Positive electrode active material for lithium secondary battery and lithium secondary battery comprising the same

Abstract

The positive electrode active material for a lithium secondary battery according to the embodiment of the present application includes composite particles including: lithium metal phosphorous oxide particles; and a carbon coating layer formed on the lithium metal phosphorous oxide particles, wherein a C / Fe ratio of the composite particles measured by X-ray photoelectron spectroscopy (XPS) analysis is 5 to 80.

Description

Technical Field

[0001] This invention relates to a positive electrode active material for lithium secondary batteries and a lithium secondary battery containing the positive electrode active material. Background Technology

[0002] Rechargeable batteries are batteries that can be recharged and discharged repeatedly. With the development of the information communication and display industries, rechargeable batteries are widely used as power sources for portable electronic communication devices such as portable cameras, mobile phones, and laptops. In addition, in recent years, battery packs that include rechargeable batteries have been developed for use as power sources in environmentally friendly vehicles such as hybrid electric vehicles.

[0003] Secondary batteries can be categorized into, for example, lithium secondary batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among them, lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous for charging speed and lightweight design, thus they are being actively researched and developed.

[0004] Lithium metal phosphorus oxide can be used as the positive electrode active material in lithium secondary batteries.

[0005] As the application scope of lithium-ion batteries expands, there is a growing demand for longer lifespan, higher capacity, and higher energy density. However, the low ionic conductivity of lithium metal phosphorus oxides may lead to a decrease in the power characteristics of lithium-ion batteries. Summary of the Invention

[0006] (a) Technical problems to be solved According to one aspect of the present invention, a positive electrode active material for lithium secondary batteries with improved power characteristics and low-temperature performance can be provided.

[0007] According to one aspect of the present invention, a lithium secondary battery with improved power characteristics and low-temperature performance can be provided.

[0008] (II) Technical Solution According to an exemplary embodiment of the present invention, a positive electrode active material for a lithium secondary battery comprises composite particles, the composite particles comprising: lithium metal phosphorus oxide particles; and a carbon coating formed on the lithium metal phosphorus oxide particles, wherein the C / Fe ratio of the composite particles as defined by Formula 1 is 5 to 80.

[0009] [Formula 1] C / Fe ratio=A C / A Fe In Equation 1, A CA is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). Fe It is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

[0010] In some implementations, the XPS analysis can be performed on the composite particles in a square region with a width of 0.9 mm and a length of 0.9 mm at a depth of 10 nm from the surface to the center.

[0011] In some embodiments, the composite particles may comprise multiple composite particles, and the square regions may comprise multiple square regions each containing two or more of the composite particles. The standard deviation of the C / Fe ratio measured by XPS analysis of five distinct square regions among the multiple square regions may be less than 20.

[0012] In some implementations, the C / Fe ratio can be between 21 and 70.

[0013] In some embodiments, the carbon coating content may be from 1.0% to 1.5% by weight relative to the total weight of the composite particles.

[0014] In some embodiments, the lithium metal phosphorus oxide particles may be represented by the following chemical formula 1.

[0015] [Chemical Formula 1] Li a M- x P y O 4+z In chemical formula 1, 0.9≤a≤1.2, 0.99≤x≤1.01, 0.9≤y≤1.2, -0.1≤z≤0.1, and M contains at least one selected from Fe, Co, Ni and Mn.

[0016] In some embodiments, the lithium metal phosphorus oxide particles may further comprise doping elements or coating elements, wherein the doping elements or coating elements may comprise at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P and Zr.

[0017] A lithium secondary battery according to an exemplary embodiment of the present invention includes: a positive electrode comprising the above-described positive electrode active material; and a negative electrode disposed opposite to the positive electrode.

[0018] A method for preparing a positive electrode active material for a lithium secondary battery according to an exemplary embodiment of the present invention includes: mixing a lithium source, a metal phosphorus oxide, a first carbon source, and a second carbon source to form a mixed solution; drying the mixed solution to form a mixture; and calcining the mixture to form a positive electrode active material for a lithium secondary battery comprising composite particles, the composite particles comprising lithium metal phosphorus oxide particles and a carbon coating formed on the lithium metal phosphorus oxide particles. The C / Fe ratio of the composite particles, as defined by Formula 1, is from 5 to 80.

[0019] [Formula 1] C / Fe ratio=A C / A Fe In Equation 1, A C A is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). Fe It is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

[0020] In some implementations, the first carbon source can be a high molecular weight carbon source, and the second carbon source can be a low molecular weight carbon source.

[0021] In some embodiments, the first carbon source may comprise at least one selected from polyethylene, polyvinyl alcohol, polyethylene glycol, polyaniline, epoxy resin, phenolic resin, furfural resin, acrylic resin, polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polyvinyl chloride, and asphalt.

[0022] In some embodiments, the second carbon source may comprise at least one selected from glucose, fructose, galactose, lactose, sucrose, and maltose.

[0023] In some embodiments, the content of the first carbon source can be from 0.5% by weight to 50% by weight relative to the weight of the second carbon source.

[0024] In some embodiments, the content of the first carbon source may be from 10% to 30% by weight relative to the weight of the second carbon source.

[0025] In some embodiments, the calcination can be carried out at 500°C to 900°C.

[0026] (III) Beneficial Effects According to one embodiment of the present invention, the energy density can be maintained or improved while increasing the conductivity and lifetime characteristics of the positive electrode active material.

[0027] According to one embodiment of the present invention, the electrical conductivity of the composite particles throughout the entire region can be improved, and the power characteristics and lifetime characteristics can be enhanced.

