KR101922920B1 - A method for manufacturing an anode active material for lithium secondary battery - Google Patents

Abstract

An embodiment of the present invention provides a method for manufacturing a cathode active material for a lithium secondary battery, which comprises the following steps: (a) dissolving an iron precursor in a solvent in which water and a polar organic solvent are mixed at a weight ratio of 1:0.1 to 1, respectively to manufacture a precursor solution; and (b) heat treating the precursor solution at the temperature of 100 to 250°C for 1 to 10 hours to manufacture spherical hematite particles. It is an object of the present invention to provide the method for manufacturing the cathode active material for the lithium secondary battery, which manufactures the hematite having a uniform structure and shape within a short time, and can improve the electrochemical characteristics and lifetime characteristics of the lithium secondary battery when applied as the cathode electrode active material for the lithium secondary battery.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a negative active material for a lithium secondary battery,

The present invention relates to a process for preparing a negative electrode active material for a lithium secondary battery, and more particularly, to a process for preparing a negative electrode active material for a lithium secondary battery, which can improve the electrochemical characteristics of a lithium secondary battery by controlling the shape and structure of the negative electrode active material, To a method for producing a negative electrode active material for a lithium secondary battery.

As the application fields of lithium secondary batteries such as portable electronic devices and electric vehicles are expanded, demand for high power, large capacity, and long life lithium secondary batteries is increasing. This leads to research and development of lithium secondary battery materials that satisfy high efficiency, low cost, and productivity.

The metal oxide used as an anode active material for a lithium secondary battery has a high theoretical capacity and is environmentally friendly, but has a problem in that the capacity is rapidly reduced as the charge and discharge are repeated.

Among these metal oxides, iron oxide (Fe _x O _y ) has an advantage in providing a rich active site in terms of porosity and specific surface area. Of these, iron oxide (Fe ₂ O ₃ ), magnetite (Fe ₂ O ₄ ) Hemite (γ-Fe ₂ O ₃ ) has been studied as an anode active material for lithium secondary batteries.

Particularly, hematite (α-Fe ₂ O ₃ ) is advantageous not only in terms of electrochemical properties such as high electric conductivity and low band gap, but also in environmental friendliness and economy.

Conventionally, a method of preparing and synthesizing hematite from an iron precursor using a hydrothermal synthesis method using water as a medium has been proposed. However, it takes about 20 hours or longer to synthesize hematite and uniform control of the structure and morphology of hematite The selectivity, reproducibility and reliability of the lithium secondary battery are low. Therefore, when the hematite prepared through the above process is applied to the anode active material for a lithium secondary battery, it is difficult to stably realize the electrochemical characteristics and life characteristics of the lithium secondary battery.

SUMMARY OF THE INVENTION The object of the present invention is to provide a method for preparing a hematite having uniform structure and shape within a short time and applying the same to an anode active material for a lithium secondary battery, And a method for producing a negative electrode active material for a lithium secondary battery capable of improving lifetime characteristics.

(A) dissolving an iron precursor in a solvent mixed with water and a polar organic solvent in a weight ratio of 1: 0.1 to 1, respectively, to prepare a precursor solution; And (b) heat treating the precursor solution at 100 to 250 ° C for 1 to 10 hours to prepare spherical hematite particles.

In one embodiment, the iron precursor may be selected from the group consisting of chloride, nitrate, sulfate, and mixtures of two or more thereof.

In one embodiment, the polar organic solvent may be an alcohol.

In one embodiment, the alcohol may be selected from the group consisting of methanol, ethanol, isopropanol, n-butanol, amyl alcohol, cyclohexanol, and mixtures of two or more thereof.

In one embodiment, the hematite particles may be mesoporous particles having an average diameter of 100 to 700 nm.

In one embodiment, the hematite particles may have a specific surface area of 50 to 500 m ² / g.

Another aspect of the present invention provides a negative active material for a lithium secondary battery, which is produced by the above method.

