Positive electrode active material for lithium secondary battery, method for producing same, and positive electrode for lithium secondary battery comprising same

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery, the positive electrode active material comprising: a lithium transition metal oxide; and a surface layer provided on the surface of the lithium transition metal oxide, wherein the surface layer comprises carbon nanotubes and a cationic polymer, and has a zeta potential of -2 to 0 mV.

Description

Cathode active material for lithium secondary batteries, method for producing same, and cathode for lithium secondary batteries containing same

[0001] The present invention relates to a positive electrode active material for a lithium secondary battery, a method for producing the same, and a positive electrode for a lithium secondary battery comprising the same.

[0002] As demand for carbon emission reduction technologies and eco-friendly energy production technologies to mitigate global warming grows, the development and production of electrochemical energy storage devices is becoming a global issue. In particular, secondary batteries, a core technology for energy storage devices, are being developed with a focus on maximizing energy density, lifespan, and environmental friendliness.

[0003] Lithium secondary batteries are widely used as power sources for portable electronic devices, electric vehicles, and energy storage systems (ESS). The market for lithium secondary batteries is increasingly demanding higher performance and lower production costs. The performance and production costs of lithium secondary batteries depend heavily on the series of manufacturing steps and the production technology that comprises the materials, electrodes, cells, and modules / packs that make up the battery.

[0004] For example, research has been conducted on the use of nickel-rich cathode materials in lithium secondary batteries. By increasing the nickel content, capacity can be increased, while decreasing the cobalt content can reduce costs. However, these nickel-rich cathode materials suffer from structural and electrochemical instability limitations, which are further exacerbated by higher nickel content.

[0005] Accordingly, various studies are being conducted to improve performance while increasing capacity at a lower cost in the cathode active material of lithium secondary batteries.

[0006] Prior patent: KR 10-1196962 (October 26, 2012)

[0007] The purpose of the present invention is to provide a cathode active material for a lithium secondary battery having a surface layer formed thereon, thereby preventing unnecessary interfacial reactions with an electrolyte, and providing a cathode active material for a lithium secondary battery having excellent electrical conductivity and thermal stability, a method for producing the same, and a cathode for a lithium secondary battery comprising the same.

[0008] One embodiment of the present invention provides a cathode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same.

[0009] In one embodiment, the cathode active material for the lithium secondary battery includes a lithium transition metal oxide in particle form; and a surface layer provided on the surface of the lithium transition metal oxide; wherein the surface layer includes carbon nanotubes and a cationic polymer and may have a zeta potential of -2 mV to 0 mV.

[0010] In one embodiment, the average thickness of the surface layer may be 10 nm to 100 nm.

[0011] In one embodiment, the carbon nanotubes may be present in an amount of 0.05 wt% to 5 wt%.

[0012] In one embodiment, the cationic polymer may be present in an amount of 0.01 wt% to 0.5 wt%.

[0013] In one embodiment, the surface layer may be provided in the form of a three-dimensional network structure.

[0014] In one embodiment, the ratio of the maximum peak intensity (ID) of the D band at 1340 nm to 1360 nm to the maximum peak intensity (IG) of the G band at 1575 nm to 1600 nm obtained by a Raman spectrum using a laser having a wavelength of 514.5 nm (ID / IG) may be 0 to 2.

[0015] In one embodiment, the carbon nanotube is formed in a bundle shape by partially converging 1 to 10 single-walled carbon nanotubes or multi-walled carbon nanotubes, and the diameter of the bundle shape is 2 nm to 35 nm, the length is 4 ㎛ to 10 mm, and the carbon nanotube may have a content of oxygen atoms relative to carbon atoms as a component of the carbon nanotube of 0.01 mol% to 10 mol%.

[0016] In one embodiment, the carbon nanotube has a specific surface area of ​​500 m 2 / g to 1500 m 2 / g may be.

[0017] In one embodiment, the positive electrode active material for a lithium secondary battery has a surface area of ​​0.1 m 2 / g to 0.8 m 2 / g may be.

[0018] In one embodiment, the cationic polymer may include at least one selected from the group consisting of poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), polyamidoamine (PAMAM), polyallylamine, quaternary ammonium polymers, cationic polysaccharides, cationic cellulose, cationic starch, cationic polyacrylamide, chitosan, and gelatin.

[0019] In one embodiment, the lithium transition metal oxide comprises a secondary particle formed from a group of a plurality of primary particles, the secondary particle being represented by the following chemical formula 1, and the secondary particle may have an average particle diameter (D50) of 2 µm to 30 µm and a BET specific surface area of ​​0.1 m2 / g to 1.0 m2 / g.

[0020] [Chemical Formula 1]

[0021] Li a Ni 1-x-y Co x M1 y O2

[0022] (In the above chemical formula 1, M1 is one or more elements selected from the group consisting of Mn, Al, Zr, Ti, Mg, Ta, Nb, W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0≤x+y≤0.5.)

[0023] In one embodiment, the lithium transition metal oxide comprises a single particle and a plurality of fine particles attached to the surface of the single particle, the single particle is represented by the following chemical formula 1, the single particle has an average particle diameter (D50) of 2 µm to 10 µm and a BET specific surface area of ​​0.5 m2 / g to 2.5 m2 / g, and the fine particle may have an average particle diameter (D50) that is 0.01 to 0.1 times that of the single particle.

[0024] [Chemical Formula 1]

[0025] Li a Ni 1-x-y Co x M1 y O2

[0026] (In the above chemical formula 1, M1 is one or more elements selected from the group consisting of Mn, Al, Zr, Ti, Mg, Ta, Nb, W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0≤x+y≤0.5.)

[0027] In one embodiment, the electrical conductivity is from 0.2 S / cm to 10 5 It could be S / cm.

[0028] In one embodiment, the method for producing a cathode active material for a lithium secondary battery may include the steps of mixing a cationic polymer, a lithium transition metal oxide, and a first solvent and physically stirring them to produce a first dispersion solution, and producing a CNT dispersion solution including graphene; mixing the first dispersion solution and the CNT dispersion solution and performing physical stirring; and drying the mixture after the physical stirring is completed to produce a lithium transition metal oxide having a surface layer formed thereon.

[0029] In one embodiment, the first dispersion solution is prepared by physically stirring the cationic polymer and lithium transition metal oxide in the first solvent to prepare an intermediate material in which a solid material is dispersed, selecting the solid material from the intermediate material, and redispersing the solid material in the first solvent, wherein the cationic polymer may be 0.5 wt% to 5 wt%, and the lithium transition metal oxide may be 10 wt% to 50 wt%.

[0030] In one embodiment, the CNT dispersion solution comprises mixing one or more additives into a second solvent, adding the carbon nanotubes to the second solvent mixed with the additives, and physically stirring the mixture, wherein the carbon nanotubes may be in an amount of 0.01 wt% to 5 wt%, and the additives may be in an amount of 0.005 wt% to 5 wt%.

[0031] In one embodiment, the first solvent may include at least one of ethanol, ultrapure water, and methanol, and the second solvent may include at least one of N-methyl-2-pyrrolidone (NMP), polypyrrolidone, isopropanol, petroleum ether, tetrahydrofuran, ethyl acetate, N,N-dimethylacetamide, N,N-dimethylformamide, n-hexane, and a halogenated hydrocarbon.

[0032] In one embodiment, the additive material includes a first additive material and a second additive material, and the weight ratio of the first additive material and the second additive material may be 1:0.8 to 1.5.

[0033] In one embodiment, the first additive may be at least one of polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, carboxymethyl cellulose (CMC), and sodium alginate, and the second additive may be at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and cetyltrimethylammonium bromide (CTAB).

[0034] In one embodiment, the zeta potential of the lithium transition metal oxide may be -50 mV to -0 mV, the zeta potential of the solid material included in the first dispersion solution may be 5 mV to 30 mV, and the zeta potential of the positive electrode active material for a lithium secondary battery may be -2 mV to 0 mV.