[0028] The positive electrode active material for lithium secondary batteries and the lithium secondary battery containing the positive electrode active material of the present invention can be widely used in green technology fields such as electric vehicles, battery charging stations, and other battery-based solar power generation and wind power generation. The positive electrode active material for lithium secondary batteries and the lithium secondary battery containing the positive electrode active material of the present invention can be used in eco-friendly electric vehicles and hybrid vehicles, which prevent climate change by suppressing air pollution and greenhouse gas emissions. Attached Figure Description

[0029] Figure 1 and Figure 2 These are schematic plan views and schematic cross-sectional views of a lithium secondary battery according to an exemplary embodiment.

[0030] Figure 3 These are X-ray photoelectron spectroscopy (XPS) analyses of the composite particles from Example 3 and Comparative Example 2. Detailed Implementation

[0031] According to an embodiment of the present invention, a positive electrode active material for a lithium secondary battery (hereinafter, simply referred to as "positive electrode active material") is provided. Furthermore, a method for preparing the positive electrode active material and a lithium secondary battery comprising the positive electrode active material (hereinafter, simply referred to as "secondary battery") are provided.

[0032] The embodiments of the present invention will now be described in detail. However, these are merely exemplary embodiments, and the present invention is not limited to the specific embodiments described herein.

[0033] In an exemplary embodiment, the positive electrode active material may comprise composite particles containing lithium metal phosphorus oxide particles. The positive electrode active material may comprise multiple composite particles.

[0034] The lithium metal phosphorus oxide particles may have an olivine structure and may include a crystal structure represented by the following chemical formula 1.

[0035] [Chemical Formula 1] Li a M- x P y O 4+z In chemical formula 1, the values ​​can be 0.9≤a≤1.2, 0.99≤x≤1.01, 0.9≤y≤1.2, -0.1≤z≤0.1, and M can contain at least one selected from Fe, Co, Ni and Mn.

[0036] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the crystal structure of the positive electrode active material, and does not exclude other additional elements. For example, M may contain Fe, Co, Ni and / or Mn, and Fe, Co, Ni and / or Mn may be provided as the main active element of the positive electrode active material. Formula 1 is provided to represent the bonding relationships of the main active elements and should be understood as including the introduction and substitution of additional elements.

[0037] In one embodiment, in addition to the primary active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the crystal structure. These auxiliary elements may be incorporated into the crystal structure and form bonds; this should be understood to also include the chemical structures represented by Formula 1.

[0038] The auxiliary element may include at least one selected from, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, and Zr. The auxiliary element may function as an auxiliary active element, working together with Fe, Co, Ni, or Mn to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0039] For example, the positive electrode active material or the lithium metal phosphorus oxide particles may include a crystal structure represented by the following chemical formula 1-1.

[0040] [Chemical Formula 1-1] Li a M-1 x M2 y P z O 4+b In chemical formula 1-1, the following values ​​can be used: 0.98≤a≤1.56, 0.99≤x≤1.01, 0≤y≤0.05, 0.86≤z≤1.2, and -0.1≤b≤0.1. In chemical formula 1-1, M1 can contain at least one selected from Fe, Co, Ni, and Mn. In chemical formula 1-1, M2 can contain at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, and Zr.

[0041] The positive electrode active material or the lithium metal phosphorus oxide particles may further comprise coating elements or doping elements. For example, elements substantially the same as or similar to the aforementioned auxiliary elements may be used as coating elements or doping elements. For example, one or more combinations of the aforementioned elements may be used as coating elements or doping elements.

[0042] For example, in chemical formula 1-1, M2 can be provided as a coating element or a dopant element.

[0043] The coating element or doping element may exist on the surface of the lithium metal phosphorus oxide particles, or may penetrate through the surface of the lithium metal phosphorus oxide particles and be contained in the bonding structure represented by chemical formula 1 or chemical formula 1-1.

[0044] In one embodiment, the lithium metal phosphorus oxide particles may contain LiFePO4.

[0045] For example, lithium metal phosphorus oxide particles can have improved stability and economy compared to other positive electrode active material particles.

[0046] In an exemplary embodiment, the composite particles may include a carbon coating formed on the lithium metal phosphorus oxide particles. This carbon coating can improve the conductivity of the positive electrode active material, thereby improving the power characteristics of the lithium secondary battery.

[0047] For example, the carbon coating may be formed on at least a portion of the surface of the lithium metal phosphorus oxide particles.

[0048] According to some embodiments, the carbon coating content can be from 1.0% to 1.5% by weight relative to the total weight of the composite particles; in one embodiment, the carbon coating content can be from 1.29% to 1.37% by weight. Within the above range, the electrical conductivity of the composite particles can be improved while preventing a decrease in capacity characteristics.

[0049] According to some embodiments, the thickness of the carbon coating can be from about 5 nm to 30 nm. Within this range, the electrical conductivity of the composite particles can be improved while preventing a decrease in capacity characteristics.

[0050] In an exemplary embodiment, the C / Fe ratio of the composite particles, as defined by Formula 1, can be from 5 to 80, and in some embodiments, the C / Fe ratio can be from 21 to 70.

[0051] [Formula 1] C / Fe ratio=A C / A Fe In Equation 1, AC A is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). Fe It is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

[0052] The XPS analysis can be performed on composite particles within a square region 0.9 mm wide and 0.9 mm long, at a depth of approximately 10 nm from the surface towards the center. Therefore, the reliability of the XPS analysis can be improved, and uniform measurement results can be obtained.

[0053] For example, the surface portion of the composite particle can refer to the square region.

