layer at 0.8 wt % to 2.4 wt %, preferably 1.0 wt % to 2.2 wt %, and more preferably 1.2 wt % to 2.0 wt %.

[0092] In addition, the fluorine-based binder may be contained in the entire positive electrode active material layer including the first positive electrode active material layer and the second positive electrode active material layer in an amount of 1.2 wt % to 2.6 wt %, preferably 1.4 wt % to 2.4 wt %, and more preferably 1.6 wt % to 2.2 wt %.

[0093] On the other hand, the rubber-based binder improves the flexibility of the positive electrode, increases the loading capacity of the positive electrode, and improves rolling performance. Furthermore, the improved flexibility prevents a portion of the positive electrode active material layer from being detached when subjected to external physical impact, thereby providing a positive electrode with excellent capacity.

[0094] According to an exemplary embodiment of the present invention, the rubber binder may be hydrogenated nitrile butadiene rubber (HNBR). The hydrogenated nitrile butadiene rubber (HNBR) is obtained by hydrogenating nitrile butadiene rubber (NBR) to convert the double bonds originally contained in the nitrile butadiene rubber (NBR) into single bonds.

[0095] The hydrogenated nitrile butadiene rubber may have a repeating unit content derived from acrylonitrile (AN) of 20% by weight to 50% by weight, more preferably 25% by weight to 45% by weight, and most preferably 30% by weight to 40% by weight, based on the total weight.

[0096] The inventors of the present invention discovered that when the weight-average molecular weight (Mw) of the hydrogenated nitrile butadiene rubber (HNBR) is 130,000 g / mol or more, the brittleness of a positive electrode containing lithium iron phosphate as a positive electrode active material is dramatically improved, leading to the completion of the present invention. The hydrogenated nitrile butadiene rubber may have a weight-average molecular weight (Mw) of 130,000 g / mol or more, preferably 150,000 g / mol to 1,000,000 g / mol, and more preferably 200,000 g / mol to 500,000 g / mol. When the weight-average molecular weight (Mw) of the second hydrogenated nitrile butadiene rubber is 130,000 g / mol or more, the improved flexibility of the positive electrode active material layer can increase the rolling density of the positive electrode, and as the rolling density increases, the positive electrode resistance can be reduced.

[0097] The rubber-based binders contained in the first and second positive electrode active material layers may be the same compound and may have the same weight-average molecular weight and chemical formula.

[0098] Meanwhile, the rubber binder contained in each of the first and second positive electrode active material layers may satisfy the following condition 1. The value of R2 / R1 below may be preferably 1 to 2.9, and more preferably 1.1 to 2.8.

[0099] [Condition 1] 1≦R2 / R1≦3

[0100] R1 represents the content (wt%) of the rubber binder contained in the first positive electrode active material layer, R2 represents the content (wt %) of the rubber binder contained in the second positive electrode active material layer.

[0101] If the value of R2 / R1 exceeds 3, the rubber-based binder contained in the second positive electrode active material layer will be excessively large, which may increase the viscosity of the slurry for the second positive electrode active material layer and reduce the coating performance, which is undesirable. Conversely, if the value is less than 1, the rubber-based binder contained in the second positive electrode active material layer will be excessively small, which may reduce the flexibility of the entire positive electrode, which is undesirable.

[0102] The rubber binder may be contained in the first positive electrode active material layer at 0.2 wt% to 0.9 wt%, preferably 0.3 wt% to 0.8 wt%, and more preferably 0.4 wt% to 0.7 wt%. The rubber binder may be contained in the second positive electrode active material layer at 0.3 wt% to 1.0 wt%, preferably 0.4 wt% to 0.9 wt%, and more preferably 0.5 wt% to 0.8 wt%. The rubber binder may be contained in the first positive electrode active material layer and the second positive electrode active material layer at 0.25 wt% to 0.85 wt%, preferably 0.35 wt% to 0.75 wt%, and more preferably 0.4 wt% to 0.65 wt% in total.

[0103] When the rubber-based binder is contained in each of the first and second positive electrode active material layers within the above range, the flexibility of the positive electrode is improved, the coating stability of the slurry is excellent, and the first and second positive electrode active material layers can be uniformly coated.