[0035] In one embodiment, a positive electrode for a lithium secondary battery is provided, which includes the positive electrode active material described above and a binder, and has a composite density of 3.8 g / cc or more.

[0036] According to the present invention as described above, a positive electrode active material for a lithium secondary battery, a method for manufacturing the same, and a positive electrode for a lithium secondary battery including the same can be mass-produced at a low unit cost while having excellent physical properties.

[0037] In addition, according to the present invention, a cathode active material for a lithium secondary battery having a surface layer including carbon nanotubes can prevent aggregation between particles, and the surface layer is maintained even by an external force applied during a process, thereby providing a cathode active material for a lithium secondary battery having improved charge / discharge characteristics and cycle characteristics, a method for producing the same, and a cathode for a lithium secondary battery including the same.

[0038] FIG. 1 is a schematic drawing of a cathode active material for a lithium secondary battery according to one embodiment of the present invention.

[0039] Figure 2 is a flow chart of a method for manufacturing a positive electrode active material for a lithium secondary battery according to one embodiment of the present invention.

[0040] Figure 3 is a drawing showing the zeta potential at each step of the process of manufacturing a positive electrode active material according to an embodiment of the present invention.

[0041] Figure 4 is a drawing showing the surface of a positive electrode active material according to Example 1 of the present invention.

[0042] Figure 5 shows the CS analysis results for the polymer and SWCNT coating contents of the surface layer of the positive electrode active material of Example 1.

[0043] Figure 6 shows the results of Raman analysis of the surface of the positive electrode active material for Example 1 and Comparative Example 1.

[0044] Figure 7 shows the electrode conductivity measurement values ​​and DC-IR measurement change values ​​of the positive electrode using the positive electrode active material for Example 1 and Comparative Example 1.

[0045] Figure 8 shows the AC impedance measurement results of the symmetrical cells for Example 1 and Comparative Example 1.

[0046] Figure 9 is an evaluation of full cell life characteristics for Example 1 and Comparative Example 1.

[0047] Figure 10 shows the results of evaluating the rate characteristics of secondary batteries according to Examples 2 to 4.

[0048] Figure 11 is a drawing showing the electrochemical performance of secondary batteries according to Examples 5 to 10.

[0049] Figure 12 shows the results of confirming the specific surface area of ​​each positive electrode active material powder.

[0050] Specific details of other embodiments are included in the detailed description and drawings.

[0051] The advantages and features of the present invention, and the methods for achieving them, will become clear with reference to the embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in various different forms, and unless otherwise specified in the following description, all numbers, values, and / or expressions expressing components, reaction conditions, and contents of components in the present invention are to be understood as being modified in all cases by the term "about" because such numbers are approximations that reflect various uncertainties of measurement that occur in obtaining such values, among other things. In addition, when a numerical range is disclosed herein, such range is continuous and includes every value from the minimum value to the maximum value inclusive, unless otherwise indicated. Furthermore, when such a range refers to an integer, every integer from the minimum value to the maximum value inclusive, unless otherwise indicated, is included.

[0052] Additionally, when a range is described for a variable in the present invention, it will be understood that the variable includes all values ​​within the described range including the described endpoints of the range. For example, the range "5 to 10" will be understood to include the values ​​5, 6, 7, 8, 9, and 10, as well as any subranges such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, etc., and also any value between integers that fall within the described range, such as 5.5, 6.5, 7.5, 5.5 to 8.5, and 6.5 to 9. For example, a range of "10% to 30%" would be understood to include all integers up to and including 30%, as well as any subranges such as 10% to 15%, 12% to 18%, 20% to 30%, etc., and any value between reasonable integers within the stated range, such as 10.5%, 15.5%, 25.5%, etc.

[0053] FIG. 1 is a schematic drawing of a cathode active material for a lithium secondary battery according to one embodiment of the present invention.

[0054] Referring to FIG. 1, a cathode active material (100) for a lithium secondary battery according to one embodiment of the present invention includes a lithium transition metal oxide (110) in particle form; and a surface layer (120) provided on the surface of the lithium transition metal oxide (110); wherein the surface layer (120) includes carbon nanotubes and a cationic polymer. The cathode active material for a lithium secondary battery may have a zeta potential of -2 mV to 0 mV.

[0055] Typically, many studies are being conducted to increase the weight content of cathode active materials in lithium secondary batteries to increase the energy density of the cathode. To increase the content of cathode active materials, it is necessary to reduce the content of conductive materials such as carbon black, graphene, and carbon nanotubes, as well as the binder. However, if the content of the conductive materials and binder is reduced, conductivity decreases and uniform current application becomes difficult, which reduces the lifespan characteristics of the lithium secondary battery. Furthermore, when the binder content is reduced, the adhesion between the cathode active material and the conductive material, as well as between the cathode active material layer and the current collector, decreases, which reduces the output and lifespan characteristics of the lithium secondary battery. Furthermore, carbon black, which is a commonly used conductive material such as Denka Black, acetylene black, and Ketjen Black, exhibits a large volume per weight, making it difficult to increase the density of the cathode mixture using the rolling process alone. If the composite density is increased by force, the positive electrode active material may be crushed or damaged, the pores within the positive electrode may be reduced, which may lower the mobility of lithium ions and deteriorate the rate characteristics and life characteristics of the lithium secondary battery.

[0056] On the other hand, the cathode active material according to one embodiment of the present invention has a high energy density and can solve the problems of the aforementioned cathode, and can reduce the content of the conductive material and binder by providing a surface layer on the lithium transition metal oxide. In addition, the cathode active material according to the present embodiment can maintain high conductivity by using carbon nanotubes provided in the surface layer while uniformly maintaining a predetermined gap between the cathode active materials.

[0057] The above surface layer (120) may be provided in the form of a three-dimensional network structure. In the surface layer (120), the cationic polymer allows the carbon nanotubes to maintain the network structure, thereby facilitating contact between the lithium transition metal oxide (110) and the electrolyte, and improving the conductivity between adjacent positive electrode active materials (100) for lithium secondary batteries. In addition, in the surface layer (110), the carbon nanotubes are firmly fixed to a predetermined size, and by directly contacting the lithium transition metal oxide (110), high rate characteristics are exhibited, and even after the cycle is performed, a decrease in conductivity due to changes in crystal structure and volume can be prevented.

[0058] The above carbon nanotubes may be provided independently in the form of a strand of fibers or a bundle of fibers, and may be connected to the lithium transition metal oxide (110) through some point contact or point surface, or may be provided in a form in which neighboring carbon nanotubes are connected to each other. In the surface layer (120), neighboring carbon nanotubes may be spaced apart from each other in the horizontal or vertical direction to have some space, but may maintain a connected form to form a mutual electric network. In addition, in the surface layer (120), the carbon nanotubes may be stably fixed to the outer surface of the lithium transition metal oxide (120) while maintaining the raw material state, such as the size and shape, of the carbon nanotubes before being provided to the lithium transition metal oxide (110) substantially the same.

[0059] The above carbon nanotubes may be formed in a bundle shape by partially converging 1 to 10 single-walled carbon nanotubes or multi-walled carbon nanotubes. The diameter of the bundle shape may be 2 nm to 35 nm, and the length may be 4 μm to 10 mm.

[0060] The above-mentioned bundle-shaped carbon nanotubes have a problem in that, when the diameter is less than 2 nm, it is difficult to attach to the surface of the lithium transition metal oxide, and when the diameter is more than 35 nm, the thickness of the surface layer increases and ion mobility decreases, which is a problem. Specifically, the diameter of the bundle-shaped carbon nanotubes may be 2 nm to 30 nm, or 3 nm to 25 nm, or 4 nm to 20 nm.

[0061] In addition, if the length of the bundle shape is less than 4 ㎛, it is difficult to maintain a network shape on the surface layer, so that an electrical network is not sufficiently provided, and if it is more than 10 mm, the viscosity of the solution in which the carbon nanotubes are dispersed is high, so that it is difficult to form a surface layer of a lithium transition metal oxide, which is a problem. Specifically, the length of the bundle shape may be 5 ㎛ to 10 mm, 10 ㎛ to 10 mm, or 20 ㎛ to 10 mm, or 20 ㎛ to 5 mm, or 50 ㎛ to 10 mm, or 50 ㎛ to 5 mm. Specifically, the carbon nanotube may include at least one of MWCNT, TWCNT, and SWCNT.

