WO2025206618A1 - Cathode active material, cathode comprising same, and lithium secondary battery - Google Patents

Abstract

The present invention relates to a cathode active material capable of improving the performance of a lithium secondary battery and, to a cathode active material, a cathode comprising same, and a lithium secondary battery, the material comprising an olivine-structured lithium iron phosphate-based compound and a coating part, which comprises carbon (C) and is formed on the lithium iron phosphate-based compound, having a degree of graphitization of 1.00-1.50 according to relation 1 described in the present specification, and having a Dip(χ) of 0.160-0.185 according to relation 2 described in the present specification.

Description

Cathode active material, and cathode and lithium secondary battery containing the same

Cross-citation with related applications

This application claims the benefit of priority from Korean Patent Application No. 10-2024-0040833, filed March 26, 2024, the entire contents of which are incorporated herein by reference.

Technology field

The present invention relates to a positive electrode active material, and a positive electrode and a lithium secondary battery including the same.

With the recent technological development and increasing demand for mobile devices and electric vehicles, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which boast high energy density and voltage, long cycle life, and low self-discharge rates, are commercialized and widely used.

Lithium secondary batteries are composed of four major components: a cathode, an anode, a separator, and an electrolyte. Among these, the cathode active material contained in the cathode plays a significant role in determining the battery's capacity, output, and lifespan. For lithium secondary batteries to achieve high energy density, output, and lifespan, improving the performance of the cathode active material is essential. Consequently, extensive research has been conducted recently to develop high-performance cathode active materials.

Lithium transition metal oxides such as lithium cobalt oxides such as LiCoO ₂ , lithium nickel oxides such as LiNiO ₂ , lithium manganese oxides such as LiMnO ₂ or LiMn ₂ O ₄ , and lithium iron phosphate compounds such as LiFePO ₄ have been developed as positive electrode active materials for lithium secondary batteries, and recently, lithium composite transition metal oxides _containing two or more _{transition metals such as Li[Ni a Co b Mn c ] O 2 , Li [} Ni _a Co _b Al c ]O ₂ , and Li[Ni a Co b Mn _c Al _d ]O 2 have been developed and are widely used.

Meanwhile, lithium iron phosphate compounds with an olivine structure are promising active materials because they have excellent structural stability, excellent life characteristics, and superior safety in all aspects, including overcharge and overdischarge.

In particular, LiFePO ₄ has excellent high-temperature stability due to the strong bonding force of PO ₄ , and because it contains iron, which is abundant and inexpensive, it is cheaper than the aforementioned LiCoO ₂ , LiNiO ₂ , or LiMn ₂ O ₄ , and has low toxicity, so it has less impact on the environment. However, since LiFePO ₄ has low electrical conductivity, there is a problem that the internal resistance of the battery increases when LiFePO ₄ is used as a positive electrode active material. This causes a problem that the polarization potential increases when the battery circuit is closed, thereby reducing the battery capacity.

To solve this problem, there is an urgent need to develop a cathode active material with improved capacity characteristics, such as by appropriately doping and coating lithium iron phosphate compounds.

[Prior Art Literature]

[Patent Document]

(Patent Document 1) Korean Patent Publication No. 10-2023-0142687

The present invention is intended to solve the above problems and to provide a positive electrode active material with improved capacity characteristics.

In addition, the present invention seeks to provide a positive electrode and a secondary battery having excellent capacity characteristics, including the positive electrode active material as described above.

(1) The present invention provides a cathode active material comprising a lithium iron phosphate compound having an olivine structure; and a coating portion including carbon (C) formed on the lithium iron phosphate compound; and having a graphitization degree of 1.00 or more and 1.50 or less according to the following formula 1 in a Raman spectrum measured by a Raman spectrometer, and a Dip(χ) of 0.160 or more and 0.185 or less according to the following formula 2.

[Formula 1]

Degree of graphitization = I _D /I _G

[Formula 2]

Dip(χ)= I _G - I _x

In the above equations 1 and 2, I _D is the maximum peak intensity of the D band, I _G is the maximum peak intensity of the G band, I _x is the intensity of the minimum point between the G band and the D band, the D band is a peak appearing at a Raman shift of 1300 cm ^-1 to 1400 cm ^-1 , and the G band is a peak appearing at a Raman shift of 1550 cm ^-1 to 1650 cm ^-1 .

(2) The present invention provides a positive electrode active material in the above (1), wherein the lithium iron phosphate compound has a manganese content of 30 mol% or more among the total transition metals.

(3) The present invention provides a positive electrode active material in any one of the above (1) to (2), wherein the lithium iron phosphate compound has a composition represented by the following chemical formula 1.

[Chemical Formula 1]

Li _1+x Fe _1-ab Mn _a M ¹ _b PO ₄

In the above chemical formula 1, M ¹ is at least one selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, and -0.5≤x≤0.5, 0.3≤a<1.0, 0≤b≤0.1.

(4) The present invention provides a positive electrode active material in which the content of carbon included in the coating portion is 1.00 wt% or more and 2.50 wt% or less with respect to the total weight of the positive electrode active material in any one of (1) to (3).

(5) The present invention provides a positive electrode active material having a graphitization degree of 1.00 or more and 1.10 or less in any one of the above (1) to (4).

(6) The present invention provides a positive electrode active material having a Dip(χ) of 0.170 or more and 0.180 or less in any one of the above (1) to (5).

(7) The present invention provides a positive electrode active material having an average particle diameter (D ₅₀ ) of 0.25 µm or more and 0.45 µm or less in any one of the above (1) to (6).

(8) The present invention provides a positive electrode active material having an average crystallite size of 110.0 nm or more and 145.0 nm or less in any one of the above (1) to (7).

(9) The present invention provides a positive electrode comprising a positive electrode active material according to any one of (1) to (8).

(10) The present invention provides a lithium secondary battery including a positive electrode according to (9).

The cathode active material of the present invention comprises a lithium iron phosphate compound having an olivine structure; and a coating portion including carbon (C) formed on the lithium iron phosphate compound. The cathode active material satisfies the following conditions: a degree of graphitization according to Equation 1 described herein is 1.00 or more and 1.50 or less in a Raman spectrum measured by a Raman spectrometer; and a Dip(χ) according to Equation 2 described herein is 0.160 or more and 0.185 or less, thereby controlling the content of disordered carbon or carbon with defects in the coating portion of the cathode active material and controlling the coating efficiency, thereby improving the capacity characteristics of a battery including the cathode active material.