[0054] In some embodiments, the composite particle may comprise multiple composite particles. For example, the square region may contain more than two of the multiple composite particles. In this case, A in Formula 1... C It can be the ratio of the sum of the number of carbon atoms on the surface of the composite particles contained within the square region to the sum of the total number of atoms, where A in Equation 1 is... Fe It can be the ratio of the sum of the number of iron atoms on the surface of the composite particles contained within the square region to the sum of the total number of atoms.

[0055] Within the aforementioned C / Fe ratio range, a carbon coating can be sufficiently formed, thereby maintaining or improving the energy density while enhancing the conductivity and lifetime characteristics of the positive electrode active material.

[0056] When the C / Fe ratio is less than 5, the surface portion of the lithium metal phosphorus oxide particles exposed to the outside increases, which may reduce power characteristics and lifetime characteristics.

[0057] When the C / Fe ratio exceeds 80, the thickness of the carbon coating increases excessively, which may reduce the energy density of the secondary battery.

[0058] As described above, the positive electrode active material may comprise multiple composite particles. The square region may include multiple square regions, each containing two or more of the composite particles.

[0059] In some embodiments, the standard deviation of the C / Fe ratio measured in five distinct square regions among the plurality of square regions can be less than 20; in one embodiment, the standard deviation of the C / Fe ratio can be less than 15. Within these ranges, the carbon coating can be uniformly formed in the distinct regions of the composite particles. Therefore, the conductivity of the composite particles throughout the entire region can be improved, and power characteristics and lifetime characteristics can be enhanced.

[0060] For example, the positive electrode active material that is the object of the above measurement method can be a positive electrode active material prepared by the following preparation method.

[0061] For example, the positive electrode active material that is the object of the above measurement method can be a positive electrode active material recovered from a lithium secondary battery or a positive electrode.

[0062] For example, a lithium secondary battery can be disassembled to obtain the positive electrode. The positive electrode can be added to an organic solvent (e.g., N-methyl-2-pyrrolidone, NMP) and kept for about 5 minutes to dissolve the binder, remove the positive electrode current collector, and then remove the organic solvent by drying.

[0063] The remaining positive electrode active material and conductive material after drying are added to a container along with distilled water, stirred for about 1 hour, and then left to stand for about 10 minutes, thereby separating the conductive material and the positive electrode active material.

[0064] By removing the conductive material separated to the upper layer of distilled water, the positive electrode active material that has settled at the bottom of the container can be obtained. Drying this positive electrode active material in a chamber at approximately 100°C for about 2 hours yields a positive electrode active material suitable for C / Fe ratio measurement.

[0065] The following provides a method for preparing a positive electrode active material comprising the above-mentioned composite particles.

[0066] A lithium source, a metal phosphorus oxide, a first carbon source, and a second carbon source can be dispersed and mixed in distilled water to form a mixed solution. In this mixing process, a ball mill can be used to pulverize the particles to the target size.

[0067] According to one embodiment, the source of the doping element and / or the source of the coating element may be further added to the mixed solution.

[0068] In some embodiments, the lithium source may comprise lithium carbonate and / or lithium hydroxide.

[0069] In some embodiments, the first carbon source can be a high-molecular-weight carbon source, and the second carbon source can be a low-molecular-weight carbon source. For example, the molecular weight of the first carbon source can be greater than that of the second carbon source. Therefore, a more uniform carbon coating can be formed.

[0070] In some embodiments, the first carbon source may comprise at least one selected from polyethylene, polyvinyl alcohol, polyethylene glycol, polyaniline, epoxy resin, phenolic resin, furfural resin, acrylic resin, polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polyvinyl chloride, and bitumen.

[0071] In some embodiments, the second carbon source may contain at least one selected from glucose, fructose, galactose, lactose, sucrose, and maltose.

[0072] By adding the first carbon source and the second carbon source with different molecular weights together in a predetermined mixing ratio, a carbon coating can be uniformly formed on the lithium metal phosphorus oxide particles.

[0073] In some embodiments, the content of the first carbon source can be from 0.5% to 50% by weight relative to the weight of the second carbon source; in one embodiment, the content of the first carbon source can be from 10% to 30% by weight. Within the above ranges, a uniform and sufficient carbon coating can be formed while preventing capacity reduction.

[0074] The mixture is dried (e.g., spray dried) to evaporate distilled water, thus forming a mixture.

[0075] Under a nitrogen atmosphere, the mixture that has undergone the drying step is calcined for about 5 to 12 hours at about 500°C to 900°C, and then graded and iron-removing processes are performed to form composite particles containing lithium metal phosphorus oxide and carbon coating.

[0076] The calcination can be carried out at a temperature of, for example, about 600°C to 750°C.

[0077] The C / Fe ratio of the composite particles can be 5 to 80 or 21 to 70.

[0078] Figure 1 and Figure 2These are schematic plan views and schematic cross-sectional views illustrating a lithium secondary battery according to an exemplary embodiment. For example, Figure 2 It is along Figure 1 A cross-sectional view taken along the thickness direction of the I-I' line.

[0079] The lithium secondary battery may include a positive electrode 100 and a negative electrode 130, wherein the positive electrode 100 contains the aforementioned positive electrode active material, and the negative electrode 130 is disposed opposite to the positive electrode 100.

[0080] The positive electrode 100 may include a positive electrode current collector 105 and a positive electrode active material layer 110 formed on at least one side of the positive electrode current collector 105.