[0104] The weight ratio of the fluorine-based binder to the rubber-based binder in the first positive electrode active material layer may be 90:10 to 75:25, preferably 8:1 to 4:1, and more preferably 7:1 to 4:1. When the weight ratio of the fluorine-based binder to the rubber-based binder satisfies the above range, the coating stability of the slurry may be excellent and the interfacial adhesion between the positive electrode current collector and the first positive electrode active material layer may be excellent.

[0105] The weight ratio of the fluorine-based binder to the rubber-based binder in the second positive electrode active material layer may be 80:20 to 60:40, and preferably 75:25 to 60:40. When the weight ratio of the fluorine-based binder to the rubber-based binder satisfies the above range, the coating stability of the slurry may be excellent and the flexibility of the positive electrode may be excellent.

[0106] The total amount of the fluorine-based binder and the rubber-based binder contained in the first and second positive electrode active material layers may be 1.8 wt % to 3.6 wt % based on the entire positive electrode active material layer.

[0107] (3) Dispersant The first positive electrode active material layer and the second positive electrode active material layer constituting the positive electrode according to the present invention each contain a rubber-based dispersant.

[0108] The dispersant is used to increase the dispersibility of the components of the active material layer, particularly the conductive material. Meanwhile, the rubber-based dispersant of the present invention also serves to adjust the viscosity of the positive electrode slurry to a suitable range.

[0109] The rubber dispersant of the present invention may be hydrogenated nitrile butadiene rubber (HNBR). The hydrogenated nitrile butadiene rubber (HNBR) is formed by subjecting nitrile butadiene rubber (NBR) to a hydrogenation reaction, thereby converting the double bonds originally contained in the nitrile butadiene rubber (NBR) into single bonds.

[0110] The rubber-based dispersant of the present invention may be hydrogenated nitrile butadiene rubber (HNBR). The hydrogenated nitrile butadiene rubber may have a weight-average molecular weight (Mw) of 10,000 g / mol to 100,000 g / mol, preferably 10,000 g / mol to 80,000 g / mol, and more preferably 10,000 g / mol to 50,000 g / mol. When the weight-average molecular weight of the hydrogenated nitrile butadiene rubber (HNBR) dispersant satisfies the above range, it can improve the solvent wetting and dispersibility of lithium iron phosphate particles and suppress particle aggregation of the lithium iron phosphate. It can also effectively and satisfactorily mix the binder and lithium iron phosphate particles, thereby contributing to improved interfacial adhesion between the positive electrode current collector and the first positive electrode active material layer.

[0111] In addition, hydrogenated nitrile butadiene rubber inhibits the agglomeration of conductive materials, improves the positive electrode conductive network, and even if the conductive materials are agglomerated, they are agglomerated into spheres rather than lines, thereby minimizing the specific surface area of ​​the agglomerated conductive materials compared to when the conductive materials are agglomerated into lines. As a result, the surface area of ​​the positive electrode active material adjacent to the agglomerated conductive materials that cannot participate in the lithium insertion / extraction reaction is minimized, thereby reducing the discharge resistance of the lithium secondary battery.

[0112] The dispersant may be contained in the first positive electrode active material layer in an amount of 0.01 wt % to 0.7 wt %, preferably 0.05 wt % to 0.6 wt %, and more preferably 0.1 wt % to 0.5 wt %. The dispersant may also be contained in the second positive electrode active material layer in the above range. When the content of the dispersant satisfies the above range, aggregation of the positive electrode active material can be suppressed, and gelation of the positive electrode slurry composition can be prevented.

[0113] (4) Conductive material The first positive electrode active material layer and the second positive electrode active material layer constituting the positive electrode according to the present invention each contain a conductive material.

[0114] The conductive material is intended to improve the conductivity of the electrode and is not particularly limited as long as it is conductive without inducing chemical changes in the battery. Examples include graphite; carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, 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. Among these, carbon nanotubes, carbon nanofibers, and carbon black are preferred as conductive materials in the present invention, with carbon nanotubes being most preferred. The conductive network of carbon nanotubes can mitigate the phenomenon of binder lifting during the drying process of the positive electrode slurry, making them the most preferred conductive material contained in the positive electrode of the present invention.

[0115] Carbon nanotubes have cylindrical graphite sheets with nanometer-sized diameters and an sp2 bond structure, and exhibit conductive or semiconductive properties depending on the graphite sheet's winding angle and structure. Carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes depending on the number of bonds forming the wall, and these carbon nanotubes can be appropriately selected depending on the application of the dispersion.