[0062] The above carbon nanotube may have a content of oxygen atoms relative to carbon atoms as a component of the carbon nanotube of 0.01 mol% to 10 mol%.

[0063] Some studies have used oxide-type carbon nanotubes to form carbon nanotubes on the surface of cathode active materials, or employed chemical methods to incorporate large quantities of oxygen atoms within them. However, the oxygen atoms contained within these carbon nanotubes have the disadvantage of reducing electronic conductivity. Furthermore, the high polarity of the oxygen atoms makes it difficult to form a network structure with spaced gaps between the carbon nanotubes.

[0064] On the other hand, in the cathode active material (100) according to the present embodiment, the carbon nanotubes included in the surface layer (110) have a very low ratio of oxygen atoms to carbon atoms, ranging from 0.01 mol% to 10 mol%. By maintaining the ratio of oxygen atoms to carbon atoms within the aforementioned range, it is possible to physically and firmly attach the carbon nanotubes while maintaining high electrical conductivity. Specifically, the carbon nanotubes may include single-walled carbon nanotubes (SWCNTs) having a negative charge.

[0065] The average thickness (t) of the surface layer (120) may be 10 nm to 100 nm. When the thickness of the surface layer (120) is less than 10 nm, it is difficult to uniformly attach the carbon nanotubes, and when it exceeds 100 nm, the content of the transition metal oxide representing the capacity per unit volume decreases, thereby causing a problem in that the specific capacity of the lithium secondary battery decreases. Specifically, the average thickness of the surface layer (120) may be about 10 nm to 90 nm, and more specifically, about 15 nm to 80 nm.

[0066] The carbon nanotubes included in the above cathode active material may be 0.05 wt% to 5 wt%. If the carbon nanotubes are less than 0.05 wt%, the effect of improving electrical conductivity by the carbon nanotubes is insignificant, and if they are more than 5 wt%, problems such as the carbon nanotubes not being uniformly dispersed in the form of a network structure and the formation of some aggregated areas may occur during the process of forming a surface layer (120) using the carbon nanotubes. Specifically, the carbon nanotubes may be present in an amount of 0.07 wt% to 5 wt%, or 0.1 wt% to 5 wt%, or 0.15 wt% to 5 wt%, or 0.2 wt% to 5 wt%, or 0.05 wt% to 4 wt%, or 0.05 wt% to 3 wt%, or 0.05 wt% to 1 wt%, or 0.05 wt% to 0.5 wt%, or 0.1 wt% to 5 wt%.

[0067] The cationic polymer included in the positive electrode active material may be 0.01 wt% to 0.5 wt%. If the content of the cationic polymer is less than 0.01 wt%, it is difficult to stably coat the carbon nanotubes on the surface of the lithium transition metal oxide, and if it exceeds 0.5 wt%, the thickness of the surface layer (120) increases, which acts as resistance to the diffusion of lithium ions and may lower the rate characteristics of the lithium secondary battery. Specifically, the cationic polymer may be 0.1 wt% to 0.4 wt%, or 0.1 wt% to 0.3 wt%, or 0.1 wt% to 0.2 wt%, or 0.15 wt% to 0.5 wt%, or 0.2 wt% to 0.5 wt%, or 0.2 wt% to 0.4 wt%.

[0068] The above carbon nanotube has a specific surface area of ​​500 m 2 / g to 1500 m 2 / g. The specific surface area of ​​the above carbon nanotube is 500 m 2 / g, the thickness of the surface layer (120) is unnecessarily increased, and 1500 m 2 / g exceeds, the carbon nanotubes may not be uniformly formed on the surface layer (120) and may be formed in agglomeration. Specifically, the specific surface area of ​​the carbon nanotubes is 600 m 2 / g to 1500 m 2 / g, or 700 m 2 / g to 1500 m 2 / g, or 800 m 2 / g to 1500 m 2 / g, or 900 m 2 / g to 1500 m 2 / g, or 1000 m 2 / g to 1500 m 2 / g, or 500 m 2 / g to 1400 m 2 / g, or 500 m 2 / g to 1300 m 2 / g, or 600 m 2 / g to 1400 m 2 / g, or 700 m 2 / g to 1300 m 2 / g, or 800 m 2 / g to 1200 m 2 / g may be.

[0069] The above-mentioned positive electrode active material (100) for lithium secondary batteries has a surface area of ​​0.1 m 2 / g to 0.8 m 2 / g. The surface area of ​​the positive electrode active material (100) for the lithium secondary battery is 0.1 m 2 / g or less, the electrical connection between adjacent positive electrode materials within the positive electrode active material layer constituting the positive electrode is reduced, and 0.8 m 2 / g exceeds, the cationic polymer may form a part in the form of an island on the surface of the cathode active material, which may lower the current efficiency of the lithium secondary battery. Specifically, the surface area of ​​the cathode active material (100) for the lithium secondary battery is 0.15 m 2 / g to 0.8 m 2 / g, or 0.2 m 2 / g to 0.8 m 2 / g, or 0.1 m 2 / g to 0.7 m 2 / g, or 0.1 m 2 / g to 0.6 m 2 / g, or 0.1 m 2 / g to 0.5 m 2 / g, or 0.15 m 2 / g to 0.5 m 2 / g, or 0.15 m 2 / g to 0.4 m 2 / g may be.

[0070] The above-described cathode active material (100) for a lithium secondary battery has a surface layer (120) formed by the cationic polymer and carbon nanotubes, and the surface area of ​​the above-described cathode active material (100) for a lithium secondary battery can be determined by the lithium transition metal oxide (110) and the surface layer (120). Accordingly, since the above-described cathode active material (100) for a lithium secondary battery and the carbon nanotubes have the above-described specific surface area, the lithium secondary battery can exhibit high current efficiency and life characteristics.

[0071] The cationic polymer may include at least one selected from the group consisting of poly(diallyldimethylammonium chloride, PDDA), polyethyleneimine (PEI), polyamidoamine (PAMAM), polyallylamine, quaternary ammonium polymers, cationic polysaccharides, cationic cellulose, cationic starch, cationic polyacrylamide, chitosan, and gelatin.

[0072] The above quaternary ammonium group may be at least one of 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHPTMAC), 2,3-epoxypropyl trimethyl ammonium chloride (EPTAC), diallyldimethyl ammonium chloride (DMDAAC), vinylbenzene trimethyl ammonium chloride, trimethylammonium ethyl methacrylate chloride, methacrylamidopropyltrimethyl ammonium chloride (MAPTAC), and tetraalkylammonium chloride.

[0073] The cationic polymer having the above quaternary ammonium group may be at least one of di(tallowcarboxyethyl)hydroxyethyl methyl ammonium methylsulfate (TET), di(oleocarboxyethyl)hydroxyethyl methyl ammonium methylsulfate (TEO), distearyl hydroxyethyl methyl ammonium methylsulfate (TES), di(hydrogenated tallow-carboxyethyl)hydroxyethyl methyl ammonium methylsulfate (TEHT), di(palmiticcarboxyethyl)hydroxyethyl methyl ammonium methylsulfate (TEP), and dimethylbis[2-[(1-oxooctadecyl)oxy]ethyl]ammonium chloride (DEEDMAC).

[0074] The cationic polysaccharide may include at least one of cationic guar and derivatives thereof, cationic cellulose and derivatives thereof, cationic starch and derivatives thereof, cationic callose and derivatives thereof, cationic xylan and derivatives thereof, cationic mannan and derivatives thereof, and cationic galactomannose and derivatives thereof.

