KR101438683B1 - Fabricating method of lithium-iron-metal-phosphate-carbon composite nanofiber cathode active material and cathode active material fabricated by the method - Google Patents

Abstract

The present invention relates to a method of producing a nanofiber lithium-iron-metal oxide-carbon composite cathode active material having a large surface area by using electrospinning, wherein LiFe _1-x M _x PO ₄ (0 <X <1, A first step of preparing a viscous solution by mixing a raw material and a chelating agent, which are quantitatively determined according to a chemical formula, according to a composition formula of M, Ni, Mn, Co, V, Cr, Cu, Ti or Zr in a solvent; A second step of electrospinning the viscous solution to produce a nanofiber precursor; A third step of drying the nanofiber precursor; And a fourth step of baking the dried nanofiber precursor.
The present invention relates to a method of manufacturing a nanofiber having a composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti, or Zr) Thereby, it is possible to manufacture a lithium-iron-metal oxide-carbon composite nano fiber cathode active material for a lithium ion secondary battery having a very large surface area while maintaining the nanofiber form in the firing process.

Description

FIELD OF THE INVENTION The present invention relates to a lithium-iron-metal composite oxide nanocomposite active material, FABRICATED BY THE METHOD}

The present invention relates to a cathode active material for a lithium ion secondary battery having a nanostructure and a method of manufacturing the same. More particularly, the present invention relates to a lithium-iron-metal oxide-carbon composite controlled by nanofiber using electrospinning To a method for producing a composite oxide for a cathode active material having uniform fiber length and diameter and stable surface morphology.

Generally, a battery can be divided into a primary battery used for one time use and a secondary battery which can be recharged. 2. Description of the Related Art [0002] As electronic devices have become more compact in recent years, such as mobile phones, notebook computers (PCs), and personal digital assistants (PDAs), interest in rechargeable battery technology is increasing. Further, as electric vehicles (EVs) and hybrid vehicles (HEVs) are put into practical use, researches on secondary batteries having high capacity and high output and high stability have been actively conducted.

The secondary battery is composed of a cathode, a cathode, and an electrolyte. The cost of the anode is the highest in the cost of various materials. The cathode material of a lithium ion secondary battery generally has a high energy density at the time of charging and discharging and at the same time the structure should not be destroyed by intercalation or deintercalation of reversible lithium ions and the electrical conductivity is high and the chemical Stability should be high. Further, it is preferable that the material is low in manufacturing cost and minimizes environmental pollution problem.

Examples of the lithium compound that exhibits the above characteristics include LiNiO ₂ , LiCoO ₂ , and LiMn ₂ O ₄ , which are layered compounds capable of intercalating and deintercalating lithium ions.

LiNiO ₂ has a high electric capacity, but has problems such as cycle characteristics and stability at the time of charging and discharging, and has not been practically used. LiCoO ₂ has the advantages of not only a large capacity but also excellent cycle life and rate capability and easy synthesis. However, LiCoO ₂ is expensive because it contains cobalt and is harmful to human body and thermally unstable at high temperature. It has disadvantages. In order to overcome the physical disadvantages of LiCoO ₂ , studies have been made on nickel-cobalt-manganese composite metal oxides having a layered crystal structure, but cost problems and harmfulness due to cobalt still remain as a problem.

Recently, studies on Li ₂ MnO ₃ -LiM _x O ₂ (here, metals such as M, Ni, Fe, Mn, Cr) published by Thackeray et al. However, the Li ₂ MnO ₃ -LiM _x O ₂ -based cathode active material has problems in terms of thermal stability and lifetime characteristics in comparison with conventional cathode active materials, except that it has a high capacity.

As a new cathode active material for a lithium ion secondary battery, attention has also been paid to a LiFePO ₄ -based material having an olivine structure. LiFePO ₄ has a relatively high theoretical capacity (170 mAh / g) and environmental friendliness compared with commercialized cathode active materials, and its price is low and its stability is very high, and research and development is proceeding as a material for HEV or EV. However, olivine-based materials require nano-particleization because they have low operating voltage and ion and ion conductivity are much slower than conventional materials.

Solid-state and coprecipitation methods are generally used as a method for producing a composite metal oxide for use as a cathode active material of a lithium ion secondary battery.

The solid phase method has a disadvantage in that it is difficult to obtain a uniform composition due to a large influx of impurities at the time of mixing, a high temperature is required in the manufacturing step, and a long manufacturing time.

The coprecipitation method is a method in which a precursor obtained by a simultaneous precipitation using an aqueous solution containing Ni, Co and Mn as a co-precipitating agent and a chelating agent as a complexing agent is mixed with a lithium salt and then calcined to obtain an active material However, since the particle size of the active material is influenced by the particle size of the precursor, and because the process parameters of the synthesis process are very large and the process is complicated, a lot of effort is required in the optimization process And time is required.