[0116] The carbon nanotubes may have a secondary shape formed by agglomeration or arrangement of a plurality of carbon nanotubes, for example, bundle- or rope-shaped carbon nanotubes in which a plurality of carbon nanotubes are aligned or arranged in a certain direction, or entangled-type carbon nanotubes in which a plurality of carbon nanotubes are tangled in a random manner, such as a sphere or potato. In terms of dispersibility, the carbon nanotubes are more preferably bundle-type carbon nanotubes.

[0117] The BET specific surface area of ​​the carbon nanotubes is 100m 2 / g~1000m 2 / g, 150m 2 / g~800m 2 / g, 150m 2 / g~500m 2 / g, 150m 2 / g~300m 2 / g, or 150m 2 / g~200m 2 / g.

[0118] When the conductive material is carbon nanotubes, the conductive material may be contained in the first positive electrode active material layer and the second positive electrode active material layer at 0.3 wt% to 2.0 wt%, 0.6 wt% to 1.5 wt%, or more specifically, 0.7 wt% to 1.3 wt%, respectively. If the conductive material content is less than 0.3 wt%, the binder may become more likely to lift off during drying of the electrode, which is undesirable. If the conductive material content exceeds 2.0 wt%, the dispersant content increases, which may result in a decrease in the positive electrode active material content, which is undesirable.

[0119] The conductive material may be contained in the first positive electrode active material layer and the second positive electrode active material layer at the same content or at different contents. When the conductive material contents in the first positive electrode active material layer and the second positive electrode active material layer are different from each other, the following condition 2 may be satisfied.

[0120] [Condition 2] 0.5≦E2 / E1≦0.9

[0121] E1 represents the content (wt%) of the linear conductive material contained in the first active material layer, E2 represents the content (wt %) of the linear conductive material contained in the second active material layer.

[0122] A positive electrode that satisfies condition 2 has a higher conductive material content in the first positive electrode active material layer that contacts the positive electrode current collector than in the second positive electrode active material layer, and a conductive network formed by the conductive material in the first positive electrode active material layer. This conductive network suppresses binder migration and improves interfacial adhesion between the positive electrode current collector and the first positive electrode active material layer. The improved interfacial adhesion can also suppress volumetric changes during charge and discharge, thereby reducing the interfacial resistance of the positive electrode and improving the resistance characteristics of the positive electrode.

[0123] When Condition 2 is satisfied, the conductive material may be contained in the first positive electrode active material layer in a range of 1.0 to 1.5% by weight, preferably 1.1 to 1.4% by weight, and in the second positive electrode active material layer in a range of 0.4 to 0.95% by weight, preferably 0.6 to 0.90% by weight.

[0124] The positive electrode according to the present invention can be manufactured by a conventional method for manufacturing a positive electrode. Specifically, the positive electrode can be manufactured by preparing a positive electrode slurry composition containing the above-described positive electrode active material, a conductive material, a binder, and a dispersant, applying the positive electrode slurry composition onto a positive electrode current collector, and then drying and rolling the composition.

[0125] Alternatively, the positive electrode may be manufactured by casting the positive electrode slurry composition on a separate support, peeling it off from the support, and laminating the resulting film on a positive electrode current collector.

[0126] The positive electrode according to one embodiment of the present invention includes a first positive electrode active material layer that maximizes adhesion and a second positive electrode active material layer that maximizes flexibility, and thus can have excellent positive electrode adhesion and flexibility even with a low binder content compared to conventional single-layer positive electrodes. In addition, the porosity can be controlled within an optimal range, thereby improving the resistance characteristics of the battery.

[0127] As a result, a high-loading positive electrode can be manufactured, and detachment of the positive electrode is prevented, thereby reducing the cell resistance of the secondary battery, improving the capacity and output characteristics of the battery, and reducing defects that occur during the manufacturing process.

[0128] The positive electrode of the present invention may have an adhesive strength between the positive electrode current collector and the positive electrode active material layer of 20 gf / 20 mm or more, specifically 23 gf / 20 mm or more, as measured by a 90° peel test.

[0129] Furthermore, the positive electrode of the present invention may have flexibility such that, during a flexibility test in which a phi measuring rod is brought into contact with the positive electrode active material layer and then the positive electrode is lifted up, the maximum phi (φ) value of the measuring rod at which cracks occur is 5 phi (φ) or less, specifically 4 phi (φ) or less, and more specifically 3 phi (φ) or less.