[0075] The cationic cellulose may be at least one of methacryloyl-ethyltrimethylammonium, methacrylamidopropyltrimethylammonium, and dimethyl-diallylammonium salt-grafted hydroxyalkylcellulose (e.g., hydroxymethylcellulose, hydroxyethylcellulose, or hydroxypropylcellulose).

[0076] The cationic starch may include a starch having cationic properties, and may be specifically prepared by reacting a cationic functional group with a starch composed of potato, glutinous potato, rice, corn, glutinous corn, barley, sweet potato, and tapioca starch, or a mixture thereof.

[0077] Specifically, the cationic polymer may include poly(diallyldimethylammonium chloride) (PDDA).

[0078] The above surface layer (120) can be formed by first coating the lithium transition metal oxide (100) using a cationic polymer to control the zeta potential, and then second coating carbon nanotubes with controlled zeta potential and size.

[0079] The lithium transition metal oxide (110) above has a negative surface charge in an aqueous medium, and can adsorb the cationic polymer onto the lithium transition metal oxide (110). The lithium transition metal oxide to which the cationic polymer is adsorbed can have a positive charge. The carbon nanotubes are adsorbed onto the surface of the lithium transition metal oxide by a force such as electrostatic attraction or ionic bonding due to the positive charge, and at the same time, are bound by the cationic polymer and stably fixed to the surface of the lithium transition metal oxide (110) to form a surface layer (120).

[0080] The zeta potential of the lithium transition metal oxide may be -50 mV to -0 mV. The zeta potential can be measured in a solvent in which the particles are dispersed, and can be performed at room temperature and neutral pH. By controlling the zeta potential of the lithium transition metal oxide within the aforementioned range, the cationic polymer can be uniformly coated. The cationic polymer can be dispersed in a solvent, and then impregnated with a lithium transition metal oxide having the zeta potential value, thereby coating the cationic polymer on the lithium transition metal oxide. Specifically, the zeta potential of the lithium transition metal oxide can be -50 mV to -5 mV, or -50 mV to -10 mV, or -40 mV to -5 mV, or -40 mV to -10 mV.

[0081] The zeta potential of the lithium transition metal oxide coated with the cationic polymer may be 5 mV to 30 mV. If the zeta potential of the lithium transition metal oxide coated with the cationic polymer is less than 5 mV, it is difficult to coat the carbon nanotubes, and if it is more than 30 mV, problems such as the lithium transition metal oxide coated with the cationic polymer clumping together may occur during the process of coating the carbon nanotubes.

[0082] After the carbon nanotubes are coated, the zeta potential of the cathode active material for the lithium secondary battery may be -2 mV to 0 mV. If the zeta potential of the cathode active material for the lithium secondary battery is less than -2 mV, the carbon nanotubes may not be stably fixed and may detach during the charge and discharge process, and if it is more than 0 mV, the electrochemical performance of the lithium secondary battery may deteriorate.

[0083] The lithium transition metal oxide may include secondary particles formed by a group of multiple primary particles. The secondary particles may be manufactured by preparing a precursor by a co-precipitation method using a transition metal hydroxide solution, and then mixing the precursor with a lithium compound and calcining the mixture.

[0084] The above secondary particles are represented by the following chemical formula 1, and the secondary particles may have an average particle diameter (D50) of 2 ㎛ to 30 ㎛ and a BET specific surface area of ​​0.1 ㎡ / g to 1.0 ㎡ / g.

[0085] [Chemical Formula 1]

[0086] Li a Ni 1-x-y Co x M1 y O2

[0087] (In the above chemical formula 1, M1 is one or more elements selected from the group consisting of Mn, Al, Zr, Ti, Mg, Ta, Nb, W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0≤x+y≤0.5.)

[0088] Alternatively, the lithium transition metal oxide may include a single particle and a plurality of fine particles attached to the surface of the single particle. The single particle is formed by approximately one particle, and fine particles may be attached to the surface. The fine particle may have an average particle diameter (D50) of 0.01 to 0.1 times that of the single particle. The fine particle may be represented by Chemical Formula 1, or may be provided in various forms, such as a compound based on residual lithium.

[0089] The above single particle is represented by the following chemical formula 1, and the above single particle may have an average particle diameter (D50) of 2 µm to 10 µm and a BET specific surface area of ​​0.5 ㎡ / g to 2.5 ㎡ / g.

[0090] [Chemical Formula 1]

[0091] Li a Ni 1-x-y Co x M1 y O2

[0092] (In the above chemical formula 1, M1 is one or more elements selected from the group consisting of Mn, Al, Zr, Ti, Mg, Ta, Nb, W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0≤x+y≤0.5.)

[0093] The above cathode active material may have a ratio (ID / IG) of the maximum peak intensity (ID) of the D band at 1340 nm to 1360 nm to the maximum peak intensity (IG) of the G band at 1575 nm to 1600 nm obtained by a Raman spectrum using a laser having a wavelength of 514.5 nm of 0 to 2. In addition, the above cathode active material may have an electrical conductivity of 0.2 S / cm to 10 5 It may be S / cm. When the carbon nanotubes provided on the surface layer do not include a functional group, the ratio of the maximum peak intensities (ID) of the D band (ID / IG) may be 0 to 0.8, and when the carbon nanotubes include a functional group, the ratio of the maximum peak intensities (ID) of the D band (ID / IG) may be 0.8 to 2.

[0094] Figure 2 is a flow chart of a method for manufacturing a positive electrode active material for a lithium secondary battery according to one embodiment of the present invention.

[0095] According to another aspect of the present invention, a method for producing a cathode active material for a lithium secondary battery according to an embodiment of the present invention may include the steps of: mixing a cationic polymer, a lithium transition metal oxide, and a first solvent, physically stirring the mixture to produce a first dispersion solution, and producing a CNT dispersion solution including graphene; mixing the first dispersion solution and the CNT dispersion solution to perform physical stirring; and drying the mixture after the physical stirring is completed to produce a lithium transition metal oxide having a surface layer formed thereon.

[0096] The above first dispersion solution can be prepared by physically stirring the cationic polymer and lithium transition metal oxide in the first solvent to prepare an intermediate material in which a solid material is dispersed, selecting the solid material from the intermediate material, and redispersing the solid material in the first solvent.

[0097] Specifically, the first dispersion solution may be prepared by first adding the cationic polymer to the first solvent to prepare a first mixed solution, and then adding the lithium transition metal oxide to the first mixed solution to prepare a second mixed solution. The cationic polymer may be added to the first solvent in an amount of 0.5 wt% to 5 wt%. If the cationic polymer is less than 0.5 wt%, problems such as loss of some of the cationic polymer during the process of preparing the positive electrode active material may occur, making it difficult to uniformly coat the surface of the lithium transition metal oxide. If it exceeds 5 wt%, the concentration of the first mixed solution is too high, making it difficult to uniformly disperse the lithium transition metal oxide. Specifically, the cationic polymer in the first solvent may be present in an amount of 0.5 wt% to 4 wt%, or 0.5 wt% to 3 wt%, or 1 wt% to 5 wt%, or 1 wt% to 4 wt%, or 1 wt% to 3 wt%.

[0098] The lithium transition metal oxide may be present in an amount of 10 wt% to 50 wt%. By providing the lithium transition metal oxide together with the cationic polymer in the aforementioned range, the cationic polymer can be uniformly coated on the surface of the lithium transition metal oxide, thereby improving process efficiency.

[0099] The first solvent may include one or more of ethanol, ultrapure water, and methanol.

[0100] By adding the lithium transition metal oxide to the first mixed solution, a cationic polymer can be coated on the surface of the lithium transition metal oxide. After selecting a solid material, which is a lithium transition metal oxide coated with a cationic polymer, from the second mixed solution, the residual solution can be removed. The residual solution can be composed of the cationic polymer remaining without being coated on the lithium transition metal oxide and the first solvent. By selecting only the solid material and then dispersing the solid material again in the first solvent, the carbon nanotubes can be uniformly coated without agglomeration during the subsequent carbon nanotube coating.