Another aspect of the present invention provides a negative electrode for a lithium secondary battery, wherein the slurry including the negative electrode active material for a lithium secondary battery, a binder, and a conductive material is applied to a current collector.

In one embodiment, the binder is selected from the group consisting of polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, , Ethylene-propylene (EPDM) rubber, styrene-butylene rubber, fluorine rubber, and mixtures of two or more thereof.

In one embodiment, the content of the negative electrode active material for the lithium secondary battery may be 70 to 95% by weight based on the total weight of the slurry.

According to one aspect of the present invention, by controlling the shape and structure of an anode active material composed of hematite (α-Fe ₂ O ₃ ) using a mixed solvent of a certain composition during hydrothermal synthesis, the electrochemical characteristics and life characteristics Can be remarkably improved.

It should be understood that the effects of the present invention are not limited to the effects described above, but include all effects that can be deduced from the description of the invention or the composition of the invention set forth in the claims.

1 is a schematic view illustrating a method for manufacturing a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention and one comparative example;
FIG. 2 is a graphical representation of the behavior of lithium ions in a lithium secondary battery according to an embodiment of the present invention; FIG.
3 is a result of X-ray diffraction (XRD) analysis of an anode active material for a lithium secondary battery according to one embodiment of the present invention and one comparative example;
4 is an FE-SEM image of an anode active material for a lithium secondary battery according to one embodiment of the present invention and one comparative example;
5 is a graph showing the results of HR-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis of an anode active material for a lithium secondary battery according to an embodiment of the present invention and one comparative example;
6 is a HR-TEM (high resolution transmission electron microscope) image of an anode active material for a lithium secondary battery according to one embodiment of the present invention and one comparative example;
7 is a nitrogen adsorption / desorption isotherm for an anode active material for a lithium secondary battery according to an embodiment of the present invention and one comparative example;
8 is a graph showing a differential capacitance with respect to a voltage of a negative electrode for a lithium secondary battery according to an embodiment of the present invention and one comparative example;
FIG. 9 and FIG. 10 show results of analysis of circulation performance of a negative electrode for a lithium secondary battery according to one embodiment of the present invention and one comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout the specification, when a part is referred to as being "connected" to another part, it includes not only "directly connected" but also "indirectly connected" . Also, when an element is referred to as "comprising ", it means that it can include other elements, not excluding other elements unless specifically stated otherwise.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(A) dissolving an iron precursor in a solvent mixed with water and a polar organic solvent in a weight ratio of 1: 0.1 to 1, respectively, to prepare a precursor solution; And (b) heat treating the precursor solution at 100 to 250 ° C for 1 to 10 hours to prepare spherical hematite particles.

Conventionally, a method of preparing and synthesizing hematite from an iron precursor using a hydrothermal synthesis method using water as a medium has been proposed. However, it takes about 20 hours or longer to synthesize hematite and uniform control of the structure and morphology of hematite The selectivity, reproducibility and reliability of the lithium secondary battery are low. Therefore, when the hematite prepared through the above process is applied to the anode active material for a lithium secondary battery, it is difficult to stably realize the electrochemical characteristics and life characteristics of the lithium secondary battery.

On the other hand, the solvent used in the step (a) is a mixture of water and a certain amount of a polar organic solvent. The polar organic solvent is added to water as a reaction medium to control the polarity of the solvent, And intermediates (nanospheres, nanoclusters, etc.) formed thereby and the shape and structure of the hematite particles as final products.

The solvent may be a mixture of water and a polar organic solvent in a weight ratio of 1: 0.1 to 1, respectively. If the weight ratio of the polar organic solvent to the water is less than 0.1, spherical mesoporous hematite particles can not be obtained. If the weight ratio is more than 1, the relative content of water as a reaction medium for hydrothermal synthesis is decreased, have.

1 is a schematic view illustrating a method for manufacturing a negative electrode active material for a lithium secondary battery according to an embodiment of the present invention and one comparative example. 1, the composition of a solvent used in the production of hematite (α-Fe ₂ O ₃ ) particles by hydrothermal synthesis, for example, a mixture of hematite (α- Fe ₂ O ₃ ) particles can be classified into spherical and cubic forms.