Accordingly, the positive electrode and secondary battery including the positive electrode active material have the advantage of improved capacity characteristics.

Figure 1 shows Raman spectrum data for each positive electrode active material manufactured in the examples and comparative examples.

Hereinafter, the present invention will be described in more detail to help understand the present invention.

Terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as meanings and concepts that conform to the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of the term to explain his or her own invention in the best possible manner.

In this specification, it should be understood that terms such as “include,” “have,” or “have” are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not preclude the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

In this specification, the term 'on' means not only when a configuration is formed directly on the top surface of another configuration, but also when a third configuration is interposed between these configurations.

In this specification, the Raman spectrum was obtained by placing the sample on a universal DXR holder, pressing it with a slide glass to perform pretreatment so that the sample surface height is uniform, and then measuring a portion corresponding to an area of 415㎛Х300㎛ of the sample placed on the DXR holder using a Raman spectrometer (HORIBA, XploRA) with a laser wavelength of 532 nm and an Ar-ion Laser.

In this specification, I _D is the maximum peak intensity of the D band appearing at a Raman shift of about 1300 cm ^-1 to 1400 cm ^-1 during Raman spectrum analysis, and I _G is the maximum peak intensity of the G band appearing at a Raman shift of about 1550 cm ^-1 to 1650 cm ^-1 during Raman spectrum analysis. A minimum point may exist between the G band and the D band in the Raman spectrum. The minimum point means a wavelength having the smallest intensity between the G band and the D band in the Raman spectrum. In this specification, I _x means a minimum value, specifically, the intensity of the minimum point.

In this specification, the degree of graphitization refers to the degree to which amorphous carbon is partially or completely transformed into crystalline graphite, and can be confirmed using Raman spectroscopy, and the structure of a carbon-based material can be analyzed with the degree of graphitization. When analyzing the Raman spectrum of a carbon-based material, the G band is a peak representing the sp ² bond of the carbon-based material, indicating a carbon crystal without structural defects, and the D band is a peak representing the sp ³ bond of the carbon-based material, and increases when an atomic bond formed by the sp ² bond is broken to become an sp ³ bond. Since the D band increases when disorder or defects are created in the carbon-based material, the degree of disorder or defects in the _{carbon crystal can be quantitatively evaluated by calculating the ratio of the maximum peak intensity of the D band (I D )} to the maximum peak intensity of the G band (I _G ) (I _D /I G ).

In this specification, Dip(χ) means the difference between the maximum peak intensity of the G band and the intensity of the minimum point between the G band and the D band.

In this specification, the carbon (C) content (weight %) relative to the total weight of the positive electrode active material may be measured through TC-IC (Total Carbon-Inorganic Carbon) analysis, a subtraction method using a carbon analyzer (Primacs, Skalar Analytical). Specifically, this is a method of measuring total carbon and inorganic carbon and calculating the difference as organic carbon.

In this specification, 'crystallite' means a particle unit having substantially the same crystal orientation.

In this specification, the 'average crystallite size' can be quantitatively analyzed using X-ray diffraction analysis (XRD) using Cu Kα X-rays. Specifically, the average crystallite size can be quantitatively analyzed by putting the particles to be measured into a holder, irradiating the particles with X-rays, and analyzing the diffraction grating that is generated. Sampling was prepared by putting the powder sample of the particles to be measured into the groove in the center of a general powder holder, smoothing the surface using a slide glass, and ensuring that the sample height is the same as the edge of the holder. Then, X-ray diffraction analysis was performed using a Bruker D8-Endeavor (light source: Cu-Kα rays, wavelength: 1.54 Å) equipped with a LynxEye XE-T position sensitive detector, in the range of FDS 0.5°, 2θ=15° to 80°, with a step size of 0.02°, a time per step of 0.2 s, and a total scan time of approximately 30 minutes. For the measured data, Rietveld refinement was performed considering the charge (+3 for metal ions at transition metal sites, +2 for Ni ions at Li sites) and cation mixing at each site. When analyzing the crystallite size, instrumental broadening was considered using the Fundamental Parameter Approach (FPA) implemented in the Bruker TOPAS program, and the entire peaks in the measurement range were used for fitting. The peak shape was fitted using only the Lorentzian contribution as the First Principle (FP) among the peak types available in TOPAS, and strain was not considered at this time.

In this specification, the average particle diameter (D ₅₀ ) can be defined as the particle diameter corresponding to 50% of the volume cumulative distribution in the particle size distribution curve (graph curve of particle size distribution). The average particle diameter (D ₅₀ ) can be measured by dispersing the powder to be measured in a dispersion medium, introducing it into a commercially available laser diffraction particle size measuring device (e.g., Microtrac, MicrotracS3500), measuring the difference in diffraction patterns according to particle size when the particles pass through a laser beam, calculating the particle size distribution, and calculating the particle diameter at the point where it becomes 50% of the volume cumulative distribution according to the particle diameter in each measuring device.

positive electrode active material

Hereinafter, the positive electrode active material according to the present invention will be described.

The cathode active material according to the present invention comprises a lithium iron phosphate compound having an olivine structure; and a coating portion including carbon (C) formed on the lithium iron phosphate compound; and in a Raman spectrum measured by a Raman spectrometer, the graphitization degree according to the following equation 1 is 1.00 or more and 1.50 or less, and the Dip(χ) according to the following equation 2 is 0.160 or more and 0.185 or less.

[Formula 1]

Degree of graphitization = I _D /I _G

[Formula 2]

Dip(χ)= I _G - I _x

In the above equations 1 and 2, I _D is the maximum peak intensity of the D band, I _G is the maximum peak intensity of the G band, I _x is the intensity of the minimum point between the G band and the D band, the D band is a peak appearing at a Raman shift of 1300 cm ^-1 to 1400 cm ^-1 , and the G band is a peak appearing at a Raman shift of 1550 cm ^-1 to 1650 cm ^-1 .

The present inventors have found that when a positive electrode active material including a lithium iron phosphate compound having an olivine structure; and a coating portion including carbon (C) formed on the lithium iron phosphate compound; has a degree of graphitization of 1.00 or more and 1.50 or less according to Equation 1 described herein in a Raman spectrum measured by a Raman spectrometer, and a Dip(χ) of 0.160 or more and 0.185 or less according to Equation 2 described herein, free electrons in the coating portion increase within an appropriate range, thereby increasing the number of electrons capable of insertion and deintercalation and providing excellent coating efficiency, the capacity characteristics of a lithium secondary battery including the positive electrode active material can be improved, and thus the present invention has been completed.