[0081] The positive electrode current collector 105 may comprise stainless steel, nickel, aluminum, titanium, or alloys thereof. The positive electrode current collector 105 may also comprise aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. For example, the thickness of the positive electrode current collector 105 may be from 10 μm to 50 μm.

[0082] The positive electrode active material layer 110 may contain the aforementioned positive electrode active material. The positive electrode active material may contain the aforementioned composite particles. For example, the positive electrode active material may contain multiple composite particles.

[0083] The content of composite particles in the total weight of the positive electrode active material can be 50% by weight or more. In some embodiments, the content of composite particles in the total weight of the positive electrode active material can be 60% by weight or more, 70% by weight or more, 80% by weight or more, or 90% by weight or more.

[0084] In one embodiment, the positive electrode active material may be substantially composed of the composite particles.

[0085] The positive electrode active material can be mixed in a solvent to prepare a positive electrode slurry. The positive electrode slurry can be coated onto at least one side of the positive electrode current collector 105, then dried and calendered to prepare a positive electrode active material layer 110. The coating process can include gravure coating, slot die coating, multilayer simultaneous die coating, embossing, doctor blade coating, dip coating, bar coating, casting, and other methods. The positive electrode active material layer 110 may further contain a binder and may selectively contain conductive materials, thickeners, etc.

[0086] The solvent can be N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

[0087] The adhesive may include polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, polyacrylonitrile, polymethyl methacrylate, nitrile rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. These can be used alone or in combination of two or more.

[0088] In one embodiment, a PVDF-based binder can be used as the positive electrode binder. In this case, the amount of binder used to form the positive electrode active material layer 110 can be reduced, and the amount of positive electrode active material can be relatively increased. Therefore, the power characteristics and capacity characteristics of the secondary battery can be improved.

[0089] The conductive material can be added to enhance the conductivity and / or the mobility of lithium ions or electrons in the positive electrode active material layer 110. For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black (e.g., Denka Black), acetylene black, Ketjen black, graphene, carbon nanotubes, vapor-grown carbon fiber (VGCF), and carbon fibers, and / or metal-based conductive materials including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3. These can be used alone or in combination of two or more.

[0090] The cathode slurry may further contain thickeners and / or dispersants. In one embodiment, the cathode slurry may contain thickeners such as carboxymethyl cellulose (CMC).

[0091] The negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 formed on at least one side of the negative electrode current collector 125.

[0092] For example, the negative electrode current collector 125 may include copper foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper, polymer substrate coated with conductive metal, etc. These can be used alone or in combination of two or more. For example, the thickness of the negative electrode current collector 125 can be from 10 μm to 50 μm.

[0093] The negative electrode active material layer 120 may include a negative electrode active material. The negative electrode active material may use a material that can adsorb and desorb lithium ions. For example, the negative electrode active material may use carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.; lithium metal; lithium alloys; silicon (Si)-containing substances or tin (Sn)-containing substances, etc. These may be used alone or in combination of two or more.

[0094] The amorphous carbon may include hard carbon, soft carbon, coke, mesocarbon microbead (MCMB), mesophase pitch-based carbon fiber (MPCF), etc.

[0095] The crystalline carbon may include graphite-based carbons such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0096] The lithium metal may include pure lithium metal and / or lithium metal formed with a protective layer for suppressing dendrite growth, etc. In one embodiment, a lithium metal-containing layer deposited or coated on the negative electrode current collector 125 may be used as the negative electrode active material layer 120. In one embodiment, a lithium thin film layer may be used as the negative electrode active material layer 120.

[0097] As the elements contained in the lithium alloy, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, etc. may be cited. These may be used alone or in combination of two or more.

[0098] The silicon-containing substance may provide further increased capacity characteristics. The silicon-containing substance may include Si, SiO x (0 < x < 2), metal-doped SiO x (0 < x < 2), silicon-carbon composites, etc.

[0099] The metal may include lithium and / or magnesium, and metal-doped SiO x (0 < x < 2) may include metal silicates.

[0100] The negative electrode active material may be mixed in a solvent to prepare a negative electrode slurry. After the negative electrode slurry is coated / deposited on the negative electrode current collector 125, it may be dried and calendered to prepare the negative electrode active material layer 120. The coating may include methods such as gravure coating, slot die coating, multi-layer simultaneous die coating, embossing, knife coating, dip coating, rod coating, casting, etc. The negative electrode active material layer 120 may further include an adhesive, and may selectively further include a conductive material, a thickener, etc.

[0101] The solvents contained in the negative electrode slurry may include water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, tert-butanol, etc. These can be used alone or in combination of two or more.

[0102] The adhesive, conductive material, and thickener may be any of the aforementioned substances that can be used in the manufacture of the positive electrode 100.

[0103] In some implementations, the negative electrode binder can be a styrene-butadiene rubber (SBR) based binder, a carboxymethyl cellulose (CMC) based binder, a polyacrylic acid based binder, or a poly(3,4-ethylenedioxythiophene) (PEDOT) based binder. These can be used alone or in combination of two or more.

[0104] In an exemplary embodiment, a separator 140 may be disposed between the positive electrode 100 and the negative electrode 130. The separator 140 may be configured to prevent short circuits between the positive electrode 100 and the negative electrode 130 and to allow ion flow. For example, the thickness of the separator may be from 10 μm to 20 μm.

[0105] For example, diaphragm 140 may comprise a porous polymer membrane or a porous nonwoven fabric.

[0106] The porous polymer membrane may include polyolefin-based polymers such as ethylene polymers, propylene polymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. These may be used alone or in combination of two or more.