[0101] The above CNT dispersion solution may include mixing one or more additives into a second solvent, adding the carbon nanotubes to the second solvent mixed with the additives, and physically stirring the mixture. The carbon nanotubes may be present in an amount of 0.01 wt% to 5 wt%, and the additives may be present in an amount of 0.005 wt% to 5 wt%.

[0102] The above additive is provided within the above-described range so that the carbon nanotubes can be uniformly dispersed in the second solvent. If the content of the additive is less than 0.005 wt%, it is difficult to uniformly disperse the carbon nanotubes, and if it exceeds 5 wt%, the problem of the carbon nanotubes and the additive agglomerating in the second solvent may occur.

[0103] If the carbon nanotubes are less than 0.01 wt%, the content of carbon nanotubes coated on the lithium transition metal oxide coated with the cationic polymer is too small, resulting in insufficient effect. If the content exceeds 5 wt%, the carbon nanotubes are not uniformly coated on the lithium transition metal oxide coated with the cationic polymer, resulting in a problem. By providing the carbon nanotubes within the aforementioned range, they can form a surface layer in the form of a network structure together with the cationic polymer on the outer surface of the lithium transition metal oxide.

[0104] The second solvent may include at least one of N-methyl-2-pyrrolidone (NMP), polypyrrolidone, isopropanol, petroleum ether, tetrahydrofuran, ethyl acetate, N,N-dimethylacetamide, N,N-dimethylformamide, n-hexane, and a halogenated hydrocarbon.

[0105] The volume ratio of the CNT dispersion solution to the first dispersion solution may be 1:0.025 to 0.15. By mixing the CNT dispersion solution with the first dispersion solution at the aforementioned volume ratio, a uniform surface layer is formed on the lithium transition metal oxide, thereby improving the electrical characteristics and specific capacity of the lithium secondary battery.

[0106] Alternatively, the additive may include a first additive and a second additive, and the weight ratio of the first additive and the second additive may be 1:0.8 to 1.5. The first additive may improve the bonding strength with the lithium transition metal oxide coated with the cationic polymer, and the second additive may improve the dispersibility of the carbon nanotubes in the CNT dispersion solution. The second additive with respect to the first additive may be provided within the above-described range, so that each additive may effectively exhibit its function while the carbon nanotubes may be uniformly coated on the lithium transition metal oxide coated with the cationic polymer.

[0107] The first additive may be at least one of polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, carboxymethyl cellulose (CMC), and sodium alginate.

[0108] The second additive may be at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and cetyltrimethylammonium bromide (CTAB).

[0109] The zeta potential of the lithium transition metal oxide may be -50 mV to -10 mV, and the zeta potential of the solid material included in the first dispersion solution may be 5 mV to 30 mV. In addition, the zeta potential of the positive electrode active material for the lithium secondary battery may be -2 mV to 0 mV.

[0110] Since the lithium transition metal oxide, the solid material in the first dispersion solution, and the cathode active material for a lithium secondary battery are each provided with a zeta potential within the aforementioned range, a surface layer is stably formed on the outer surface of the lithium transition metal oxide, and the conductivity of the powder and the rate characteristics of the lithium secondary battery can be improved. In addition, the cathode active material according to the present embodiment can exhibit high capacity and stable life characteristics even with a low binder and conductive material content compared to a conventional cathode.

[0111] In the method for manufacturing a cationic polymer, carbon nanotubes, etc. for a lithium secondary battery according to the present invention, some of the cationic polymers, carbon nanotubes, etc. added may be lost during the manufacturing process due to the type of equipment or the process of performing each step, and it is not easy for the added amount to be provided as a cathode active material as it is due to the relationship with the solvent used or the substances added together. In the method for manufacturing a cathode active material for a lithium secondary battery according to the present invention, a numerical range was described that took into account the performance of the cathode active material while having high process efficiency through multiple experiments.

[0112] According to one embodiment of the present invention, a cathode active material can provide a cathode having a composite density of 4.0 g / cc or more by using 95 wt% to 99.7 wt% of the cathode active material and 0.3 wt% to 5 wt% of a binder without separately adding a conductive material such as carbon black, graphene, or carbon nanotubes to increase the energy density of the cathode. Specifically, a cathode having a composite density of 3.8 g / cc to 4.1 g / cc can be provided by using 99.5 wt% of the cathode active material and 0.5 wt% of a binder.

[0113] In addition, the cathode active material according to the present invention has a surface that is physically and uniformly coated with carbon nanotubes, enabling uniform current application to the cathode active material layer, and can improve lithium ion mobility at a high composite density without the addition of a separate conductive agent. Accordingly, a lithium secondary battery using the cathode active material according to the present embodiment can have a high specific capacity while performing an overall uniform electrochemical reaction, thereby improving the life characteristics of the lithium secondary battery.

[0114] Additionally, the cathode active material according to the present embodiment can have a binder content that is reduced compared to the content of the binder typically included. The surface layer can simultaneously maintain the position of the adjacent cathode active materials by functioning similar to a buffer between adjacent cathode active materials.

[0115] According to the present embodiment, even when shrinkage and expansion of the volume of the positive electrode active material occurs during the charging and discharging process, the positive electrode for a lithium secondary battery can reduce the binder content by reducing the positional deformation of the positive electrode active material due to the surface layer.

[0116] The conductive material may include at least one of carbon nanotubes, graphene, graphite, carbon black, and carbon fibers. For example, the conductive material may include carbon nanotubes, graphene, graphite such as natural graphite and artificial graphite, carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, superp, toka black, and denka black, carbon fibers, and the like. More specifically, the conductive material may be carbon nanotubes or graphene.

[0117] The above binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), and polyamideimide (PAI).

[0118] Hereinafter, examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the scope of the present invention is not limited by the following examples.

[0119] 1. Manufacturing of positive electrode active material

[0120] Manufacturing Example 1

[0121] 0.1 wt% of PAN and 0.1 wt% of PVP were mixed in N-methyl pyrrolidone solvent for 100 wt% of the total, and then 0.2 wt% of single-wall carbon nanotubes (SWCNTs) were added and mixed. After that, single-wall carbon nanotubes (SWCNTs) (OSCiAl, TUBALL, BET specific surface area = 1160 m) were sonicated for 12 h using a tip sonicator. 2 / g) was dispersed. When coating with lithium transition metal oxide, 0.3 wt% of SWCNTs out of the total 100 wt% were mixed in ethanol solvent and a carbon nanotube dispersion was prepared using a bath sonicator for 10 min.

[0122] PDDA was added to deionized water (DI) to prepare a 35 wt% solid PDDA solution, and using the prepared 35 wt% solid PDDA solution, 1.75 wt% of PDDA and lithium transition metal oxide (average particle size (D)) were prepared among 100 wt% of the total. 50 ) is 9 ㎛ Li[Ni0. 85 Mn 0.07 Co 0.05 Al 0.03 ]O215 wt% was added to the ethanol solvent and stirred at 500 rpm for 10 min. The ethanol solvent containing the dissolved PDDA was separated using a centrifuge (2000 rpm, 30 s), and the lithium transition metal oxide coated with the PDDA polymer was recovered.

[0123] After mixing 15 wt% of the lithium transition metal oxide coated with PDDA polymer out of the total 100 wt% in ethanol solvent, the carbon nanotube dispersion prepared previously was added to the ethanol solvent, and the coating was performed by stirring at 500 rpm for 1 min. The remaining ethanol was discarded using a centrifuge (2000 rpm, 30 s), and the prepared solid material was dried in a vacuum oven at 120°C for 12 h to prepare a cathode active material. The prepared cathode active material is shown in Table 1.

[0124] Manufacturing Example 2

[0125] In order to confirm the reproducibility of Manufacturing Example 1, Manufacturing Example 2 was manufactured. Using the same content of PDDA (0.35 g) and lithium transition metal oxide (NCMA) (3 g) as in Manufacturing Example 1, the positive electrode active material was manufactured in the same manner, but in an environment with different temperature and humidity, and this is shown in Table 1.

[0126] Manufacturing Example 3

[0127] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that 0.7 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and the results are shown in Table 1.