Meanwhile, when a cathode active material including a lithium iron phosphate compound having an olivine structure; and a coating portion including carbon (C) formed on the lithium iron phosphate compound; has a graphitization degree of less than 1 according to Equation 1 described herein in a Raman spectrum measured by a Raman spectrometer, there was a problem that disordered carbon or defective carbon was insufficient and thus free electrons were not sufficient in the carbon layer, and when the graphitization degree according to Equation 1 described herein exceeds 1.5, there was a problem that disordered carbon or defective carbon was excessive and thus life characteristics were inferior. When Dip(χ) according to Equation 2 described herein exceeds 0.185, there was a problem that the content of carbon included in the coating portion was excessive, which acted as resistance, or the coverage of the coating portion was low, and the charge/discharge capacity was low at room temperature. When Dip(χ) according to Equation 2 described herein was less than 0.160, there was a problem that the charge/discharge capacity was low at room temperature. In addition, when the degree of graphitization was less than 1.00 and Dip(χ) exceeded 0.185, there was a problem that the capacity characteristics were inferior because the coating portion, which had relatively few free electrons in the carbon layer, existed excessively and unevenly on the lithium iron phosphate compound.

In the present invention, the degree of graphitization was evaluated and expressed as a parameter represented by the following Equation 1. The degree of graphitization referred to in the present invention is a value controlled according to the maximum peak intensity of the G band (I _G ) and the maximum peak intensity of the D band (I _D ), and the greater the content of disordered carbon or carbon with defects on the surface of the lithium iron phosphate compound, the greater the graphitization value.

For example, in the case of a carbonaceous material in which no disordered carbon or carbon with defects exists, the maximum peak of the D band does not exist, and therefore the degree of graphitization is 0.

[Formula 1]

Degree of graphitization = I _D /I _G

In the above equation 1, I _D is the maximum peak intensity of the D band, I _G is the maximum peak intensity of the G band, I _x is the intensity of the minimum point between the G band and the D band, the D band is a peak appearing at a Raman shift of 1300 cm ^-1 to 1400 cm ^-1 , and the G band is a peak appearing at a Raman shift of 1550 cm ^-1 to 1650 cm ^-1 .

The above-described positive electrode active material has a graphitization degree of 1.00 or more and 1.50 or less according to the above-described formula 1. Specifically, the graphitization degree may be 1.00 or more, or 1.01 or more, and may be 1.04 or less, 1.05 or less, 1.06 or less, 1.07 or less, 1.08 or less, 1.09 or less, 1.10 or less, 1.20 or less, 1.30 or less, 1.40 or less, or 1.50 or less. When the graphitization degree satisfies the above range, the number of free electrons in the carbon layer increases to an appropriate degree, thereby increasing the number of electrons that can be inserted and deintercalated, thereby providing an effect of improving capacity characteristics. In particular, when the graphitization degree is 1.00 or more and 1.10 or less, there is an effect of being physically and chemically stable while having better electrical conductivity.

In the present invention, the coating efficiency is evaluated from the content of carbon included in the coating portion and the coverage of the coating portion, and is expressed as a parameter represented by the following Equation 2. Dip(χ) as referred to in the present invention is a value that is adjusted according to the maximum peak intensity of the G band (I _G ) and the intensity of the minimum point between the G band and the D band (I _x ), and the higher the content of carbon included in the coating portion or the lower the coverage of the coating portion, the larger the Dip(χ) value appears.

The above coating efficiency represents the relationship between the carbon content contained in the coating and the coverage of the coating. Specifically, high coating efficiency means that the carbon content contained in the coating is minimized while the coating has high coverage of the carbon formed on the lithium iron phosphate compound. High coating coverage means that the coating is continuous and uniform in thickness and composition.

For example, as the carbon content in the coating portion is low and the coverage of the coating portion is high, the Dip(χ) does not differ from the G band maximum peak intensity (I _G ).

[Formula 2]

Dip(χ)= I _G - I _x

In the above equation 2, I _G is the maximum peak intensity of the G band, I _x is the intensity of the minimum point between the G band and the D band, and the G band is a peak appearing at a Raman shift of 1550 cm ^-1 to 1650 cm ^-1 .

The above positive electrode active material has a Dip(χ) of 0.16 or more and 0.185 or less according to the above formula 2. Specifically, the Dip(χ) may be 0.160 or more, 0.161 or more, 0.162 or more, 0.163 or more, 0.164 or more, 0.165 or more, 0.166 or more, 0.167 or more, 0.168 or more, 0.169 or more, 0.170 or more, 0.171 or more, or 0.172 or more, and may be 0.179 or less, 0.180 or less, 0.181 or less, 0.182 or less, 0.183 or less, 0.184 or less, or 0.185 or less. When the above Dip(χ) satisfies the above range, the electronic conductivity of the positive electrode active material is improved without acting as a resistor, thereby improving the capacity characteristics. In particular, when the Dip(χ) is 0.170 or more and 0.180 or less, the coating portion does not interfere with the insertion and deintercalation reactions of lithium ions and electrons, thereby promoting ion movement.

According to one embodiment of the present invention, the lithium iron phosphate compound may have a manganese content of 30 mol% or more among the total transition metals. Specifically, the lithium iron phosphate compound may have a manganese content of 30 mol% or more, 35 mol% or more, 40 mol% or more, 45 mol% or more, 50 mol% or more, 55 mol% or more, or 60 mol% or more among the total transition metals, and may have a manganese content of 65 mol% or less, 70 mol% or less, 75 mol% or less, 80 mol% or less, 85 mol% or less, 95 mol% or less, or 99 mol% or less. When the manganese content is within the above range, a high energy density may be exhibited, thereby realizing high-capacity characteristics.

According to one embodiment of the present invention, the lithium iron phosphate compound may have a composition represented by the following chemical formula 1.

[Chemical Formula 1]

Li _1+x Fe _1-ab Mn _a M ¹ _b PO ₄

In the above chemical formula 1,

M ¹ is at least one selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y, and -0.5≤x≤0.5, 0.30≤a<1.00, 0≤b≤0.1.

The above M ¹ is a doping element, and is at least one selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y. The above M ¹ is not essential, but when included in an appropriate amount, the particle shape of the positive electrode active material can be improved, and the stability of the crystal structure can be enhanced.