[0107] The porous nonwoven fabric may include high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0108] The diaphragm 140 may also comprise a ceramic-based material. For example, inorganic particles may be coated on or dispersed in the polymer membrane to improve heat resistance.

[0109] The diaphragm 140 may have a single-layer structure or a multi-layer structure including the polymer membrane and / or nonwoven fabric described above.

[0110] According to an exemplary embodiment, the battery cell can be defined by a positive electrode 100, a negative electrode 130, and a separator 140, and an electrode assembly 150 can be formed, for example, in the form of a jelly roll, by stacking multiple battery cells. For example, the electrode assembly 150 can be formed by winding, stacking, z-folding, stack-folding, etc. of the separator 140.

[0111] The electrode assembly 150 can be housed together with the electrolyte in the housing 160, thereby defining a lithium secondary battery. According to an exemplary embodiment, the electrolyte can be a non-aqueous electrolyte.

[0112] Non-aqueous electrolytes may contain a lithium salt as the electrolyte and an organic solvent. For example, the lithium salt may be made from Li... + X – This is represented, for example, by the anion (X) of the lithium salt. - ), can be exemplified by F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - (CF3CF2SO2)2N - wait.

[0113] The organic solvent may be, for example, propylene carbonate (PC), ethylene carbonate (EC), butene carbonate, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, vinylene carbonate, methyl acetate (MA), ethyl acetate (EA), n-propylacetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), ethyl fluoroacetate (FEA), ethyl difluoroacetate (DFEA), ethyl trifluoroacetate (TFEA), dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), and diethylene glycol dimethyl ether. These include ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethyl alcohol, isopropyl alcohol, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, γ-butyrolactone, propylene sulfite, etc. These can be used alone or in combination of two or more.

[0114] The non-aqueous electrolyte may further contain additives. These additives may include, for example, cyclic carbonate compounds, fluorinated carbonate compounds, sulfonyl lactone compounds, cyclic sulfate compounds, cyclic sulfite compounds, phosphate compounds, borate compounds, etc. These may be used alone or in combination of two or more.

[0115] The cyclic carbonate-based compound may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc.

[0116] The fluorine-substituted cyclic carbonate compounds may include fluoroethylene carbonate (FEC), etc.

[0117] The sulfonyl compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0118] The cyclic sulfate-based compounds may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0119] The cyclic sulfite-based compounds may include ethylene sulfite, butylene sulfite, etc.

[0120] The phosphate-based compounds may include lithium difluorobis-oxalato phosphate, lithium difluorophosphate, etc.

[0121] The borate-based compounds may include lithium bis(oxalate) borate, etc.

[0122] In some embodiments, a solid electrolyte can be used instead of the non-aqueous electrolyte. In this case, the lithium secondary battery can be made into an all-solid-state battery. Furthermore, a solid electrolyte layer can be disposed between the positive electrode 100 and the negative electrode 130 instead of the separator 140.

[0123] The solid electrolyte may include a sulfide-based electrolyte. As a non-limiting example, the sulfide-based electrolyte may include Li₂S-P₂S₅, Li₂S-P₂S₅-LiCl, Li₂S-P₂S₅-LiBr, Li₂S-P₂S₅-LiCl-LiBr, Li₂S-P₂S₅-Li₂O, Li₂S-P₂S₅-Li₂O-LiI, Li₂S-SiS₂, Li₂S-SiS₂-LiI, Li₂S-SiS₂-LiBr, Li₂S-SiS₂-LiCl, Li₂S-SiS₂-B₂S₃-LiI, Li₂S-SiS₂-P₂S₅-LiI, Li₂S-B₂S₃, and Li₂S-P₂S₅-Z. m Sn (m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, M is P, Si, Ge, B, Al, Ga, or In), Li7-xPS6-xCl x (0≤x≤2), Li7-xPS6-xBr x (0≤x≤2), Li7-xPS6-xI x (0≤x≤2), etc. These can be used individually or in combination of two or more.

[0124] In one embodiment, the solid electrolyte may further include oxide-based amorphous solid electrolytes such as Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, and Li2O-B2O3-ZnO.

[0125] like Figure 2 As shown, the tabs (positive tab and negative tab) can protrude from the positive current collector 105 and negative current collector 125 belonging to each cell and extend to one side of the housing 160. The tabs can be fused to said side of the housing 160 to form electrode leads (positive lead 107 and negative lead 127) extending to or exposed outside the housing 160.

[0126] The lithium secondary battery can be manufactured in shapes such as cylindrical, prismatic, pouch, or coin, for example, using a can.

[0127] The embodiments of the present invention described above include the following aspects, and can be implemented by at least one of the following aspects.

[0128] The positive electrode active material for a lithium secondary battery according to a first aspect of the present invention comprises composite particles, the composite particles comprising: lithium metal phosphorus oxide particles; and a carbon coating formed on the lithium metal phosphorus oxide particles, wherein the C / Fe ratio of the composite particles as defined by Formula 1 is 5 to 80.

[0129] [Formula 1] C / Fe ratio=A C / A Fe In Equation 1, A C A is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). FeIt is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

[0130] In the first aspect, according to the second aspect, the XPS analysis can be performed on the composite particles in a square region with a width of 0.9 mm and a length of 0.9 mm at a depth of 10 nm from the surface to the center.

[0131] In the second aspect, according to the third aspect, the composite particle may comprise a plurality of composite particles, the square region may comprise a plurality of square regions each containing two or more of the composite particles, and the standard deviation of the C / Fe ratio measured by XPS analysis of five distinct square regions among the plurality of square regions may be less than 20.