Korean Patent No. 10-1106269

"Li2MnO3-stabilized LiMO2 (M = Mn, Ni, Co) electrodes for lithium-ion batteries" - Michael M. Thackeray, Sun-Ho Kang, Christopher S. Johnson, John T. Vaughey, Roy Benedek and SA Hackney, Materials Chemistry, Issue 30, 2007, 17, 3112-3125

Disclosure of Invention Technical Problem [8] The present invention has been made to solve the above problems of the prior art, and it is an object of the present invention to provide a method for producing a nanofiber lithium-iron-metal oxide-carbon composite cathode active material having a large surface area using an electrospinning method, And an object of the present invention is to provide the improved cathode active material.

In order to accomplish the above object, the present invention provides a method for producing a lithium-iron-metal oxide-carbon composite nano-fiber cathode active material, comprising the steps of: LiFe _1-x M _x PO ₄ (0 <X <1, M = A first step of preparing a viscous solution by mixing a raw material and a chelating agent, which are quantitatively determined according to a chemical formula, according to a composition formula of Co, V, Cr, Cu, Ti, or Zr in a solvent; A second step of electrospinning the viscous solution to produce a nanofiber precursor; A third step of drying the nanofiber precursor; And a fourth step of baking the dried nanofiber precursor.

Electrospinning is a technique that is basically applied to the polymer industry and is a method that can effectively form fibers having diameters ranging from a submicrometer to a nanometer unit. Therefore, it was expected that a uniform nanostructure material could be synthesized as compared with the solid phase method or coprecipitation method. However, since the problem of maintaining a uniform microstructure in a synthesis condition and a post-synthesis heat treatment can not be solved, It was not used for synthesis.

In the present invention, the chelating agent may be selected from the group consisting of polyvinylpyrrolidine (PVP), polyvinylacetate (PVAc), polyvinyl alcohol (PVA), polyethylene (PE), polypropylene Poly (3,4-ethylenedioxythiophene), PEDOT). It is preferable that the viscosity of the viscous solution prepared in the first step is in the range of 0.1 to 1.5 Pa · s .

Since the present invention uses electrospinning, a viscous solution must be prepared by mixing raw materials. Therefore, the present invention controls the viscosity by using a polymer material as a chelating agent, and adjusts the amount of the chelating agent to be in the range of 0.1 to 1.5 Pa · s, which is a viscosity suitable for electrospinning nano-unit fibers.

Further, the chelating agent of the present invention is an organic high molecular material, which enables the formation of a structure in which a carbon layer is coated on the surface of nanofibers by supplying carbon during firing.

The raw material used in the present invention may be a Li salt, a Fe salt, a salt of a metal (M) to be added and a phosphate, and may be exemplified by lithium nitrate (LiNO ₃ ), manganese nitrate tetrahydrate (MnNO ₄ 4H ₂ O), iron nitrate tetrahydrate (FeNO ₄ .9H ₂ O), and ammonium phosphate (NH ₄ H ₂ PO ₄ ).

As the solvent, it is preferable to use a complex solvent in which distilled water, an acid and an alcohol are mixed. Especially, a complex solvent in which distilled water, nitric acid and methanol are mixed is preferably used.

In the second step, the electrospinning is preferably carried out in a voltage range of 10 to 30 kV and a tip-to-collector distance (TCD) of 5 to 20 cm. If it is outside this range, it is difficult to form nanofibers, and the formed nanofibers are also destroyed during the firing process.

The third step may be carried out in a vacuum atmosphere or an atmospheric pressure state, and it is required to be performed at a temperature of 100 ° C or more for 8 hours or more to sufficiently remove impurities and organic substances contained in the nanofiber precursor produced in the second step, Can be manufactured.

The fourth step is preferably composed of a first heat treatment performed in a range of 450 to 550 ° C and a second heat treatment performed in a range of 600 to 900 ° C. From this, it is preferable that the nanofiber precursor maintains the nanofiber form, And a nanofiber structure in which a carbon layer is coated on the surface.

The fourth step may be performed in an inert gas atmosphere such as argon or nitrogen, an oxygen atmosphere, or an atmosphere in which an inert gas and oxygen are mixed.

The lithium-iron-metal oxide-carbon composite nanofiber cathode active material according to another embodiment of the present invention is produced by the above-described method and has a composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti, or Zr) is coated with a carbon layer, and the carbon of the carbon layer is supplied from a chelating agent.