As used herein, the term "spherical" can be understood not only as a complete sphere but also as a concept involving the case where one region is deformed or one side is oval or the overall shape is substantially spherical. As used herein, the term "cube shape" includes not only a complete cube shape but also a case in which the curved surface includes a part of a curved surface in a vertex or an edge area, but includes a case where the overall shape is substantially a cube or a rectangular parallelepiped. .

First, when the iron precursor and ammonia are dissolved in a mixed solvent of water and ethanol, ammonia and ethanol are simultaneously bonded to the surface of the iron ion, and they are mutually agglomerated to reduce the surface energy of the particles under hydrothermal synthesis conditions, (nanoclusters) are formed. The hematite nanocluster increases in size under hydrothermal synthesis according to predetermined conditions, and finally spherical mesoporous hematite particles of about 600 nm in size are produced. The process by which these spherical hematite particles are produced can be explained by Ostwald ripening.

On the other hand, when the iron precursor and the ammonia are dissolved in a single solvent composed of water, only ammonia is bonded to the surface of the iron ion, and the hematite nanocrystals under the hydrothermal synthesis conditions form hexahedral hematite particles by oriented attachment.

The iron precursor may be one selected from the group consisting of iron chloride, nitrate, sulfate, and a mixture of two or more thereof, preferably iron chloride, but is not limited thereto.

The polar organic solvent may be an alcohol and may be selected from the group consisting of methanol, ethanol, isopropanol, n-butanol, amyl alcohol, cyclohexanol and mixtures of two or more thereof, Ethanol, but is not limited thereto.

The hematite particles may be mesoporous particles having an average diameter of 100 to 700 nm and a specific surface area of 50 to 500 m ² / g. As used herein, the term "mesoporous particle" means a particle comprising a plurality of pores having an average diameter of about 2-50 nm. When the average diameter of the hematite particles is less than 100 nm or the specific surface area is more than 500 m ² / g, the reactivity with the electrolyte becomes excessively large, so that it is difficult to control the electrochemical characteristics of the battery and the average diameter exceeds 700 nm or the specific surface area is 50 m ² / g , It is difficult to provide sufficient active sites for storage and transportation of lithium ions.

Another aspect of the present invention provides a negative electrode for a lithium secondary battery, wherein the slurry containing the negative electrode active material for a lithium secondary battery, the binder and the conductive material prepared by the above method is applied to a current collector.

The binder may be selected from the group consisting of polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ) Rubber, styrene-butylene rubber, fluorine rubber, and a mixture of two or more thereof, preferably polyvinylidene fluoride, but is not limited thereto. The content of the binder may be 1 to 15% by weight, preferably 5 to 13% by weight, based on the total weight of the slurry.

The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, the conductive material may be graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Polyphenylene derivatives, and the like. The content of the conductive material may be 1 to 30 wt%, preferably 5 to 20 wt%, based on the total weight of the slurry.

The content of the negative electrode active material for a lithium secondary battery may be 70 to 95% by weight based on the total weight of the slurry. If the content of the negative electrode active material for a lithium secondary battery is less than 70% by weight, the life characteristics and conductivity of the battery may be deteriorated. If the content is more than 95% by weight, the content of the binder and the conductive material may be decreased.

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

The lithium secondary battery may have a structure in which a lithium salt-containing electrolyte is impregnated in an electrode assembly having a separator interposed between an anode and a cathode.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength can be used. The pore diameter of the separator is generally 0.01 탆 to 10 탆, and the thickness may be 5 탆 to 300 탆. For example, the separator may be an olefinic polymer such as a chemical resistant and hydrophobic polypropylene; A sheet or a nonwoven fabric made of glass fiber, polyethylene, or the like. In addition, when a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may serve as a separator.

The lithium salt-containing electrolytic solution is composed of an electrolytic solution and a lithium salt, and the electrolytic solution may be water, a non-aqueous organic solvent, an organic solid electrolyte, an inorganic solid electrolyte, or the like, but is not limited thereto.