The above x may be -0.5 or more, -0.4 or more, -0.3 or more, -0.2 or more, -0.1 or more, or 0 or more, and may be 0.1 or less, 0.2 or less, 0.3 or less, 0.4 or less, or 0.5 or less. When x satisfies the above range, high capacity characteristics and high energy density per unit volume can be realized.

The above a is the mole fraction of manganese (Mn) among the total metals excluding lithium in the lithium iron phosphate compound, and may be 0.30 or more, 0.35 or more, 0.40 or more, 0.45 or more, 0.50 or more, 0.55 or more, or 0.60 or more, and may be 0.65 or less, 0.70 or less, 0.75 or less, 0.80 or less, 0.85 or less, 0.90 or less, 0.95 or less, or less than 1.00. When a is within the above range, the capacity characteristics of the positive electrode active material can be improved. In particular, when a is 0.30 or more and 0.65 or less, the structural stability and operating voltage range of the positive electrode active material can be increased and the energy density can be improved.

The above b is the molar fraction of M ¹ among all metals excluding lithium in the lithium iron phosphate compound, and may be 0.00 or more, 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, or 0.05 or more, and may be 0.06 or less, 0.07 or less, 0.08 or less, 0.09 or less, or 0.10 or less. When b is within the above range, the particle shape of the positive electrode active material may be improved, the stability of the crystal structure may be improved, and the electrical conductivity may be improved, thereby improving the capacity characteristics.

The above 1-a-b is the molar fraction of Fe among all metals excluding lithium in the lithium iron phosphate compound. When 1-a-b is within the above range, the capacity characteristics of the positive electrode active material can be improved. In particular, when 1-a-b is 0.35 or more and 0.70 or less, the structural stability and thermal stability of the positive electrode active material can be improved.

According to one embodiment of the present invention, the content of carbon included in the coating portion may be 1.00 wt% or more and 2.50 wt% or less with respect to the total weight of the positive electrode active material. Specifically, the content of carbon included in the coating portion may be 1.00 wt% or more, 1.10 wt% or more, 1.20 wt% or more, 1.30 wt% or more, 1.40 wt% or more, 1.50 wt% or more, 1.60 wt% or more, 1.70 wt% or more, or 1.80 wt% or more, and may be 2.20 wt% or less, 2.30 wt% or less, 2.40 wt% or less, or 2.50 wt% or less. When the content of carbon is within the above range, the electrical conductivity of the positive electrode active material may be improved without acting as a resistor.

According to one embodiment of the present invention, the shape of the coating portion may be a film type, an island type, or a combination thereof. The film type may be a continuous shape, and the island type may be a discontinuous shape. In particular, when the coating portion is a film type, the mobility of lithium ions is enhanced, thereby improving capacity characteristics.

According to one embodiment of the present invention, the positive electrode active material may have an average particle diameter (D ₅₀ ) of 0.25 μm or more and 0.45 μm or less. Specifically, the average particle diameter (D ₅₀ ) may be 0.25 μm or more, 0.26 μm or more, 0.27 μm or more, 0.28 μm or more, 0.29 μm or more, or 0.3 μm or more, 0.32 μm or less, 0.33 μm or less, 0.34 μm or less, 0.35 μm or less, 0.36 μm or less, 0.37 μm or less, 0.38 μm or less, 0.39 μm or less, 0.4 μm or less, 0.41 μm or less, 0.42 μm or less, 0.43 μm or less, 0.44 μm or less, or 0.45 μm or less. When the above average particle diameter (D ₅₀ ) is within the above range, the electrode density can be increased.

According to one embodiment of the present invention, the positive electrode active material may have an average crystallite size of 110.0 nm or more and 145.0 nm or less. Specifically, the average crystallite size may be 110.0 nm or more, 140.0 nm or less, 141.0 nm or less, 142.0 nm or less, 143.0 nm or less, 144.0 nm or less, or 145.0 nm or less. When the average crystallite size is within the above range, side reactions between the positive electrode active material and the electrolyte are reduced, thereby improving life characteristics, and the lithium diffusion path within the particles is shortened, thereby improving resistance characteristics.

The cathode active material of the present invention can be manufactured by the following method. For example, it can be manufactured by mixing raw materials and firing them. The mixing can be performed by dry mixing or wet mixing. When mixing each component through dry mixing, the firing process can be performed without a separate drying process. When mixing each component through wet mixing, it can be manufactured by adding it to a solvent, specifically water, or a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water. Alternatively, a solution containing each raw material, specifically an aqueous solution, is prepared, and then the mixed components are mixed, spray-dried, and then the firing process is performed. Each raw material can be used in an appropriate amount considering the content of each metal element in the lithium iron phosphate compound to be finally manufactured.

According to one embodiment of the present invention, the present invention can be manufactured by mixing a lithium-containing raw material, a phosphorus-containing raw material, an iron-containing raw material, and a manganese-containing raw material to manufacture a slurry, and then further mixing a carbon-containing raw material into the slurry and then calcining the slurry.

Specifically, the positive electrode active material of the present invention can be manufactured by a method for manufacturing a positive electrode active material, which comprises: (a) a step of preparing a slurry by mixing a lithium-containing raw material, a phosphorus-containing raw material, an iron-containing raw material, and a manganese-containing raw material with a solvent; and (b) a step of preparing a mixture by mixing the slurry and a carbon-containing raw material, and then calcining the mixture.

The lithium-containing raw material may be lithium-containing sulfate, nitrate, acetate, carbonate, oxalate, citrate, halide, hydroxide or oxyhydroxide, and is not particularly limited as long as it can be dissolved in water. Specifically, the lithium-containing raw material may be Li ₂ CO ₃ , LiNO ₃ , LiNO ₂ , LiOH, LiOHㆍH ₂ O, LiH, LiF, LiCl, LiBr, LiI, CH ₃ COOLi, Li ₂ O, Li ₂ SO ₄ , CH ₃ COOLi, or Li ₃ C ₆ H ₅ O ₇ , and any one of these or a mixture of two or more thereof may be used.

The above phosphorus (P)-containing raw material may be at least one selected from the group consisting of (NH ₄ ) ₃ PO ₄ , NH ₄ H ₂ PO ₄ , LiH ₂ PO ₄ , and H ₃ PO ₄ , and a mixture of one or two or more of these may be used.