[0128] Manufacturing Example 4

[0129] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that 1.05 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and the results are shown in Table 1.

[0130] Manufacturing Example 5

[0131] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that the content of SWCNT included in the carbon nanotube dispersion was set to 0.001 g, and 0.35 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and this is shown in Table 1.

[0132] Manufacturing Example 6

[0133] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that the content of SWCNT included in the carbon nanotube dispersion was set to 0.002 g, and 0.35 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and this is shown in Table 1.

[0134] Manufacturing Example 7

[0135] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that the content of SWCNT included in the carbon nanotube dispersion was set to 0.003 g, and 0.35 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and this is shown in Table 1.

[0136] Manufacturing Example 8

[0137] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that the content of SWCNT included in the carbon nanotube dispersion was set to 0.004 g, and 0.35 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and this is shown in Table 1.

[0138] Manufacturing Example 9

[0139] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that the content of SWCNT included in the carbon nanotube dispersion was set to 0.005 g, and 0.35 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and this is shown in Table 1.

[0140] Manufacturing Example 10

[0141] A cathode active material was manufactured in the same manner as in Manufacturing Example 1, except that the content of SWCNT included in the carbon nanotube dispersion was set to 0.006 g, and 0.35 g of PDDA and 3 g of lithium transition metal oxide (NCMA) were used, and this is shown in Table 1.

[0142] Classification PDDANCMASWCNT Manufacturing Example 10.35 g3 g0.006 g Manufacturing Example 20.35 g3 g0.006 g Manufacturing Example 30.7 g3 g0.006 g Manufacturing Example 41.05 g3 g0.006 g Manufacturing Example 50.35 g3 g0.001 g Manufacturing Example 60.35 g3 g0.002 g Manufacturing Example 70.35 g3 g0.003 g Manufacturing Example 80.35 g3 g0.004 g Manufacturing Example 90.35 g3 g0.005 g Manufacturing Example 100.35 g3 g0.006 g

[0143] 2. Manufacturing of secondary batteries

[0144] Example 1

[0145] The positive electrode active material, conductive material, and PVDF binder according to Manufacturing Example 1 manufactured by the above-described method were mixed in a weight ratio of 99.5:0:0.5 in N-methyl pyrrolidone to prepare a positive electrode slurry. The positive electrode slurry was applied to one surface of an aluminum current collector, dried at 120°C, and then rolled to prepare a positive electrode.

[0146] Additionally, artificial graphite, carbon black (SUPER C-65), CMC, and SBR were mixed in a weight ratio of 96:1:1.5:1.5 and added to water as a solvent to prepare a negative electrode slurry. The prepared negative electrode slurry was applied onto a copper current collector, dried, and then rolled to prepare a negative electrode.

[0147] The composite density of the positive electrode manufactured in this way is 4.0 g / cc, and the composite density of the negative electrode is 1.5 g / cc, as shown in Tables 2 and 3.

[0148] Comparative Example 1

[0149] Average particle diameter (D 50 ) is 9㎛ Li[Ni0. 85 Mn 0.07 Co 0.05 Al 0.03 ]O2 lithium transition metal oxide (NCMA), carbon black (Super-P), and PVDF binder were mixed in a weight ratio of 96:2:2 in N-methyl pyrrolidone to prepare a cathode slurry. The prepared cathode slurry was applied to one surface of an aluminum current collector, dried at 120°C, and then rolled to prepare a cathode.

[0150] Additionally, artificial graphite, carbon black (SUPER C-65), CMC, and SBR were mixed in a weight ratio of 96:1:1.5:1.5 and added to water as a solvent to prepare a negative electrode slurry. The prepared negative electrode slurry was applied onto a copper current collector, dried, and then rolled to prepare a negative electrode.

[0151] The composite density of the positive electrode manufactured in this way is 3.6 g / cc, and the composite density of the negative electrode is 1.5 g / cc, as shown in Tables 2 and 3.

[0152] Comparative Example 2

[0153] Average particle diameter (D 50 ) is 9㎛ Li[Ni0. 85 Mn 0.07 Co 0.05 Al 0.03 ]O2, lithium transition metal oxide (NCMA), carbon black (Super-P), and PVDF binder were prepared in the same manner as in Comparative Example 1, except that the weight ratio was 98:1:1, and the positive and negative electrodes are shown in Tables 2 and 3.

[0154] Current density of cathode slurry composition (mA / cm) 2 ) Mixture density (g / cc) Solid content (%) Example 1 NCMA@SWCNT : C : binder = 99.5 : 0 : 0.53465 Comparative example 1 NCMA : C : binder = 96 : 2 : 233.665 Comparative example 2 NCMA : C : binder = 98 : 1 : 133.765

[0155] Current density of cathode slurry composition (mA / cm) 2 ) Mixture density (g / cc) Solid content (%) Example 1 AG : C : binder = 96 : 1 : 1.5 : 1.5 3.3 1.545 Comparative example 1 AG : C : binder = 96 : 1 : 1.5 : 1.5 3.3 1.545

[0156] Example 2

[0157] A secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode slurry was manufactured using the positive electrode active material, carbon black (Super-P), and PVDF binder according to Manufacturing Example 2 manufactured by the above-described method at a weight ratio of 98:1:1. The manufactured positive electrode slurry is shown in Table 4.

[0158] Example 3

[0159] A secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode slurry was manufactured using the positive electrode active material, carbon black (Super-P), and PVDF binder according to Manufacturing Example 3 manufactured by the above-described method at a weight ratio of 98:1:1. The manufactured positive electrode slurry is shown in Table 4.

[0160] Example 4

[0161] A secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode slurry was manufactured using the positive electrode active material, carbon black (Super-P), and PVDF binder according to Manufacturing Example 4 manufactured by the above-described method at a weight ratio of 98:1:1. The manufactured positive electrode slurry is shown in Table 4.

[0162] Examples 5 to 10

[0163] A secondary battery was manufactured in the same manner as in Example 1, except that the positive electrode slurry was manufactured using the positive electrode active material, carbon black (Super-P), and PVDF binder of each of Manufacturing Examples 5 to 10 in a weight ratio of 98:1:1, as shown in Table 5 below.

[0164] Classification 2 Cathode active material current density (mA / cm) 2 ) Mixing density (g / cc) Solid content (%) Example 2 NCMA@PDDA (0.35g) Manufacturing example 233.765 Example 3 NCMA@PDDA (0.7g) Manufacturing example 333.765 Example 4 NCMA@PDDA (1.05g) Manufacturing example 433.765

[0165] Classification 2 Cathode active material current density (mA / cm) 2 ) Mixture density (g / cc) Solid content (%) Example 5 SWCNT 0.001g Preparation example 533.765 Example 6 SWCNT 0.002g Preparation example 633.765 Example 7 SWCNT 0.003g Preparation example 733.765 Example 8 SWCNT 0.004g Preparation example 833.765 Example 9 SWCNT 0.005g Preparation example 933.765 Example 10 SWCNT 0.006g Preparation example 1033.765

[0166] Figure 3 is a diagram showing the zeta potential during the process of manufacturing a positive electrode active material according to an embodiment of the present invention.

[0167] FIG. 3 is a graph showing the zeta potential values ​​for each surface charge value of the cathode active material according to Manufacturing Example 1, a lithium transition metal oxide (NCMA cathode), a lithium transition metal oxide after cationic polymer coating (positive NCMA cathode), a dispersed single-walled carbon nanotube dispersion solution (SWCNT), and a cationic polymer and single-walled carbon nanotube-coated lithium transition metal oxide (SWCNT@NCMA).

[0168] Referring to FIG. 3, the zeta potentials for the surface charge values ​​for the lithium transition metal oxide, the lithium transition metal oxide after cationic polymer coating, the dispersed single-walled carbon nanotube dispersion solution, and the particles coated with single-walled carbon nanotubes on the lithium transition metal oxide coated with cationic polymer were -33.8 mV, 14.2 mV, -35 mV, and -1.92 mV, respectively.