Wherein the non-aqueous organic solvent is at least one selected from the group consisting of N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -butylolactone, 1,2-dimethoxyethane, The organic solvent is selected from the group consisting of franc, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, , Trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate But may not be limited to, non-magnetic organic solvents.

Wherein the organic solid electrolyte is selected from the group consisting of a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene, But it is not limited thereto.

Wherein the inorganic solid electrolyte is selected from the group consisting of Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Li ₄ SiO ₄ -LiI -LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ and the like, but the present invention is not limited thereto.

The lithium salt is a substance which can be dissolved in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , _{LiAsF 6, LiSbF 6, LiAlCl 4} , CH 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate, but already the like de, but are not limited to .

The electrolytic solution may contain at least one selected from the group consisting of pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, hexametriacetamide, nitrobenzene derivative, sulfur, quinoneimine Dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride and the like may be added. Further, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride can be further added to impart nonflammability to the electrolyte solution, and carbon dioxide gas, FEC (Fluoro-Ethylene Carbonate), PRS (Propene sultone ) May be further added.

Hereinafter, embodiments of the present invention will be described in detail.

Example

0.2 M iron chloride (FeCl ₃ .6H ₂ O) and ammonia water were added to a solvent composed of 50 ml of deionized water and 25 ml of ethanol, followed by stirring for 30 minutes. The prepared solution was put into an 80 ml Teflon-coated stainless steel autoclave and heat-treated at 180 ° C for 3 hours, cooled to room temperature, and then a dark brown precipitate was obtained using a centrifuge. The precipitate was washed three times with deionized water and ethanol, dried at 100 ° C for 10 hours until only the precipitate remained, and then calcined at 500 ° C for 4 hours to obtain spherical hematite (α-Fe ₂ O ₃ ) particles , "SFO").

Comparative Example

0.2M iron chloride (FeCl ₃ .6H ₂ O) and ammonia water were added to 50 ml of deionized water, and the mixture was stirred for 30 minutes. The prepared solution was put into an 80 ml Teflon-coated stainless steel autoclave and heat-treated at 180 ° C for 3 hours, cooled to room temperature, and then a dark brown precipitate was obtained using a centrifuge. The precipitate was washed with deionized water three times, dried at 100 ° C. for 10 hours until only the precipitate remained, and then calcined at 500 ° C. for 4 hours to obtain cubic hematite (α-Fe ₂ O ₃ ) "CFO").

Experimental Example  One: XRD  analysis

Figure 3 shows XRD (X-ray diffraction) analysis results for SFO and CFO. Referring to FIG. 3, both SFO and CFO are 24.29 DEG and 33.28 DEG. Peaks appear at 35.74 °, 40.97 °, 43.69 °, 49.55 °, 54.23 °, 57.71 °, 62.54 °, 64.08 °, 72.03 ° and 75.51 ° and this is due to the formation of hematite with a rhombohedral crystal system (JCPDS-13 -0534). ≪ / RTI > The peaks observed in SFO and CFO indicate good crystallinity and purity of both materials.

Experimental Example  2: FE- SEM  analysis

Figure 4 is an FE-SEM image for SFO and CFO. Referring to Figures 4 (a) - (i) to (iv), the CFO has some curved surfaces at corners or vertices, but has a cube shape as a whole, while Figures 4 (c) - (i) SFO is a spherical shape with no edges or vertices at all. Thus, it can be seen that the composition of the solvent used in the hydrothermal synthesis process is an important factor determining the nucleation and orientation of the crystal structure, and thus the shape of the particles.

Experimental Example  3: Raman Spectrum  analysis

Figure 5 (a) is the HR-Raman spectrum for SFO and CFO. When reference to Fig. 5 (a), SFO and CFO are all ^{225cm -1, 245cm -1, 293cm -1} , 410cm -1, 395cm -1, 613cm -1 , and the seven peaks appeared at a wave number of 1,320cm ^-1 , Indicating that SFO and CFO are hematite (α-Fe ₂ O ₃ ), not any other iron oxide, iron hydroxide.