The above iron (Fe)-containing raw material may be at least one selected from the group consisting of iron-containing oxides, sulfur oxides, carbonates, nitrates, and phosphates, and a mixture of one or two or more of these may be used.

The above manganese (Mn)-containing raw material may be at least one selected from the group consisting of MnSO ₄ , MnCO ₃ , Mn ₂ O ₃ , MnO ₂ , Mn ₃ O ₄ , Mn(NO ₃ ) ₂ , manganese acetate, manganese dicarboxylate, manganese citrate, manganese salts of fatty acid manganese, oxyhydroxides, and halides of manganese chloride, and a mixture of one or two or more of these may be used.

As the above solvent, a mixture of water and an organic solvent (specifically, alcohol, etc.) that can be uniformly mixed with water can be used.

When preparing the slurry in the above step (a), a raw material containing a doping element, for example, a raw material containing zirconium, a raw material containing yttrium, etc., may be further mixed.

According to the present invention, in the step (a), the lithium-containing raw material, the phosphorus-containing raw material, the iron-containing raw material, and the manganese-containing raw material can be mixed in an amount such that the lithium iron phosphate compound has a composition represented by the chemical formula 1 described herein.

The above carbon-containing raw material may be at least one selected from the group consisting of sucrose, glucose, polyethylene glycol, polyvinyl alcohol, and polyvinyl acetate.

The above carbon-containing raw material may be mixed in an amount of 1 wt% or more and 7 wt% or less based on the total weight of the positive electrode active material.

The above step (b) may be a step of manufacturing a cathode active material including a lithium iron phosphate compound and a coating portion including carbon on the lithium iron phosphate compound by firing the mixture at a temperature of 600°C or higher and less than 730°C, wherein the coating portion includes amorphous carbon and has a shape of a film type, an island type, or a combination thereof.

By manufacturing in the above manner, a positive electrode active material satisfying the degree of graphitization according to Equation 1 described herein and the Dip(χ) according to Equation 2 can be manufactured. The degree of graphitization and the Dip(χ) are technical characteristics that appear by comprehensively controlling the type of raw material, the sintering temperature, and the mixing amount of the raw material.

anode

Next, the anode according to the present invention will be described.

The positive electrode according to the present invention comprises a positive electrode active material layer comprising a positive electrode material according to the present invention. Specifically, the positive electrode comprises a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and comprising the positive electrode material. Since the positive electrode material has been described above, a detailed description thereof will be omitted, and only the remaining components will be described in detail below.

The positive electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the positive electrode current collector may typically have a thickness of 3 µm to 500 µm, and fine unevenness may be formed on the surface of the positive electrode current collector to increase the adhesive strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

The above-mentioned positive electrode active material layer may include a positive electrode material, a conductive material, and a binder. At this time, the positive electrode material may be included in an amount of 80 wt% to 99 wt%, more specifically 85 wt% to 98.5 wt%, based on the total weight of the positive electrode active material layer, and excellent capacity characteristics may be exhibited within this range.

The conductive material is used to provide conductivity to the electrode, and in the battery to be formed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without any special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like. One type alone or a mixture of two or more types of these may be used. The conductive material may be included in an amount of 0.1 wt% to 15 wt% based on the total weight of the positive electrode active material layer.

The above binder serves to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode material and the current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluoroelastomer, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof. The binder may be included in an amount of 0.1 wt% to 15 wt% based on the total weight of the positive electrode active material layer.

The above-mentioned positive electrode can be manufactured according to a conventional positive electrode manufacturing method, except that the above-mentioned positive electrode material is used. Specifically, the positive electrode can be manufactured by coating a composition for forming a positive electrode active material layer, which is manufactured by dissolving or dispersing the above-mentioned positive electrode material and optionally a binder and a conductive agent in a solvent, on a positive electrode current collector, followed by drying and rolling. At this time, the types and contents of the positive electrode material, binder, and conductive agent are as described above. Alternatively, the positive electrode can be manufactured by casting the composition for forming a positive electrode active material layer on a separate support, and then laminating the film obtained by peeling it from the support on a positive electrode current collector.

The solvent may be a solvent generally used in the relevant technical field, such as dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, or water. One of these may be used alone or a mixture of two or more thereof may be used. The amount of the solvent used is sufficient to dissolve or disperse the positive electrode active material, conductive material, and binder, taking into account the coating thickness and manufacturing yield of the slurry, and to have a viscosity that can exhibit excellent thickness uniformity when applied thereafter for manufacturing the positive electrode.

lithium secondary battery

Next, a lithium secondary battery according to the present invention will be described.

The present invention can manufacture an electrochemical device including the above-described positive electrode. The electrochemical device may be, specifically, a battery, a capacitor, or the like, and more specifically, a lithium secondary battery.

The lithium secondary battery specifically includes a positive electrode, a negative electrode positioned opposite the positive electrode, and a separator and electrolyte interposed between the positive electrode and the negative electrode. Since the positive electrode is the same as described above, a detailed description thereof will be omitted, and only the remaining components will be described in detail below.

In addition, the lithium secondary battery may optionally further include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

In the above lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer positioned on the negative electrode current collector.

The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the negative electrode current collector can typically have a thickness of 3 ㎛ to 500 ㎛, and like the positive electrode current collector, fine unevenness can be formed on the surface of the current collector to strengthen the bonding strength of the negative electrode active material. For example, it can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a non-woven fabric, etc.

The above negative electrode active material layer optionally includes a binder and a conductive material together with the negative electrode active material.

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples thereof include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, and Al alloy; metallic oxides capable of doping and dedoping lithium, such as SiO _β (0<β<2), SnO ₂ , vanadium oxide, and lithium vanadium oxide; or composites comprising the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these may be used. In addition, a metallic lithium thin film may be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon may be used as the carbonaceous material. Representative examples of low-crystalline carbon include soft carbon and hard carbon, and representative examples of high-crystalline carbon include natural graphite or artificial graphite in the form of amorphous, plate-like, flaky, spherical, or fiber-like forms, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, meso-carbon microbeads, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes.

The above negative electrode active material may be included in an amount of 80% to 99% by weight based on the total weight of the negative electrode active material layer.