[0130] <Lithium secondary battery> Next, the lithium secondary battery according to the present invention will be described.

[0131] A lithium secondary battery according to an embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0132] In the lithium secondary battery, the positive electrode may be as described above. For example, the positive electrode may include a positive current collector, a first positive active material layer formed on one or both sides of the positive current collector, and a second positive active material layer formed on the first positive active material layer, the first positive active material layer and the second positive active material layer each including lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant, the ratio (P2 / P1) of the weight of fluorine contained in the second positive active material layer to the weight of fluorine contained in the first positive active material layer (P1) being 1 or less, and the porosity calculated by the following formula 1 may be 25% to 28%, preferably 25.5% to 27.5%:

[0133] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0134] The rubber binder may be a hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) of 130,000 g / mol or more.

[0135] The rubber dispersant may be hydrogenated nitrile butadiene rubber (HNBR) having a weight average molecular weight (Mw) ranging from 10,000 g / mol to 100,000 g / mol.

[0136] Furthermore, the rubber binder contained in each of the first and second positive electrode active material layers can satisfy the following condition 1.

[0137] [Condition 1] 1≦R2 / R1≦3

[0138] R1 represents the content (wt%) of the rubber binder contained in the first positive electrode active material layer, R2 represents the content (wt %) of the rubber binder contained in the second positive electrode active material layer.

[0139] The lithium iron phosphate, fluorine-based binder, rubber-based binder, and rubber-based dispersant contained in the first and second positive electrode active material layers have been described in detail above, so a duplicated description will be omitted.

[0140] The negative electrode can be manufactured, for example, by preparing a negative electrode-forming composition containing a negative electrode active material, a negative electrode binder, and a negative electrode conductive material on a negative electrode current collector, and then coating the composition on the negative electrode current collector.

[0141] The negative electrode active material is not particularly limited, and may generally be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and highly crystalline carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and composites containing metallic compounds and carbonaceous materials. In addition, examples of low-crystalline carbon include soft carbon and hard carbon, and examples of high-crystalline carbon include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbon such as petroleum or coal tar pitch-derived cokes. One or a mixture of two or more of these may be used, and a metallic lithium thin film may also be used as the negative electrode active material.

[0142] The negative electrode conductive material is used to impart conductivity to the electrode and can be any material that is electronically conductive without causing chemical changes in the battery. Specific examples include graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, and carbon nanotubes; metal powder or fiber, such as copper, nickel, aluminum, or silver; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The negative electrode conductive material is typically present in an amount of 1 wt % to 30 wt %, specifically 1 wt % to 20 wt %, and more specifically 1 wt % to 10 wt %, based on the total weight of the negative electrode active material layer.

[0143] The negative electrode binder serves to improve adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The negative electrode binder may be included in an amount of 1 wt% to 30 wt%, specifically 1 wt% to 20 wt%, more specifically 1 wt% to 10 wt%, based on the total weight of the negative electrode active material layer.

[0144] Meanwhile, the negative electrode current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used.

[0145] The negative electrode current collector may typically have a thickness of 3 μm to 500 μm, and like the positive electrode current collector, the negative electrode current collector may have fine irregularities on its surface to enhance the binding force of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0146] Meanwhile, in the lithium secondary battery, the separator may be any material commonly used as a separator in lithium secondary batteries, with no particular limitations. In particular, a material that exhibits low resistance to ion migration of the electrolyte and excellent electrolyte humidification capability is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Typical porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. The separator may be a porous thin film having a pore diameter of 0.01 μm to 10 μm and a thickness of 5 μm to 300 μm.

[0147] Meanwhile, in the lithium secondary battery, the electrolyte may include an organic solvent and a lithium salt typically used in electrolytes, and is not particularly limited.

[0148] The organic solvent may be any organic solvent without particular limitation, as long as it can function as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent that can be used include ester-based solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether-based solvents such as dibutyl ether and tetrahydrofuran; ketone-based solvents such as cyclohexanone; aromatic hydrocarbon-based solvents such as benzene and fluorobenzene; and carbonate-based solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC).

[0149] Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0150] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O), etc. The lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%.