[0132] In any one of the first to the third aspects, according to the fourth aspect, the C / Fe ratio may be from 21 to 70.

[0133] In any one of the first to the fourth aspects, according to the fifth aspect, the content of the carbon coating may be from 1.0% to 1.5% by weight relative to the total weight of the composite particles.

[0134] In any one of the first to the fifth aspects, according to the sixth aspect, the lithium metal phosphorus oxide particles may be represented by the following chemical formula 1.

[0135] [Chemical Formula 1] Li a M- x P y O 4+z In chemical formula 1, 0.9≤a≤1.2, 0.99≤x≤1.01, 0.9≤y≤1.2, -0.1≤z≤0.1, and M contains at least one selected from Fe, Co, Ni and Mn.

[0136] In any one of the first to the sixth aspects, according to the seventh aspect, the lithium metal phosphorus oxide particles may further comprise a doping element or a coating element, wherein the doping element or the coating element may comprise at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, and Zr.

[0137] The lithium secondary battery according to an eighth aspect of the present invention comprises: a positive electrode, the positive electrode comprising a positive electrode active material according to any one of the first to seventh aspects; and a negative electrode disposed opposite to the positive electrode.

[0138] A method for preparing a positive electrode active material for a lithium secondary battery according to a ninth aspect of the present invention includes mixing a lithium source, a metal phosphorus oxide, a first carbon source, and a second carbon source to form a mixed solution. The mixed solution is dried to form a mixture, and the mixture is calcined to form a positive electrode active material for a lithium secondary battery comprising composite particles, the composite particles comprising lithium metal phosphorus oxide particles and a carbon coating formed on the lithium metal phosphorus oxide particles. The C / Fe ratio of the composite particles, as defined by Formula 1, is from 5 to 80.

[0139] [Formula 1] C / Fe ratio=A C / A Fe In Equation 1, A C A is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). Fe It is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

[0140] In the ninth aspect, according to the tenth aspect, the first carbon source may be a high molecular weight carbon source, and the second carbon source may be a low molecular weight carbon source.

[0141] In the ninth or tenth aspect, according to the eleventh aspect, the first carbon source may comprise at least one selected from polyethylene, polyvinyl alcohol, polyethylene glycol, polyaniline, epoxy resin, phenolic resin, furfural resin, acrylic resin, polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polyvinyl chloride, and bitumen.

[0142] In the ninth or eleventh aspect, according to the twelfth aspect, the second carbon source may comprise at least one selected from glucose, fructose, galactose, lactose, sucrose, and maltose.

[0143] In the ninth or twelfth aspect, according to the thirteenth aspect, the content of the first carbon source may be from 0.5% by weight to 50% by weight relative to the weight of the second carbon source.

[0144] In the ninth or thirteenth aspect, according to the fourteenth aspect, the content of the first carbon source may be from 10% by weight to 30% by weight relative to the weight of the second carbon source.

[0145] In the ninth or fourteenth aspect, according to the fifteenth aspect, the calcination can be carried out at 500°C to 900°C.

[0146] The embodiments of the present invention will be further described below with reference to specific experimental examples. The examples and comparative examples included in the experimental examples are only for illustrating the present invention and do not limit the scope of the claims. Various changes and modifications can be made to the embodiments within the scope of the present invention and the technical concept, which is obvious to those skilled in the art, and such variations and modifications are naturally within the scope of the claims.

[0147] Examples 1, 5, 6, 8, and 9, and Comparative Examples 1 to 4 Preparation of composite particles Lithium carbonate as a lithium source, iron phosphate as a metal phosphorus oxide, polyethylene as a first carbon source, and glucose as a second carbon source are added to distilled water, and the particles are mixed and pulverized by a ball mill to form a mixed solution containing LiFePO4.

[0148] As shown in Table 1, the content of the first carbon source is adjusted relative to the weight of the second carbon source.

[0149] The resulting mixed solution containing LiFePO4 was dried using a spray dryer equipped with micro-nozzles.

[0150] Under a nitrogen atmosphere, the dried powder is calcined at about 600°C to 750°C for about 5 to 12 hours, followed by classification and iron removal processes, thereby preparing composite particles with a carbon coating formed on lithium metal phosphorus oxide particles.

[0151] Manufacturing of lithium secondary batteries The composite particles are used as the positive electrode active material to manufacture lithium secondary batteries.

[0152] Specifically, the positive electrode active material, acetylene black (Denka Black) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder are mixed in a mass ratio of 93:5:2 to prepare a positive electrode slurry. A positive electrode comprising a layer of the positive electrode active material is manufactured, wherein the positive electrode active material layer is formed by coating the positive electrode slurry onto an aluminum current collector, followed by drying and calendering. After calendering, the target electrode density of the positive electrode is adjusted to 2.45 g / cm³ (cc).

[0153] Lithium metal is used as the negative electrode active material.

[0154] The positive and negative electrodes manufactured as described above are notched into circular shapes with diameters of Φ14 and Φ16, respectively, and stacked. A separator (polyethylene, 13 μm thick) cut to Φ19 is placed between the positive and negative electrodes to form a battery cell. ΦN (N is a positive number) represents a circle with a diameter of N mm.

[0155] The battery cell is assembled by placing it into a coin-shaped battery casing material with a diameter of 20 mm and a height of 1.6 mm and injecting electrolyte, and then aging it for more than 12 hours to allow the electrolyte to penetrate into the electrode.

[0156] The electrolyte used was a 1M LiPF6 solution formed using a mixed solvent of EC / EMC (30 / 70; volume ratio).