Experimental Example  4: XPS  analysis

5 (b) to 5 (d) show X-ray photoelectron spectroscopy (XPS) analysis results for SFO.

FIG. 5 (b) shows the result of the total analysis for SFO, where a peak corresponding to Fe, O, Si, N is observed. The N and Si peaks are from the atmospheric nitrogen and the substrate, respectively.

Figure 5 (c) shows the XPS spectrum of the SFO Fe 2p _3/2 and Fe 2p _1/2 orbital. Peak of Fe 2p _1/2 orbital spin orbital (ji) by coupling smaller than the _{Fe 2p 3/2, Fe 2p} 3/2 and 2p Fe _1/2 orbital is four and two degenerate state (degeneracy each state. In general, Fe 2p _3/2 peak of Fe2O3 is 710.6eV to 711.2eV is observed in the range of, than that of the peak at about 718eV 8eV high of about or approximately 719eV satellite peaks of α-Fe ₂ O ₃ of the Fe 2p _3/2 (satellite peak). The satellite peaks were clearly distinct from the Fe 2p _3/2 and Fe 2p _1/2 peaks do not overlap at all.

The O 1s spectrum of FIG. 5 (d) is separated into two peaks (deconvoluted), with peaks at 528.8 eV indicating the presence of lattice oxygen and peaks at 531.5 eV indicating the presence of hydroxide ions.

Experimental Example  5: HR- TEM  Image analysis

Figure 6 is a HR-TEM (high resolution transmission electron microscope) image for SFO and CFO. 6 (a) to 6 (d), CFO is a cube having an average diameter of about 800 nm, void space is not observed on the surface of CFO, and a lattice having an interplanar spacing of 0.269 nm A lattice fringe is observed. Referring to Figures 6 (e) to 6 (h), SFO is a spherical shape consisting of a plurality of nanospheres having an average diameter of about 5 nm, and a lattice fringe having an interplanar spacing of 0.269 nm is observed do. 6 (k) to 6 (p), all of the CFOs of FIGS. 6 (k) to 6 (p) And a green component are observed.

Experimental Example  6: Surface area and pore size

Figures 7 (a) and 7 (b) are nitrogen adsorption / desorption isotherms for SFO and CFO, respectively. Referring to FIG. 7 (a), it can be seen that SFO is a mesoporous particle. The specific surface areas of SFO and CFO are 100.81 m ² / g and 27.368 m ² / g, respectively, and the pore size distribution of SFO is lower than that of CFO. As such, the micropores formed in the SFO provide a number of active sites for the storage of lithium ions and can promote ion transport of the electrolyte.

Manufacturing example

A mixed material of SFO, polyvinylidene difluoride (PVDF, binder), and Super P (conductive material) at a weight ratio of 80: 10: 10 was mixed with N-methylpyrrolidone to prepare a slurry, And dried sequentially in an oven (120 ° C), air (100 ° C, 5 hours) and vacuum (120 ° C, 5 hours) to form a lithium secondary battery cathode (hereinafter referred to as "SFO cathode ").

Comparative Manufacturing Example

A mixed material of CFO, polyvinylidene difluoride (PVDF, binder), and Super P (conductive material) at a weight ratio of 80: 10: 10 was mixed with N-methylpyrrolidone to prepare a slurry, And then dried in an oven (120 ° C), air (100 ° C, 5 hours) and vacuum (120 ° C, 5 hours) in succession to obtain a negative electrode for a lithium secondary battery ").

The electrode assembly was manufactured using the porous separator made of the negative electrode, the positive electrode made of lithium foil, and the polyethylene / polypropylene / polyethylene triple layer film manufactured in the above Production Examples and Comparative Production Examples. The electrode assembly was inserted into a pouch and lead wires were connected. Then, a 1: 2 volume ratio of ethylene carbonate (EC) and diethylene carbonate (DEC) solution containing 1 M of LiPF ₆ salt was injected into the electrolyte and sealed, .