The above binder is a component that assists in bonding between the conductive material, the active material, and the current collector, and can typically be added in an amount of 0.1 wt% to 10 wt% based on the total weight of the negative electrode active material layer. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

The conductive agent is a component for further improving the conductivity of the negative electrode active material, and may be added in an amount of 10 wt% or less, specifically, 5 wt% or less, based on the total weight of the negative electrode active material layer. The conductive agent is not particularly limited as long as it has conductivity and does not cause a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber or metal fiber; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

The negative electrode active material layer may be manufactured by applying and drying a composition for forming a negative electrode active material layer prepared by dissolving or dispersing a negative electrode active material, and optionally a binder and a conductive material in a solvent, on a negative electrode current collector, or by casting the composition for forming a negative electrode active material layer on a separate support, and then laminating the film obtained by peeling it off from the support on a negative electrode current collector.

Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without special restrictions, and in particular, one having low resistance to ion movement of the electrolyte and excellent electrolyte moisture absorption capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer, or a laminated structure of two or more layers thereof, can be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. can also be used. In addition, a coated separator containing a ceramic component or a polymer material to secure heat resistance or mechanical strength can be used, and can optionally be used in a single-layer or multi-layer structure.

In addition, examples of the electrolyte used in the present invention include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

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

As the organic solvent, any solvent that can serve as a medium through which ions involved in the electrochemical reaction of the battery can move may be used without particular limitation. Specifically, the organic solvent includes ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); Alcohol solvents such as ethyl alcohol, isopropyl alcohol, etc.; nitriles such as R-CN (R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may include a double-bonded aromatic ring or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolanes can be used. Among these, carbonate solvents are preferable, and a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and high dielectric constant that can improve the charge/discharge performance of the battery and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) is more preferable. In this case, the performance of the electrolyte can be excellent when the cyclic carbonate and the linear carbonate are mixed and used in a volume ratio of about 1:1 to about 1:9.

The lithium salt may be used without any particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, the lithium salt may be LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , LiSbF ₆ , LiAl0 ₄ , LiAlCl ₄ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN(C ₂ F ₅ SO ₃ ) ₂ , LiN(C ₂ F ₅ SO ₂ ) ₂ , LiN(CF ₃ SO ₂ ) ₂ . LiCl, LiI, or LiB(C ₂ O ₄ ) ₂ . The concentration of the lithium salt is preferably used within the range of 0.1 to 5.0 M, specifically, 0.1 to 3.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance and lithium ions can move effectively.

In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery. At this time, the additive may be included in an amount of 0.1 to 10 wt%, specifically, 0.1 to 5 wt%, based on the total weight of the electrolyte.

As described above, a lithium secondary battery including a cathode material according to the present invention exhibits excellent capacity and efficiency characteristics, and is therefore useful in portable devices such as mobile phones, laptop computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

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

The above battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as a power tool; an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

There is no particular limitation on the external shape of the lithium secondary battery of the present invention, but it may be a cylindrical shape using a can, a square shape, a pouch shape, or a coin shape.

The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but can also be preferably used as a unit battery in a medium- to large-sized battery module including a plurality of battery cells.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement them. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein.

Examples and Comparative Examples

Hereinafter, the present invention will be described in more detail by way of examples. However,

The following examples are intended to illustrate the present invention and are not intended to limit the scope of the present invention.

Example 1

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and the mixture was mixed with water so that the solid concentration was 20 wt%, and then wet-ground using a bead mill or a ball mill to prepare a slurry (D ₅₀ : 0.21 μm). Thereafter, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is mixed in an amount of 4.5 wt% based on the total weight of Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ .

After spray drying the above mixture (inlet temperature 230°C, outlet temperature 95°C), the mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to produce a cathode active material comprising a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound. The coating portion including carbon includes amorphous carbon and has a film type shape.

Example 2

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and mixed with water so that the solid concentration was 20 wt%, and then wet-ground with a bead mill or ball mill to prepare a slurry (D ₅₀ : 0.25 μm). Then, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ It is mixed at a content of 5% by weight based on the total weight.

The above mixture was spray-dried (inlet temperature 230°C, outlet temperature 95°C) and then calcined at 700°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material comprising a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound.

Example 3

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.7:0.3:1, and the mixture was mixed with water so that the solid concentration was 20 wt%, and then wet-ground using a bead mill or a ball mill to prepare a slurry (D ₅₀ : 0.25 μm). Thereafter, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is mixed in an amount of 5 wt% based on the total weight of Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ .

After spray drying the above mixture (inlet temperature 230°C, outlet temperature 95°C), the mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material comprising a lithium iron phosphate compound having a composition represented by LiFe _0.7 Mn _0.3 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound. The coating portion including carbon includes amorphous carbon and has a film type shape.

Comparative Example 1

Li ₂ CO ₃ , LiH ₂ PO ₄ , FePO ₄ and MnO were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and mixed with water so that the solid concentration was 20 wt%, and then wet-ground with a bead mill to prepare a slurry (D ₅₀ : 0.21 μm). Then, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is Li ₂ CO ₃ , LiH ₂ PO ₄ , FePO ₄ and MnO It is mixed at a content of 5% by weight based on the total weight.

The above mixture was spray-dried (inlet temperature 230°C, outlet temperature 95°C) and then calcined at 700°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material comprising a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound.

Comparative Example 2

Li ₂ CO ₃ , LiH ₂ PO ₄ , FePO ₄ and Mn ₃ O ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and then mixed with water so that the solid concentration was 20 wt%, and then wet-ground with a bead mill to prepare a slurry (D ₅₀ : 0.25 μm). Then, sucrose was mixed with the slurry to prepare a mixture. At this time, sucrose is Li ₂ CO ₃ , LiH ₂ PO ₄ , FePO ₄ and Mn ₃ O ₄ It is mixed at a content of 5% by weight based on the total weight.

The above mixture was spray-dried (inlet temperature 230°C, outlet temperature 95°C) and then calcined at 700°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material comprising a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound.

Comparative Example 3

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and mixed with water so that the solid concentration was 20 wt%, and then wet-ground with a bead mill to prepare a slurry (D ₅₀ : 0.21 μm). Then, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ It is mixed at a content of 4.8% by weight based on the total weight.

After spray drying the above mixture (inlet temperature 230°C, outlet temperature 95°C), the mixture was calcined at a temperature of 730°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material including a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound.

Comparative Example 4

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and mixed with water so that the solid concentration was 20 wt%, and then wet-ground with a bead mill to prepare a slurry (D ₅₀ : 0.21 μm). Then, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ It is mixed at a content of 5.2% by weight based on the total weight.