[0169] Figure 4 is a drawing showing the surface of a positive electrode active material according to Example 1 of the present invention.

[0170] Referring to Fig. 4, FE-SEM images of the surface of the lithium transition metal oxide (left) and the surface of the cathode active material (right) and their respective AFM-topology images were confirmed.

[0171] Referring to Fig. 4, it can be confirmed that the surface of the cathode active material is coated with carbon nanotubes in a network structure, and it can be confirmed that the carbon nanotubes are uniformly coated throughout the entire surface along the shape of the surface of the lithium transition metal oxide. In addition, it can be confirmed that the carbon nanotubes on the surface of the lithium transition metal oxide are interconnected and coated.

[0172] Figure 5 shows the CS analysis results for the polymer and SWCNT coating contents of the surface layer of the positive electrode active material of Example 1.

[0173] In Fig. 5, the CS analysis was measured based on a combustion method. The lithium transition metal hydroxide (NCMA) used in Example 1 and the lithium transition metal hydroxide were coated using only cationic polymers excluding carbon nanotubes in the same manner, and then they were completely combusted using oxygen gas at a temperature of 1300°C to 1500°C, respectively. At this time, the carbon corresponding to the cationic polymer was converted to CO2 gas, and the generated CO2 gas was captured to confirm the content of the cationic polymer. Here, carbon nanotubes were excluded in order to confirm only the content of the cationic polymer coated on the surface of the lithium transition metal oxide.

[0174] Since the PDDA polymer, a cationic polymer used in Example 1, contains other elements such as hydrogen (H), nitrogen (N), and chlorine (Cl) in addition to carbon (C), the content of the PDDA polymer coated on the cathode active material was confirmed by inversely calculating the molecular weight of each element of the PDDA polymer and the carbon mass occupied by the carbon in the monomer unit. As shown in Fig. 5, when the polymer content was inversely calculated by manufacturing in the same manner as in Example 1, it was confirmed that PDDA was coated at approximately 0.064 wt%.

[0175] Figure 6 shows the results of Raman analysis of the surface of the positive electrode active material for Example 1 and Comparative Example 1.

[0176] Figure 6 shows the results of Raman analysis of the cathode active materials according to Example 1 and Comparative Example 1. Example 1 is for particles in which a cationic polymer is sequentially coated on a lithium transition metal oxide as described above and then single-walled carbon nanotubes are coated thereon, while Comparative Example 1 is for particles composed only of a lithium transition metal oxide without coating either a cationic polymer or single-walled carbon nanotubes.

[0177] Referring to FIG. 6, the ratio (ID / IG) of the maximum peak intensity of the D band at 1340 nm to 1360 nm to the maximum peak intensity (IG) of the G band at 1575 nm to 1600 nm obtained by a Raman spectrum using a laser having a wavelength of 514.5 nm was approximately 0.1 for Example 1, and approximately 1 for Comparative Example 1, showing different intensities. Specifically, Example 1 showed that the maximum peak intensity of the D band at 1340 nm to 1360 nm was approximately 0, and the maximum peak intensity (IG) of the G band at 1575 nm to 1600 nm was approximately 10. On the other hand, Comparative Example 1 showed that the maximum peak intensity of the D band at 1340 nm to 1360 nm was approximately 1, and the maximum peak intensity (IG) of the G band at 1575 nm to 1600 nm was approximately 1. Example 1 and Comparative Example 1 of the present invention exhibited different Raman spectra.

[0178] Figure 7 shows the electrode conductivity measurement values ​​and DC-IR measurement change values ​​of the positive electrode using the positive electrode active material for Example 1 and Comparative Example 1.

[0179] Fig. 7 shows the electrode conductivity measurement values ​​of the anode for Example 1 and Comparative Example 1. Example 1 was 0.4 S / cm and Comparative Example 1 was 0.17 S / cm, showing that Example 1 was approximately twice as high as Comparative Example 1. In addition, in the DC-IR measurement change value, Example 1 showed a lower value than Comparative Example 1.

[0180] Figure 8 shows the AC impedance measurement results of the symmetrical cell for Example 1 and Comparative Example 1.

[0181] Figure 9 shows the AC impedance measurement results of the symmetrical cells of the positive electrodes for Example 1 and Comparative Example 1, and shows that although Example 1 has a higher composite density than Comparative Example 1, the lithium ion resistance value within the electrode is lower.

[0182] Figure 9 is an evaluation of full cell life characteristics for Example 1 and Comparative Example 1.

[0183] Figure 9 confirms the cycle characteristics of the secondary battery according to Example 1 and the secondary battery according to Comparative Example 1. In the room temperature capacity retention rate (250 cycles) of the secondary batteries manufactured in Example 1 and Comparative Example 1, the charge conditions are CC / CV, 0.5C, 4.3V, 0.05C cut off, and the discharge conditions are CC, 0.5C, 2.8V cut off. It was confirmed that the cycle characteristics of Example 1 and Comparative Example 1 were similar, but the capacity of Example 1 was higher.

[0184] Figure 10 shows the results of evaluating the rate characteristics of secondary batteries according to Examples 2 to 4.

[0185] Referring to Fig. 10, Example 2 (NCMA@PDDA (0.35 g)), Example 3 (NCMA@PDDA (0.7 g)), and Example 4 (NCMA@PDDA (1.05 g)) all exhibited excellent characteristics up to 2 C in the rate characteristics. On the other hand, overvoltage was observed only in Example 4 under the condition of 5 C. This is believed to have occurred due to the high content of PDDA.

[0186] Figure 11 is a drawing showing the electrochemical performance of secondary batteries according to Examples 5 to 10.

[0187] Referring to Fig. 11, the content of the PDDA cationic polymer binder was kept the same based on the weight of 3 g of the cathode active material, and the SWCNT content was changed to 0.001 g, 0.002 g, 0.003 g, 0.004 g, 0.005 g, and 0.006 g, respectively, to manufacture cathode active materials, and then secondary batteries were manufactured using them. As a result of evaluating the powder conductivity and electrode conductivity after manufacturing each electrode, it was confirmed that the best powder and electrode conductivity was exhibited when 0.005 g of SWCNT was coated. In this figure, PA1-1 refers to the condition where 0.001 g of SWCNT was coated against 0.35 g of PDDA cationic polymer.

[0188] In Table 6 below, the powder density and powder conductivity are shown as values ​​when an external pressure of 20 kN is applied to the powder. In addition, it was confirmed that even when SWCNTs were coated in an amount of 0.004 g or more, higher powder and electrode conductivity were exhibited compared to the case where carbon black was included, as in Comparative Example 1.

[0189] Powder Density (g / cc) @ 20kN Powder Conductivity (S / cm) SWCNT 0.001g 3.634 0.03085 SWCNT 0.002g 3.621 0.05546 SWCNT 0.003g 3.49 0.10183 SWCNT 0.004g 3.521 0.12019 SWCNT 0.005g 3.358 0.20243 SWCNT 0.006g 3.358 0.18232 NCMA4.06 0.0215698:2 (NCMA:CB) Mixed 3.19 0.119799:1 (NCMA:CB) Mixed 3.597 0.06766 Super-P1.13539.17 SWCNT1.159313.5

[0190] Electrode Conductivity (S / cm) bare 0.16594 W / O 0.07351 PA1-10.08809 PA1-20.12111 PA1-30.22142 PA1-40.37989 PA1-50.3956 PA1-60.34279

[0191] Figure 12 shows the results of confirming the specific surface area of ​​each positive electrode active material powder.

[0192] In Table 8, NCMA is the average particle diameter (D 50 ) is 9 ㎛ Li[Ni0. 85 Mn 0.07 Co 0.05 Al 0.03 ]O2 is a lithium transition metal oxide, NCMA@SWCNT is a lithium transition metal oxide coated with PDDA and SWCNT manufactured according to Manufacturing Example 1, and NCMA:CB (96:2) is the result of mixing NCMA and carbon black in a weight ratio.