[0151] In addition to the electrolyte components, the electrolyte may further contain one or more additives such as pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the battery's life characteristics, suppressing battery capacity reduction, improving the battery's discharge capacity, etc. In this case, the additives may be contained in an amount of 0.1 wt % to 5 wt % based on the total weight of the electrolyte.

[0152] The lithium secondary battery of the present invention may be manufactured by forming an electrode assembly by disposing a separator between a positive electrode and a negative electrode, and then placing the electrode assembly in a cylindrical or prismatic battery case and then injecting an electrolyte into the case. Alternatively, the electrode assemblies may be stacked, impregnated with an electrolyte, and then placed in a battery case and sealed.

[0153] When fabricating the lithium secondary battery of the present invention, the electrode assembly may be dried to remove one or more organic solvents used in fabricating the positive electrode, such as N-methyl-2-pyrrolidone (NMP), acetone, ethanol, propylene carbonate, ethyl methyl carbonate, ethylene carbonate, and dimethyl carbonate. If an electrolyte having the same organic solvent as that used in fabricating the positive electrode is used, the step of drying the electrode assembly may be omitted.

[0154] Unlike the above-described lithium secondary batteries, the lithium secondary battery according to another embodiment of the present invention may be an all-solid-state battery.

[0155] The battery case may be one typically used in the art, and may have any shape depending on the intended use of the battery, such as a cylindrical can, a square can, a pouch can, or a coin can.

[0156] The lithium secondary battery according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, energy storage systems (ESS), and electric vehicles such as hybrid electric vehicles (HEV), etc.

[0157] The present invention will be described in more detail below with reference to examples. However, the following examples are for illustrative purposes only and the scope of the present invention is not limited to these examples.

[0158] Example 1: Preparation of the positive electrode (1) Preparation of first positive electrode slurry

[0159] Carbon nanotubes (CNT, BET specific surface area: 250 m) were used as conductive materials. 2 A dispersion containing a polymer (polymer of 100% cellulose, ...) and a rubber-based dispersant, hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 30,000 g / mol, was prepared.

[0160] Average particle size D as lithium iron phosphate 50 A first positive electrode slurry was prepared by adding LiFePO4, a 1.0 μm diameter, monolithic primary particles, polyvinylidene fluoride (PVDF) with a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, hydrogenated nitrile butadiene rubber with a weight average molecular weight (Mw) of 310,000 g / mol as a rubber-based binder, and the dispersion to an N-methylpyrrolidone (NMP) solvent and mixing at 2500 rpm for 90 minutes using Homo-disperse. The lithium iron phosphate, conductive material, fluorine-based binder, rubber-based binder, and dispersant in the first positive electrode slurry were present in a weight ratio of 96.26:0.8:2.2:0.5:0.24.

[0161] (2) Preparation of second positive electrode slurry A second positive electrode slurry was prepared using the same method as the first positive electrode slurry, except that the weight ratios of the lithium iron phosphate, conductive material, fluorine-based binder, rubber-based binder, and dispersant in the positive electrode slurry were changed as shown in Table 1.

[0162] (3) Manufacturing of the positive electrode Using a dual slot die coater, the first positive electrode slurry was applied to an aluminum foil at a thickness of 300 mg / 25 cm. 2 , the second positive electrode slurry was 300 mg / 25 cm 2 and the second positive electrode slurry was coated so as to be layered on the first positive electrode slurry.

[0163] The cathode slurry was then dried with hot air at 130°C for 5 minutes to achieve a solids content of 99.0 wt% or more. The cathode was then rolled to achieve a porosity of 26.6% for the cathode active material layer, thereby producing a cathode. The porosity is calculated using the following equation 1. In equation 1, the "measured density of the cathode active material layer" is a value calculated by measuring the weight and volume of the cathode active material layer separated from the cathode current collector of the rolled cathode, and the "true density of the cathode active material" is the density relative to the volume of only the cathode active material particles themselves, excluding pores, and can be measured using a Pycnometer (AccuPycII 1340).

[0164] [Formula 1] Porosity = {1 - (measured density of positive electrode active material layer / true density of positive electrode active material)} × 100

[0165] Examples 2 to 3, Comparative Examples 1 to 6: Production of Positive Electrode Positive electrodes were prepared in the same manner as in Example 1, except that the weight ratios of the lithium iron phosphate, conductive material, fluorine-based binder, rubber-based binder, and dispersant in the first positive electrode slurry and the second positive electrode slurry were changed as shown in Table 1, and the positive electrodes were rolled to have porosities as shown in Table 1.