Experimental Example  7: Lithium secondary battery  Performance Evaluation 1

8 (a) and 8 (b) are graphs showing the differential capacitance with respect to the voltages of the SFO cathode and the CFO cathode, respectively. The measurement was made at a current density of 100 mA / g, 0.01 V to 3 V. In general, there is a significant difference between the initial cycle and the subsequent cycle. Referring to FIG. 8, a reduction peak appears at 0.75 V for both the SFO cathode and the CFO cathode during the initial circulation. This is due to the decomposition of the electrolyte and the reversible conversion mechanism in which lithium ions are intercalated into Fe ₂ O ₃ to form Li ₂ O. The peak intensity decreases after the first cycle, which means that some irreversible reaction occurs at the electrode. On the other hand, the oxidation peak observed at about 1.70 V is due to the reversible oxidation of Fe ⁰ to Fe ^{3 +} . The area of the redox peak between the reduction and oxidation process decreases, which means the capacity loss between charge and discharge. The reduction / oxidation peak at the next cycle is shifted to 0.95 and 2.01V, respectively. Also, the redox peak area is larger than that of the SFO negative electrode and the CFO negative electrode, which means that the non-capacity and the reactivity between the electrode and the electrolyte are good.

Experimental Example  8: Lithium secondary battery  Performance Evaluation 2

The performance evaluation was performed by repeating the constant current charge-discharge cycle at a current density of 100 mA / g in a voltage range of 0.01 V to 3 V for each lithium secondary battery including the SFO cathode and the CFO cathode, and the result is shown in FIG. .

9 (a), in the case of the SFO cathode, the first circulating constant current discharge capacity and the charge capacity were measured to be 1,306 mAh / g and 927 mAh / g respectively, and the Coulomb efficiency was 75%. Referring to FIG. 9 (c), in the case of the CFO cathode, the first circulating constant current discharge capacity was 1,163 mAh / g, and the Coulomb efficiency was 65%. Coulomb efficiency for subsequent cycles is 95% and 97% for SFO cathodes and 89% and 91% for CFO cathodes. The linear behavior at 0.95 V in the discharge region and 2.01 V in the charging region is due to the reduction of Fe ^{2 +} to Fe and the oxidation of Fe to iron oxide.

9 (b) and 9 (d), the SFO cathode carries a higher reversible capacity than the CFO cathode. Specifically, referring to FIG. 9 (b), the SFO cathode delivers a discharge capacity of 968 mAh / g after 100 cycles while maintaining a Coulomb efficiency of 98%. This is a significant improvement over CFO cathodes delivering a discharge capacity of 617 mAh / g at a Coulomb efficiency of 93% under the same conditions. After 100 cycles, the capacity retention rates of SFO cathode and CFO cathode were 91.4% and 63.0%, respectively. The high capacity retention of the SFO cathode is due to uniform, homogeneously dispersed particles of nanospheres. In the case of SFO, a void space exists between the nanospheres and the porous SFO, and this void space can inhibit the formation of a solid electrolyte interface (SEI) layer. Also, referring to FIG. 9 (b), the amount of charge after 30 cycles is slightly increased because the active material is activated by the large surface area of the nanospheres and the rich nano pores forming the SFO.

Experimental Example  9: Lithium secondary battery  Performance Evaluation 3

The results of measurement of the rate capability of each lithium secondary battery including the SFO cathode and the CFO cathode in the range of 100 to 1,600 mA / g are shown in FIGS. 10 (a) and 10 (b) .