After spray drying the above mixture (inlet temperature 230°C, outlet temperature 95°C), the mixture was calcined at a temperature of 730°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material including a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound.

Comparative Example 5

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.4:0.6:1, and mixed with water so that the solid concentration was 20 wt%, and then wet-ground with a bead mill to prepare a slurry (D ₅₀ : 0.25 μm). Then, citric acid was mixed with the slurry to prepare a mixture. At this time, citric acid is Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ It is mixed at a content of 5.62% by weight based on the total weight.

After spray drying the above mixture (inlet temperature 230°C, outlet temperature 95°C), the mixture was calcined at a temperature of 730°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material including a lithium iron phosphate compound having a composition represented by LiFe _0.4 Mn _0.6 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound.

Comparative Example 6

Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ were mixed so that the molar ratio of Li:Fe:Mn:PO ₄ was 1.02:0.7:0.3:1, and the mixture was mixed with water so that the solid concentration was 20 wt%, and then wet-ground using a bead mill or a ball mill to prepare a slurry (D ₅₀ : 0.25 μm). Thereafter, sucrose was mixed into the slurry to prepare a mixture. At this time, sucrose is mixed in an amount of 5 wt% based on the total weight of Li ₂ CO ₃ , FePO ₄ , MnCO ₃ and NH ₄ H ₂ PO ₄ .

After spray drying the above mixture (inlet temperature 230°C, outlet temperature 95°C), the mixture was calcined at 700°C for 10 hours under a nitrogen atmosphere to produce a positive electrode active material comprising a lithium iron phosphate compound having a composition represented by LiFe _0.7 Mn _0.3 PO ₄ ; and a coating portion including carbon (C) formed on the lithium iron phosphate compound. The coating portion including carbon includes amorphous carbon and has a film type shape.

Experimental example

Experimental Example 1: Raman Spectrum Analysis

The Raman spectra of each of the positive electrode active materials manufactured in the above Examples and Comparative Examples were analyzed using a Raman spectrometer (HORIBA, XploRA) (laser wavelength: 532 nm, Ar-ion Laser). Specifically, the peak appearing at a Raman shift of 1300 cm ^-1 to 1400 cm ^-1 was designated as the D band, and the peak appearing at a Raman shift of 1550 cm ^-1 to 1650 cm ^-1 was designated as the G band. The graphitization degree according to Equation 1 below and the Dip(χ) according to Equation 2 below were calculated and shown in Table 1 below. In addition, the Raman spectrum data for the positive electrode active materials of Example 1 and Comparative Example 1 and the Dip(χ) according to Equation 2 described herein were indicated by arrows on the Raman spectrum data.

[Formula 1]

Degree of graphitization = I _D /I _G

[Formula 2]

Dip(χ)= I _G - I _x

In the above equations 1 and 2,

The above I _D is the maximum peak intensity of the D band, I _G is the maximum peak intensity of the G band, and I _x is the intensity of the minimum point between the G band and the D band.

Figure 1 shows Raman spectrum data for each positive electrode active material manufactured in Examples and Comparative Examples. The arrows below represent the Dip(χ) for each positive electrode active material manufactured in Example 1 and Comparative Example 1.

division Graphitization degree (I _D /I _G ) Dip(χ) Example 1 1.04 0.179 Example 2 1.01 0.178 Example 3 1.04 0.172 Comparative Example 1 0.87 0.466 Comparative Example 2 0.91 0.451 Comparative Example 3 1.05 0.156 Comparative Example 4 1.03 0.190 Comparative Example 5 0.99 0.184 Comparative Example 6 0.91 0.460

Through Table 1, it was confirmed that the positive electrode active materials of Examples 1 to 3 had a graphitization degree of 1.00 or more and 1.50 or less according to the present invention, and a Dip(χ) of 0.160 or more and 0.185 or less according to the present invention. In contrast, it was confirmed that the positive electrode active materials of Comparative Examples 1 and 2 had a graphitization degree of less than 1.00 and a Dip(χ) of greater than 0.185 according to the present invention, and it was confirmed that the positive electrode active materials of Comparative Examples 3 and 4 had a graphitization degree of 1.00 or more and 1.50 or less according to the present invention, and a Dip(χ) of less than 0.160 or greater than 0.185 according to the present invention. And, it was confirmed that the positive electrode active material of Comparative Example 5 had a graphitization degree of less than 1.00 according to the present invention and a Dip(χ) of 0.160 or more and 0.185 or less according to the present invention, and it was confirmed that the positive electrode active material of Comparative Example 6 had a graphitization degree of less than 1.00 according to the present invention and a Dip(χ) of more than 0.185 according to the present invention.

Experimental Example 2: ICP Analysis

Each of the positive electrode active materials manufactured in the above examples and comparative examples is taken in an amount of 3 g to prepare an analysis sample. Using a carbon analyzer (Primacs, Skalar Analytical Co.), the analysis sample is subjected to high-temperature combustion oxidation at 1,100°C to convert the carbon present in the sample to _CO2 , which is measured with an NDIR detector. IC is detected by acidification in which inorganic carbon is converted to _CO2 in an IC reactor. Data is collected using Windows-based PRIMACS MCS software, and the TOC (Total Organic Carbon) concentration of the sample is calculated using the formula TC-IC=TOC. Regarding the carbon (C) present in the analysis sample, the content (weight%) of carbon (C) with respect to the total weight of the positive electrode active material is shown in Table 2 below.

Experimental Example 3: Particle Size Analysis

The average particle diameter (D ₅₀ ) of each positive electrode active material manufactured in the above examples and comparative examples was measured using PSA (Microtrac, MicrotracS3500) and is shown in Table 2 below.

Experimental Example 4: XRD Analysis

For each positive electrode active material manufactured in the above examples and comparative examples, the average crystallite size (nm) of the positive electrode active material after XRD measurement is shown in Table 2 below.

division Carbon (C) content
(weight%) Average particle size (D ₅₀ )
(㎛) Average crystallite size
(nm) Example 1 1.82 0.32 139.9 Example 2 2.05 0.31 110.2 Example 3 2.15 0.30 143.4 Comparative Example 1 2.57 1.69 98.3 Comparative Example 2 2.49 1.30 106.9 Comparative Example 3 1.91 0.33 153.9 Comparative Example 4 2.12 0.39 150.7 Comparative Example 5 1.24 1.65 164.1 Comparative Example 6 2.25 0.43 152.1

Through Table 2, it was confirmed that the content of carbon (C) in the positive electrode active materials of Examples 1 to 3 was 1.00 wt% or more and 2.50 wt% or less based on the total weight of the positive electrode active materials, the average particle diameter was 0.25 µm or more and 0.45 µm or less, and the average crystallite size was 110.0 nm or more and 145.0 nm or less. In comparison, it was confirmed that the average crystallite size of the positive electrode active materials of Comparative Examples 1 and 2 was less than 110 nm, and the average crystallite size of the positive electrode active materials of Comparative Examples 3 to 6 was more than 145 nm.