[0166] Comparative Example 7

[0167] Carbon nanotubes (CNT, BET specific surface area: 250 m) were used as conductive materials. 2 A dispersion containing a polymer (polymer of 100% cellulose, ...) and a rubber-based dispersant (hydrogenated nitrile butadiene rubber having a weight-average molecular weight (Mw) of 30,000 g / mol) was prepared.

[0168] Average particle size D as lithium iron phosphate 50The cathode slurry was prepared by adding LiFePO4, a 1.0 μm diameter, monolithic primary particles, polyvinylidene fluoride (PVDF) with a weight average molecular weight (Mw) of 630,000 g / mol as a fluorine-based binder, and the dispersion to an N-methylpyrrolidone (NMP) solvent and mixing at 2500 rpm for 90 minutes using Homo-disperse. The lithium iron phosphate, conductive material, fluorine-based binder, and dispersant in the cathode slurry were present in a weight ratio of 96.96:0.8:3.0:0.24.

[0169] 600mg / 25cm of positive electrode slurry was applied to the aluminum foil. 2 The positive electrode slurry was dried with hot air at 130°C for 5 minutes so that the solid content of the positive electrode slurry was 99.0 wt% or more. Then, the positive electrode was manufactured by rolling the positive electrode active material layer so that the porosity was 29.2%.

[0170] [Table 1]

[0171] Experimental Example 1: Viscosity measurement of positive electrode slurry

[0172] The viscosity of the positive electrode slurries produced in Examples 1 to 3 and Comparative Examples 1 to 7 was measured, and the results are shown in Table 2.

[0173] Specifically, the positive electrode slurries prepared in Examples 1 to 3 and Comparative Examples 1 to 7 were cooled for 1 hour at room temperature and a relative humidity of 1%, and then the viscosities of the positive electrode slurry compositions were measured at 25°C and a shear rate of 2.5 / s using a Brookfield viscometer. The viscosity measurements were performed within 2 hours after the preparation of the positive electrode slurry compositions, including the cooling time.

[0174] Experimental example 2: Positive electrode adhesion test

[0175] The positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 7 were vacuum-dried at 130°C for 2 hours, and then the adhesive strength between the positive electrode active material layer and the positive electrode current collector was measured. The results are shown in Table 2.

[0176] Specifically, the positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 7 were cut to a length of 150 mm and a width of 20 mm, and the positive electrode surfaces were attached to a 75 mm long, 25 mm wide glass slide in the longitudinal direction using double-sided tape. That is, the glass slide was attached to an area corresponding to half of the positive electrode in the longitudinal direction. Then, evaluation samples were produced by rolling the double-sided tape 10 times with a roller to ensure uniform adhesion.

[0177] Next, the glass slide portion of the evaluation sample was fixed to the sample stage of a Universal Testing Machine (UTM) (product name: LS5, manufacturer: LLOYD), and the half of the positive electrode without the glass slide was connected to the load cell of the UTM device. The load cell was applied at a 90° angle at a speed of 100 mm / min and moved up to 50 mm, while the load applied to the load cell was measured. The average load measured over a 20 mm to 40 mm section of the travel section was calculated, and this was repeated five times. The average value was evaluated as the positive electrode adhesive strength (gf / 20 mm) of each sample.

[0178] Experimental example 3: Positive electrode flexibility test

[0179] The positive electrodes produced in Examples 1 to 3 and Comparative Examples 1 to 7 were vacuum-dried at 130° C. for 2 hours, and then the flexibility of each positive electrode was measured. The results are shown in Table 2.

[0180] Specifically, the positive electrodes prepared in Examples 1 to 3 and Comparative Examples 1 to 7 were cut into 10 cm wide x 30 cm long pieces. Measuring rods with diameters of 2.5, 3, 4, 5, 6, 7, 8, 9, and 10 φ were prepared. Then, with the positive electrode current collector surface facing the measuring rod, each cut positive electrode was bent in half and both ends of the positive electrode were lifted at a rate of 10 mm per minute. The lifting force was continued until the force measured by the UTM reached 5 N. Measurements were made at each diameter, and the electrodes were observed under an optical microscope to determine whether cracks had occurred. If no cracks were found, the test was continued with a smaller diameter. The diameters (φ) of the measuring rods at which cracks occurred are listed in Table 2.