[0157] The secondary battery manufactured as described above was subjected to formation charging and discharging (charging conditions: CC-CV 0.1C 3.8V 0.05C cut-off, discharging conditions: CC 0.1C 2.5V cut-off).

[0158] Example 2 Composite particles and lithium secondary batteries were prepared using the same method as in Example 1, except that polyvinyl alcohol was used instead of polyethylene as the first carbon source, fructose was used instead of glucose as the second carbon source, and the content of the first carbon source relative to the weight of the second carbon source was adjusted as shown in Table 1.

[0159] Example 3 Composite particles and lithium secondary batteries were prepared using the same method as in Example 1, except that polyethylene glycol was used instead of polyethylene as the first carbon source, galactose was used instead of glucose as the second carbon source, and the content of the first carbon source relative to the weight of the second carbon source was adjusted as shown in Table 1.

[0160] Example 4 Composite particles and lithium secondary batteries were prepared using the same method as in Example 1, except that polyaniline was used instead of polyethylene as the first carbon source, lactose was used instead of glucose as the second carbon source, and the content of the first carbon source relative to the weight of the second carbon source was adjusted as shown in Table 1.

[0161] Example 7 Composite particles and lithium secondary batteries were prepared using the same method as in Example 1, except that they were calcined at approximately 550°C for approximately 12 hours under a nitrogen atmosphere, and the content of the first carbon source relative to the weight of the second carbon source was adjusted as shown in Table 1.

[0162] Experimental Example (1) XPS analysis - Measurement of C / Fe ratio After the composite particles prepared according to the above examples and comparative examples were coated onto the substrate, XPS analysis was performed under the following conditions. The XPS analysis was performed using an ESCALAB250Xi instrument from Thermo Fisher Scientific.

[0163] [XPS Analysis Conditions] i) X-ray type: Al kα, 1486.68 eV, beam size 900 μm ii) Analyzer: Constant analyzer energy (CAE) mode iii) Number of scans: 2 (full scan), 50 (narrow scan) iv) Pass energy: 150 eV (full scan), 20 eV (narrow scan) Specifically, XPS analysis was performed on the composite particles within a square region with a width of 0.9 mm and a length of 0.9 mm, at a depth of approximately 10 nm from the surface towards the center. This square region contained multiple composite particles.

[0164] A measured by XPS analysis C and A Fe Substitute into Equation 1 to calculate the C / Fe ratio.

[0165] Figure 3 These are XPS analysis graphs of the composite particles from Example 3 and Comparative Example 2.

[0166] See Figure 3 XPS analysis can obtain the C1s peak and Fe2p peak. Using these C1s and Fe2p peaks, the A in Equation 1 can be measured. C and A Fe .

[0167] The above XPS analysis was performed on five distinct square regions to measure the C / Fe ratio and calculate the standard deviation of the C / Fe ratio.

[0168] (2) Measurement of carbon content The carbon content relative to the total weight of the composite particles prepared according to the above examples and comparative examples was measured using a C / S analyzer (carbon / sulfur analyzer, CS844, LECO).

[0169] Specifically, the carbon content in the sample is quantitatively analyzed by detecting the CO2 produced when 1g of positive electrode active material sample is burned.

[0170] (3) Measurement of conductivity The electrical conductivity of the composite particles prepared according to the above examples and comparative examples was measured using a powder resistance measuring device (MCP-PD51, Nittoseiko Analytical Technology Co., Ltd.).

[0171] Specifically, 2g of positive electrode active material sample was pressurized to a density of approximately 2.0g / cm³, and then the resistance and conductivity of the sample were measured.

[0172] The analysis conditions are as follows.

[0173] Start lane: 1 ohm Voltage limiter: 10V Probe: Four-pin probe (electrode distance: 3.0 mm / electrode radius: 0.7 mm / sample radius: 10.0 mm) (4) Measurement of energy density For the lithium secondary batteries manufactured according to the above embodiments and comparative examples, the energy density is measured using a program within the battery charging and discharging device based on the capacity measured during formation charging (CC-CV 0.1C 3.8V 0.05C cutoff).

[0174] The calculation method within the program is shown in the following formula.

[0175] Energy density (mWh / g) = Formation charge capacity (mAh / g) × Average voltage (V) (5) Measurement of the ratio of 1C discharge capacity to 0.1C discharge capacity For the lithium secondary batteries manufactured according to the above embodiments and comparative examples, charging (CC-CV 0.5C 3.8V 0.05C cutoff) and discharging (CC 0.1C 2.5V cutoff) were performed at room temperature (25°C), and the discharge capacity was measured. The measured discharge capacity was evaluated as the 0.1C discharge capacity.

[0176] For the lithium secondary batteries manufactured according to the above embodiments and comparative examples, charging (CC-CV 0.5C 3.8V 0.05C cutoff) and discharging (CC 1C 2.5V cutoff) were performed at room temperature (25°C), and the discharge capacity was measured. The measured discharge capacity was evaluated as the 1C discharge capacity.

[0177] The ratio (%) of 1C discharge capacity to 0.1C discharge capacity is calculated by dividing the 1C discharge capacity by the 0.1C discharge capacity and then multiplying by 100.

[0178] (6) Evaluation of capacity retention (500 cycles) For the lithium secondary batteries of the above embodiments and comparative examples, 500 charge (CC-CV 0.5C 3.8V 0.05C cutoff) and discharge (CC 0.5C 2.5V cutoff) cycles were repeated at room temperature (25°C). The capacity retention rate was evaluated by dividing the discharge capacity of the 500th cycle by the discharge capacity of the 1st cycle and then multiplying by 100.