[0193] SampleSpecific Surface Area(m 2 g -1 )Total Pore Volume(cm 3 g -1 )Average Pore Size(nm)NCMA0.11490.00072237.33NCMA@SWCNT0.21040.00107237.36NCMA: CB (96:2)3.12480.0088436.50

[0194] Comparing NCMA before SWCNT coating (NCMA), NCMA@SWCNT after coating, and NCMA mixed with carbon black, it was confirmed that the specific surface area due to SWCNT increased before and after coating. On the other hand, the specific surface area was lower than when carbon black was mixed into NCMA.

[0195] That is, the cathode active material usually contains carbon black and exhibits a very high specific surface area due to the carbon black. A high specific surface area compared to NCMA@SWCNT, such as NCMA:CB (96:2), causes side reactions such as gas generation due to oxidation reaction with the electrolyte, but the NCMA@SWCNT according to the present example was equipped with a relatively low specific surface area, thereby suppressing side reactions with the electrolyte.

[0196] Those skilled in the art will appreciate that the present invention can be implemented in other specific forms without altering its technical spirit or essential characteristics. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims below rather than the detailed description above, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

Claims

1. Lithium transition metal oxide in particle form; and A surface layer provided on the surface of the lithium transition metal oxide; The above surface layer comprises carbon nanotubes and cationic polymers, A cathode active material for a lithium secondary battery having a zeta potential of -2 mV to 0 mV.

2. In paragraph 1, A cathode active material for a lithium secondary battery having an average thickness of the surface layer of 10 nm to 100 nm.

3. In paragraph 1, A cathode active material for a lithium secondary battery, wherein the carbon nanotubes are present in an amount of 0.05 wt% to 5 wt%.

4. In paragraph 1, A cathode active material for a lithium secondary battery, wherein the cationic polymer is present in an amount of 0.01 wt% to 0.5 wt%.

5. In paragraph 1, A cathode active material for a lithium secondary battery, wherein the surface layer is formed in the form of a three-dimensional network structure.

6. In paragraph 1, A cathode active material for a lithium secondary battery, wherein a ratio of the maximum peak intensity (ID) of the D band at 1340 nm to 1360 nm to the maximum peak intensity (IG) of the G band at 1575 nm to 1600 nm obtained by a Raman spectrum using a laser having a wavelength of 514.5 nm (ID / IG) is 0 to 2.

7. In paragraph 1, The above carbon nanotube is formed in a bundle shape by partially converging 1 to 10 single-walled carbon nanotubes or multi-walled carbon nanotubes, and the diameter of the bundle shape is 2 nm to 35 nm and the length is 4 ㎛ to 10 mm. The above carbon nanotube is a cathode active material for a lithium secondary battery having a content of oxygen atoms relative to carbon atoms as a component of the carbon nanotube of 0.01 mol% to 10 mol%.

8. In paragraph 1, The above carbon nanotube has a surface area of ​​500 m 2 / g to 1500 m 2 / g is a cathode active material for a lithium secondary battery.

9. In paragraph 1, The above cathode active material for lithium secondary batteries has a surface area of ​​0.1 m 2 / g to 0.8 m 2 / g is a cathode active material for a lithium secondary battery.

10. In paragraph 1, A cationic polymer is a cathode active material for a lithium secondary battery, the cationic polymer including at least one selected from the group consisting of poly(diallyldimethylammonium chloride) (PDDA), polyethyleneimine (PEI), polyamidoamine (PAMAM), polyallylamine, quaternary ammonium polymers, cationic polysaccharides, cationic cellulose, cationic starch, cationic polyacrylamide, chitosan, and gelatin.

11. In paragraph 1, The above lithium transition metal oxide comprises a secondary particle composed of a group of multiple primary particles, The above secondary particle is represented by the following chemical formula 1: The above secondary particles are a cathode active material for a lithium secondary battery having an average particle diameter (D50) of 2 ㎛ to 30 ㎛ and a BET specific surface area of ​​0.1 ㎡ / g to 1.0 ㎡ / g: [Chemical Formula 1] Li a Ni 1-x-y Co x M1 y O 2 (In the chemical formula 1 above, M1 is one or more elements selected from the group consisting of Mn, Al, Zr, Ti, Mg, Ta, Nb, W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0≤x+y≤0.5.) 12. In paragraph 1, The above lithium transition metal oxide comprises a single particle and a plurality of fine particles attached to the surface of the single particle, The above single particle is represented by the following chemical formula 1, The above single particles have an average particle diameter (D50) of 2 ㎛ to 10 ㎛ and a BET specific surface area of ​​0.5 ㎡ / g to 2.5 ㎡ / g, The above fine particles are a cathode active material for a lithium secondary battery, wherein the average particle diameter (D50) is 0.01 to 0.1 times that of the single particles: [Chemical Formula 1] Li a Ni 1-x-y Co x M1 y O 2 (In the chemical formula 1 above, M1 is one or more elements selected from the group consisting of Mn, Al, Zr, Ti, Mg, Ta, Nb, W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0≤x+y≤0.5.) 13. In paragraph 1, Electrical conductivity is 0.2 S / cm to 10 5 Cathode active material for lithium secondary batteries with S / cm.

14. A step of preparing a first dispersion solution by physically mixing a cationic polymer, a lithium transition metal oxide, and a first solvent, and preparing a CNT dispersion solution including graphene; A step of mixing the first dispersion solution and the CNT dispersion solution and performing physical stirring; and A method for producing a cathode active material for a lithium secondary battery according to any one of claims 1 to 13, comprising the step of producing a lithium transition metal oxide having a surface layer formed by drying after the above physical stirring is completed.

15. In paragraph 14, The above first dispersion solution is, In the first solvent, the cationic polymer and lithium transition metal oxide are physically stirred to produce an intermediate material in which a solid material is dispersed, It is manufactured by selecting the solid material from the above intermediate material and redispersing the solid material in the first solvent, A method for producing a cathode active material for a lithium secondary battery, wherein the cationic polymer is present in an amount of 0.5 wt% to 5 wt% and the lithium transition metal oxide is present in an amount of 10 wt% to 50 wt%.

16. In paragraph 14, The above CNT dispersion solution is, Mixing one or more additives into a second solvent, Comprising adding the carbon nanotube to a second solvent mixed with the above additive and physically stirring it, The above carbon nanotubes are 0.01 wt% to 5 wt%, A method for producing a cathode active material for a lithium secondary battery, wherein the additive is 0.005 wt% to 5 wt%.

17. In paragraph 16, The first solvent comprises at least one of ethanol, ultrapure water, and methanol, A method for producing a cathode active material for a lithium secondary battery, wherein the second solvent comprises at least one of N-methyl-2-pyrrolidone (NMP), polypyrrolidone, isopropanol, petroleum ether, tetrahydrofuran, ethyl acetate, N,N-dimethylacetamide, N,N-dimethylformamide, n-hexane, and a halogenated hydrocarbon.

18. In paragraph 14, The above additives include a first additive and a second additive, A method for manufacturing a cathode active material for a lithium secondary battery, wherein the weight ratio of the first additive and the second additive is 1:0.8 to 1.

5.

19. In Article 17, The above first additive is at least one of polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), styrene butadiene rubber (SBR), polyimide (PI), polyacrylic acid (PAA), polyacrylate, polyolefin, carboxymethyl cellulose (CMC), and sodium alginate, A method for producing a cathode active material for a lithium secondary battery, wherein the second additive is at least one of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), and cetyltrimethylammonium bromide (CTAB).

20. In paragraph 14, The zeta potential of the above lithium transition metal oxide is -50 mV to -0 mV, The zeta potential of the solid material included in the first dispersion solution is 5 mV to 30 mV, A method for producing a cathode active material for a lithium secondary battery, wherein the zeta potential of the cathode active material for the lithium secondary battery is -2 mV to 0 mV.

21. Containing a cathode active material according to any one of clauses 1 to 14 and a binder, A cathode for lithium secondary batteries having a composite density of 3.8 g / cc or more.

Images