[0181] Experimental Example 4: Measurement of fluorine weight

[0182] For each of the positive electrodes according to the Examples and Comparative Examples, the first positive electrode active material layer and the second positive electrode active material layer were scraped off to obtain 100 mg samples. The weight of fluorine contained in each sample was measured using a combustion reaction technique, and the ratio (P2 / P1) of the weight (P1) of fluorine contained in the second positive electrode active material layer to the weight (P2) of fluorine contained in the first positive electrode active material layer was calculated. The results are shown in Table 2.

[0183] To measure the weight of fluorine contained in each sample, the first and second positive electrode active material layer samples were completely combusted under high-pressure oxygen conditions (40 atm), and the generated gas was collected in an absorbent. The gas collection absorbent was a 1 L solution containing 2.52 g NaHCO3 and 2.52 g Na2CO3. Once the fluorine ions were absorbed or dissolved in the gas collection absorbent, the weight of fluorine in each sample was measured using ion chromatography. The ion chromatography column and analytical conditions were as follows:

[0184] Ion chromatography: ICS-3000 Column: IonPac AS18 (4 x 250 mm) Detector: suppressed conductivity detector SRS current:76Ma Injection volume: 20uL

[0185] The same test was repeated three times and the average values ​​were shown.

[0186] Table 2 shows the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer based on the analysis results of ion chromatography.

[0187] [Table 2]

[0188] The positive electrodes of Examples 1 to 3 satisfy the condition that the ratio (P2 / P1) of the weight P2 of fluorine contained in the second positive electrode active material layer to the weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less, and therefore have far superior adhesive strength compared to the positive electrode of Comparative Example 3 which does not satisfy the above condition.

[0189] The positive electrodes according to Examples 1 to 3, which contain a rubber-based binder, were shown to have far superior flexibility compared to the positive electrode of Comparative Example 4, which does not contain a rubber-based binder.

[0190] The positive electrode of Comparative Example 1, which has a porosity of less than 25%, was shown to have poorer flexibility than the positive electrodes of the Examples, and the positive electrode of Comparative Example 2, which has a porosity of more than 28%, was shown to have poorer adhesive strength than the positive electrodes of the Examples. Therefore, it is clear that it is preferable to control the porosity to 25% to 28% from the perspectives of flexibility and adhesive strength.

[0191] On the other hand, in the case of Comparative Example 5, since the first positive electrode slurry and the second positive electrode slurry did not contain a rubber-based dispersant, the lithium iron phosphate and conductive material contained in the slurry were severely coagulated, and it was not possible to manufacture an electrode.

[0192] Experimental Example 5: Battery Resistance Evaluation Using the positive electrodes produced in Examples 1 to 3, Comparative Examples 1 to 2, and Comparative Examples 6 and 7, lithium secondary batteries were produced.

[0193] The negative electrode of the lithium secondary battery was prepared by adding a mixture of artificial graphite (GT), natural graphite (AGP8), and SiO (KSC6027D) (85:10:5 wt%) as the negative electrode active material, a mixture of carbon black (Super-C65) and CNT (NCL-H5) as the conductive material, SBR (BM-L302) as the binder, and CMC (Daicel 2200) as the thickener to distilled water in a weight ratio of 95.1:1.5:2.3:1.1 to prepare a negative electrode slurry. The negative electrode slurry was coated on one side of a 6 μm-thick copper foil and dried and rolled under the same conditions as the positive electrode.

[0194] The electrolyte solution was prepared by mixing 0.1 wt% tetravinylsilane (VS2), 1 wt% ethylene sulfate (ESa), 0.5 wt% 1,3-propene sultone (PS), 1 wt% lithium difluorophosphate (DFP), and 0.2 wt% LiBF4 in a 30:70 (volume ratio) organic solvent, and dissolving 0.7M LiPF6 and 0.5M LiFSI in the solvent. A separator (DB0905 / BA1, 8μm) was placed between the positive and negative electrodes, and the electrolyte was then injected to fabricate a lithium secondary battery.

[0195] The resistance of each secondary battery containing the positive electrode produced in Examples 1 to 3, Comparative Examples 1 to 2, and Comparative Examples 6 and 7 was measured using the following method. The battery was discharged at a current of 0.2 C at 25°C from an SOC of 100% until the SOC reached 30%. A pulse current of 2.0 C was then applied for 10 seconds at an SOC of 30%. DCIR was calculated using the following formula, and the results are shown in Table 3.