Current density
(mA / g) 100 400 800 1,200 1,600 Manufacturing example
(mAh / g) 1,306 925 837 698 631 Comparative Manufacturing Example
(mAh / g) 1,163 643 526 428 259

In general, as the current density increases, the capacity of the electrode material decreases. At high current densities, lithium ions diffuse only at the surface of the active material, while at low current densities, lithium ions diffuse into or between the particles, resulting in increased cost. Initially, at low current densities, both cathodes carry high capacities, but at high current densities, the value of the CFO cathode is lower. On the other hand, the SFO cathode maintains a good level of non-capacity and, in particular, delivers approximately the same capacity when the current density returns to 100 mA / g. In addition, even when the current density is increased to 1,600 mA / g, the specific capacity of the SFO cathode is significantly higher than that of the CFO cathode, which means that SFO is structurally stable compared to CFO.

10 (c) and 10 (d) show electrochemical impedance spectra before circulation of SFO cathode and CFO cathode and after full circulation after 100 cycles, respectively. Measurements were made in the frequency range of 100 kHz to 10 mHz. Referring to Figures 10 (c) and 10 (d), the spectrum consists of two regions. The semicircle at high frequency corresponds to the charge transfer resistance (R _ct ) and the line at low frequency is related to the solid state diffusion and migration of Li ⁺ ions from the electrolyte to the electrode surface, Impedance (Warburg impedance, R _w ) behavior. Referring again to FIG. 6, the diameter of CFO is relatively large compared to SFO. The R _ct minimum value of the SFO cathode means good contact between the electrode and the electrolyte. This is consistent with the excellent results of the SFO rate characteristics compared to the CFO, which allows us to understand the rapid reaction rate and easy ion accessibility at the SFO cathode.

In addition, since SFO is abundant in the nanospheres and active sites therebetween, the bug impedance ( _Rw ) after 100 cycles is larger than that of CFO. The low frequency region of the impedance spectrum corresponds to the limited diffusion of ions within the particle. The geometric structure and pore size distribution of nanoparticles can affect this limited diffusion impedance. Specifically, the shape of the SFO (spherical shape) and the low pore size distribution facilitates ion diffusion compared to CFO in electrochemical charge and discharge processes, and these porous materials can improve the storage stability of electrochemical energy.

It will be understood by those skilled in the art that the foregoing description of the present invention is for illustrative purposes only and that those of ordinary skill in the art can readily understand that various changes and modifications may be made without departing from the spirit or essential characteristics of the present invention. will be. It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. For example, each component described as a single entity may be distributed and implemented, and components described as being distributed may also be implemented in a combined form.

The scope of the present invention is defined by the appended claims, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be construed as being included within the scope of the present invention.

Claims (10)

(a) dissolving an iron precursor in a mixed solvent of water and a polar organic solvent in a weight ratio of 1: 0.1 to 0.3945, respectively, to prepare a precursor solution; And
(b) heat treating the precursor solution at 100 to 250 ° C for 1 to 10 hours to have a plurality of pores having a maximum peak of a pore size distribution between 10 and 100 nm, and having a specific surface area of 50 to 500 m ² / g And forming a spherical hematite particle. The method for producing a negative electrode active material for a lithium secondary battery according to claim 1, The method according to claim 1,
Wherein the iron precursor is one selected from the group consisting of iron chloride, nitrate, sulfate, and a mixture of two or more thereof. The method according to claim 1,
Wherein the polar organic solvent is an alcohol. The method of claim 3,
Wherein the alcohol is selected from the group consisting of methanol, ethanol, isopropanol, n-butanol, amyl alcohol, cyclohexanol, and mixtures of two or more thereof. The method according to claim 1,
Wherein the hematite particles are mesoporous particles having an average diameter of 100 to 700 nm. delete A negative electrode active material for a lithium secondary battery, which is produced by the method of any one of claims 1 to 5. A negative electrode for a lithium secondary battery according to claim 7, wherein a slurry containing a negative electrode active material for a lithium secondary battery, a binder and a conductive material is applied to the current collector. 9. The method of claim 8,
The binder may be selected from the group consisting of polyvinylidene fluoride, polyvinyl fluoride, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ) Rubber, styrene-butylene rubber, fluorine rubber, and mixtures of two or more thereof. 9. The method of claim 8,
Wherein the content of the negative electrode active material for a lithium secondary battery is 70 to 95% by weight based on the total weight of the slurry.

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