Experimental Example 5: Battery Characteristics Evaluation

Coin-type half-cell manufacturing

A positive electrode slurry was prepared by mixing 90 wt% of the positive electrode active material manufactured in the above examples and comparative examples, 5.0 wt% of carbon black as a conductive agent, and 5.0 wt% of polyvinylidene fluoride (PVDF) as a binder in an N-methylpyrrolidone (NMP) solvent. The prepared positive electrode slurry was applied to one surface of an aluminum current collector, dried at 130°C, and then rolled to prepare a positive electrode.

An electrode assembly was manufactured using a lithium metal electrode as the negative electrode and a porous polyethylene separator interposed between the positive and negative electrodes. This was placed inside a battery case and an electrolyte solution containing 1.0 M LiPF ₆ dissolved in an organic solvent containing ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) in a volume ratio of 3:4:3 was injected to manufacture a coin-type half-cell.

After manufacturing the coin-type half-cell, the capacity characteristics of the cell were measured using the following method.

Evaluation of battery capacity characteristics

Immediately after performing the activation process, the battery was charged (0.1C, cut-off current: 0.05C) to 4.25 V at 25°C using the CC-CV method, and then discharged (0.1C) to 2.5 V using the CC method, and the charge/discharge capacity (mAh/g) at this time was measured. The measured charge/discharge capacity (mAh/g) and the percentage of discharge capacity to charge capacity (efficiency (%)) are shown in Table 3 below.

Charging capacity (mAh/g) Discharge capacity (mAh/g) Efficiency (%) Example 1 149.2 147.0 98.5 Example 2 148.4 146.6 98.8 Example 3 158.5 158.3 99.8 Comparative Example 1 137.5 135.3 98.4 Comparative Example 2 135.3 135.5 98.2 Comparative Example 3 137.9 144.7 98.3 Comparative Example 4 135.5 139.6 97.9 Comparative Example 5 122.1 111.2 91.1 Comparative Example 6 155.5 153.0 98.4

Through Table 3, it was confirmed that the batteries including the positive active materials of Examples 1 to 3, that is, the positive active materials having the degree of graphitization of 1.00 or more and 1.50 or less and the Dip(χ) of 0.160 or more and 0.185 or less, had a charge/discharge capacity of 145.0 mAh/g or more and an efficiency of 98.5% or more at room temperature. In contrast, the batteries including the positive active materials of Comparative Examples 1 and 2, that is, the positive active materials having the degree of graphitization of less than 1.00 and the Dip(χ) of greater than 0.185, had a problem of low charge/discharge capacity at room temperature. In addition, it was confirmed that the batteries including the positive electrode active materials of Comparative Examples 3 and 4, that is, the positive electrode active materials having the degree of graphitization of 1.00 or more and 1.50 or less and the Dip(χ) of less than 0.160 or greater than 0.185, had problems of low charge/discharge capacity and efficiency at room temperature. In addition, it was confirmed that the batteries including the positive electrode active materials of Comparative Examples 5 and 6, that is, the positive electrode active materials having the degree of graphitization of less than 1, had problems of low efficiency at room temperature.

For reference, the cathode active material according to the present invention having a graphitization degree of more than 1.50 was not subjected to battery characteristic evaluation because secondary phases in the form of Fe ₂ P or Fe ₃ P are generated by firing at a high temperature of 800°C or higher during manufacturing.

In conclusion, it can be seen that the positive electrode active material according to the present invention can increase the number of electrons that can be inserted and deintercalated by controlling the content of disordered carbon or carbon with defects in the coating portion and controlling the coating efficiency, and that irreversible capacity loss occurs less during charge and discharge.

Claims (10)

Lithium iron phosphate compound having an olivine structure; and A coating portion including carbon (C) formed on the lithium iron phosphate compound; In the Raman spectrum measured by a Raman spectrometer, the degree of graphitization according to Equation 1 below is 1.00 or more and 1.50 or less, A cathode active material having a Dip(χ) of 0.160 or more and 0.185 or less according to the following equation 2: [Formula 1] Degree of graphitization = I _D /I _G [Formula 2] Dip(χ)= I _G - I _x In the above equations 1 and 2, The above I _D is the maximum peak intensity of the D band, I _G is the maximum peak intensity of the G band, and I _x is the intensity of the minimum point between the G band and the D band. The above D band is a peak that appears at a Raman shift of 1300 cm ^-1 to 1400 cm ^-1 , The above G band is a peak that appears at a Raman shift of 1550 cm ^-1 to 1650 cm ^-1 . In claim 1, The above lithium iron phosphate compound is a cathode active material in which the proportion of manganese among the total transition metals is 30 mol% or more. In claim 1, The above lithium iron phosphate compound is a positive electrode active material having a composition represented by the following chemical formula 1: [Chemical Formula 1] Li _1+x Fe _1-ab Mn _a M ¹ _b PO ₄ In the above chemical formula 1, M ¹ is at least one selected from the group consisting of Al, Mg, Ni, Co, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y, -0.5≤x≤0.5, 0.3≤a<1.0, 0.0≤b≤0.1. In claim 1, A positive electrode active material having a carbon content included in the above coating portion of 1.00 wt% or more and 2.50 wt% or less based on the total weight of the positive electrode active material. In claim 1, A positive electrode active material having a graphitization degree of 1.00 or more and 1.10 or less. In claim 1, A positive electrode active material having the above Dip(χ) of 0.170 or more and 0.180 or less. In claim 1, A positive electrode active material having an average particle size (D ₅₀ ) of 0.25 ㎛ or more and 0.45 ㎛ or less. In claim 1, A cathode active material having an average crystallite size of 110.0 nm or more and 145.0 nm or less. A positive electrode comprising a positive electrode active material according to any one of claims 1 to 8. A lithium secondary battery comprising a positive electrode according to claim 9.