[0196] [Table 3]

[0197] The initial resistance of the batteries including the positive electrodes of Examples 1 to 3, which have a porosity of 25% to 28%, is lower than that of the batteries including the positive electrodes of Comparative Examples 1, 2, 6, and 7, which do not satisfy the above range of porosity. Therefore, it is expected that the lithium secondary battery including the positive electrode according to the present invention will have improved resistance characteristics.

[0198] Although the preferred embodiments of the present invention have been described above, it will be understood that those skilled in the art or those with ordinary knowledge in the art can make various modifications and changes to the present invention without departing from the spirit and technical scope of the present invention as set forth in the claims.

[0199] Therefore, the technical scope of the present invention is not limited to the content described in the Summary of the Invention of the specification, but can be defined by the claims.

Claims

1. a positive electrode current collector; a first positive electrode active material layer formed on one or both surfaces of a positive electrode current collector; a second positive electrode active material layer formed on the first positive electrode active material layer, the first positive electrode active material layer and the second positive electrode active material layer each contain lithium iron phosphate, a conductive material, a fluorine-based binder, a rubber-based binder, and a rubber-based dispersant; the rubber binder and the rubber dispersant are each hydrogenated nitrile butadiene rubber (HNBR); The rubber binder has a weight average molecular weight (Mw) of 130,000 g / mol or more, The rubber-based dispersant has a weight average molecular weight (Mw) in the range of 10,000 g / mol to 100,000 g / mol, a ratio (P2 / P1) of a weight P2 of fluorine contained in the second positive electrode active material layer to a weight P1 of fluorine contained in the first positive electrode active material layer is 1 or less; The porosity calculated by the following formula 1 is 25% to 28%, In Formula 1, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer. [Formula 1] Porosity={1−(measured density of positive electrode active material layer / true density of positive electrode active material)}×100

2. 2. The positive electrode of claim 1, wherein the porosity calculated by Equation 1 is 25.5% to 27.5%.

3. 2. The positive electrode of claim 1, wherein the rubber binder has a weight average molecular weight (Mw) in the range of 150,000 g / mol to 1,000,000 g / mol.

4. Fluorine-based binders are contained in the first positive electrode active material layer in a range of 1.5 wt % to 3.0 wt %; The positive electrode according to claim 1 , wherein the second positive electrode active material layer contains the cation exchange material in an amount ranging from 0.8% by weight to 2.4% by weight.

5. The positive electrode according to claim 1 , wherein the rubber binder contained in each of the first positive electrode active material layer and the second positive electrode active material layer satisfies the following condition 1: [Condition 1] 1≦R2 / R1≦3 R1 represents the content (wt%) of the rubber-based binder contained in the first positive electrode active material layer, and R2 represents the content (wt%) of the rubber-based binder contained in the second positive electrode active material layer.

6. The rubber binder is contained in the first positive electrode active material layer in a range of 0.2 wt % to 0.9 wt %; The positive electrode according to claim 1 , wherein the second positive electrode active material layer contains the cation exchange material in an amount of 0.3% by weight to 1.0% by weight.

7. The positive electrode of claim 1 , wherein the conductive material is a carbon nanotube.

8. 2. The positive electrode according to claim 1, wherein the conductive material is contained in each of the first positive electrode active material layer and the second positive electrode active material layer in an amount ranging from 0.3% by weight to 2.0% by weight.

9. The positive electrode according to claim 1 , wherein the lithium iron phosphate is a compound represented by the following Chemical Formula 1: [Chemical formula 1] Li １＋ａ Fe １－ｘ M ｘ (PO ４－ｂ )X ｂ (In the above Chemical Formula 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are −0.5≦a≦0.5, 0≦b≦0.1, and 0≦x≦0.5, respectively.)

10. The average particle diameter D of the lithium iron phosphate ５０ The positive electrode according to claim 1, wherein the thickness is 0.6 μm to 3.0 μm.

11. The positive electrode according to claim 1 , wherein the fluorine-based binder is a polyvinylidene fluoride (PVDF)-based polymer binder.

12. A lithium secondary battery comprising the positive electrode according to any one of claims 1 to 11.

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