[0179] The content, measurement results, and evaluation results of the first carbon source relative to the weight of the second carbon source in the examples and comparative examples are shown in Tables 1 and 2 below.

[0180] [Table 1] [Table 2] Referring to Tables 1 and 2, in the examples with C / Fe ratios of 5 to 80, the conductivity, energy density, power characteristics, and capacity retention were all improved compared to the comparative examples.

[0181] In Example 5, where the carbon content of the composite particles was less than 1.0% by weight relative to the total weight, the electrical conductivity, power characteristics, and lifetime characteristics were relatively reduced.

[0182] In Example 6, where the carbon content of the composite particles exceeds 1.5% by weight relative to the total weight, the energy density is relatively reduced.

[0183] In Example 7, where the standard deviation of the C / Fe ratio exceeds 20, the capacity retention is relatively reduced.

[0184] In Example 8, where the content of the first carbon source relative to the weight of the second carbon source is less than 0.5% by weight, the conductivity, power characteristics, and lifetime characteristics are relatively reduced.

[0185] In Example 9, where the content of the first carbon source exceeds 50% by weight relative to the weight of the second carbon source, the energy density is relatively reduced.

[0186] In Comparative Example 1 and Comparative Example 2, where the C / Fe ratio exceeded 80, the energy density decreased.

[0187] In Comparative Examples 3 and 4, where the C / Fe ratio was less than 5, the conductivity, power characteristics, and lifetime characteristics decreased.

Claims

1. A positive electrode active material for lithium secondary batteries, comprising composite particles, The composite particles comprise: Lithium metal phosphorus oxide particles; and A carbon coating is formed on the lithium metal phosphorus oxide particles. in, The C / Fe ratio of the composite particles, as defined by Formula 1, is between 5 and 80. [Formula 1] C / Fe ratio = A C / A Fe In Equation 1, A C A is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). Fe It is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

2. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The XPS analysis was performed on the composite particles in a square region with a width of 0.9 mm and a length of 0.9 mm at a depth of 10 nm from the surface to the center.

3. The positive electrode active material for lithium secondary batteries according to claim 2, wherein, The composite particles comprise multiple composite particles, and the square regions include multiple square regions each containing two or more of the composite particles. The standard deviation of the C / Fe ratio measured by XPS analysis of five distinct square regions among the multiple square regions is less than 20.

4. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The C / Fe ratio is between 21 and 70.

5. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The carbon coating content is 1.0% to 1.5% by weight relative to the total weight of the composite particles.

6. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The lithium metal phosphorus oxide particles are represented by the following chemical formula 1: [Chemical Formula 1] Li a M- x P y Oh 4+z In chemical formula 1, 0.9≤a≤1.2, 0.99≤x≤1.01, 0.9≤y≤1.2, -0.1≤z≤0.1, and M contains at least one selected from Fe, Co, Ni and Mn.

7. The positive electrode active material for lithium secondary batteries according to claim 1, wherein, The lithium metal phosphorus oxide particles further comprise doping elements or coating elements, wherein the doping elements or coating elements comprise at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P and Zr.

8. A lithium secondary battery, comprising: The positive electrode comprises the positive electrode active material for lithium secondary batteries as described in claim 1; as well as The negative electrode is positioned opposite to the positive electrode.

9. A method for preparing a positive electrode active material for lithium secondary batteries, comprising the following steps: A lithium source, a metal phosphorus oxide, a first carbon source, and a second carbon source are mixed to form a mixed solution; The mixed solution is dried to form a mixture; as well as The mixture is calcined to form a positive electrode active material for lithium secondary batteries comprising composite particles, the composite particles comprising lithium metal phosphorus oxide particles and a carbon coating formed on the lithium metal phosphorus oxide particles. Wherein, the C / Fe ratio of the composite particles, as defined by the following formula 1, is 5 to 80: [Formula 1] C / Fe ratio = A C / A Fe In Equation 1, A C A is the ratio of the number of carbon atoms on the surface of the composite particle to the total number of atoms on the surface, as determined by X-ray photoelectron spectroscopy (XPS). Fe It is the ratio of the number of iron atoms in the surface portion to the total number of atoms in the surface portion, as determined by XPS analysis.

10. The method for preparing the positive electrode active material for lithium secondary batteries according to claim 9, wherein, The first carbon source is a high molecular weight carbon source, and the second carbon source is a low molecular weight carbon source.

11. The method for preparing the positive electrode active material for lithium secondary batteries according to claim 9, wherein, The first carbon source comprises at least one selected from polyethylene, polyvinyl alcohol, polyethylene glycol, polyaniline, epoxy resin, phenolic resin, furfural resin, acrylic resin, polyethylene oxide, polyvinylidene fluoride, polyacrylonitrile, polyvinyl chloride, and asphalt.

12. The method for preparing the positive electrode active material for lithium secondary batteries according to claim 9, wherein, The second carbon source comprises at least one selected from glucose, fructose, galactose, lactose, sucrose, and maltose.

13. The method for preparing the positive electrode active material for lithium secondary batteries according to claim 9, wherein, The content of the first carbon source is 0.5% to 50% by weight relative to the weight of the second carbon source.

14. The method for preparing the positive electrode active material for lithium secondary batteries according to claim 9, wherein, The content of the first carbon source is 10% to 30% by weight relative to the weight of the second carbon source.

15. The method for preparing the positive electrode active material for lithium secondary batteries according to claim 9, wherein, The calcination is carried out at 500°C to 900°C.

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