The cathode active material of the present invention has an average diameter in the range of 50 to 800 nm and has a very large surface area, and the thickness of the carbon layer coated on the surface is preferably 50% or less of the total diameter. If the thickness of the carbon layer is too wide, it can not function as a cathode active material of the lithium ion secondary battery.

The cathode active material of the present invention may be one of 1 to 3 axes and may be hollow in the inside of the nanofiber by shrinking in the firing process.

According to another aspect of the present invention, there is provided a method of manufacturing a lithium ion secondary battery comprising a positive electrode plate, a negative electrode plate, and an electrolyte, the method comprising the steps of: preparing a positive electrode active material; And applying the positive electrode active material to a current collector to produce a positive electrode plate.

The lithium ion secondary battery includes a positive electrode plate, a negative electrode plate and an electrolytic solution. In the process of manufacturing the positive electrode plate, the positive electrode active material is manufactured by the above-described method and then applied to the collector to manufacture a positive electrode plate. have. Other configurations of the lithium ion secondary battery and the manufacturing steps thereof are not particularly limited, and all known configurations and manufacturing steps are applicable, so a detailed description thereof will be omitted.

In the present invention constructed as described above, the composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti, or Zr) It is possible to produce lithium-iron-metal oxide-carbon composite nanofiber cathode active material for a lithium ion secondary battery having a very large surface area while maintaining the nanofiber form in the firing process.

Further, the present invention has an effect of simultaneously acting as a carbon source in the baking step and a viscosity adjusting agent for electrospinning by using an organic polymer material as a chelating agent.

The lithium-iron-metal oxide-carbon composite nanofiber cathode active material of the present invention has an extremely large surface area in the range of 50 to 800 nm while maintaining an olivine structure irrespective of the amount of iron and metal to be substituted, A carbon layer having a thickness of 50% or less of the thickness of the cathode active material layer is coated to solve the problems of low operating voltage and slow ion conductivity, which are disadvantages of olivine-based materials.

FIG. 1 is a result of X-ray diffraction analysis of the cathode active material prepared according to this embodiment.
FIGS. 2 to 4 are field scanning electron microscope (FE-SEM) photographs of the cathode active material prepared according to this embodiment.
FIG. 5 is a result of performing Fourier transform infrared spectroscopy (FT-IR) on the cathode active material manufactured according to the present embodiment.
6 to 8 show the results of EDS analysis of the cathode active material prepared according to this embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the accompanying drawings, embodiments of the present invention will be described in detail.

The method for producing a positive electrode active material according to the present invention is characterized in that at first a positive stoichiometric ratio is selected in accordance with the composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti, or Zr) The viscous solution is prepared by mixing the quantified raw material and the chelating agent in a solvent. (First step)

In the first step, the raw material and the chelating agent are quantified so as to be applicable to the electrospinning and mixed with a solvent to prepare a viscous solution.

The raw materials are substances capable of forming a substance of the composition formula by a sol-gel method, and Li salts, Fe salts, salts of added metals (M) and phosphates are used. The metal (M) to be added is a substance to be substituted with Fe, and the additive amount (X) has a value of 0 to 1.

The chelating agent simultaneously acts as a viscosity controlling agent for controlling the viscosity of the viscous solution, and the viscosity thereof is controlled so that the viscosity of the viscous solution is adjusted in the range of 0.1 to 1.5 Pa · s so as to be suitable for electrospinning.

Furthermore, the chelating agent of the present invention is preferably a polymer organic material so as to control the viscosity and to supply carbon in the baking step. Specifically, the chelating agent may be polyvinylpyrrolidine (PVP), polyvinyl acetate polyvinylacetate, PVAc), polyvinyl alcohol (PVA), polyethylene (PE), polypropylene (PP), poly (3,4-ethylenedioxythiophene) Select one or more.

The solvent is a complex solvent composed of water, alcohol and acid.

The nanofiber precursor is prepared by electrospinning the viscous solution prepared in the first step. (Second step)

Electrospinning is performed using a high voltage electrospinner, SUS as a collector, and a voltage range of 10 to 30 kV and a TCD range of 5 to 20 cm. Since electrospinning is a general technique, detailed description is omitted.

Next, the nanofiber precursor produced in the second step is dried. (Third step)

The third step is a step of removing organic matters and impurities contained in the nanofiber precursor, and is performed by drying in an oven at atmospheric pressure or a vacuum oven at 100 DEG C for at least 8 hours.

Finally, the nanofiber precursor dried in the third step is fired to obtain a composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti, or Zr) Carbon composite nanofiber cathode active material coated with a carbon layer on the surface of the nanofibers of lithium-iron-metal. (Step 4)

The fourth step is a step in which the precursor material substantially forms the LiFe _1-x M _x PO ₄ phase, and simultaneously the carbon of the chelating agent is coated on the surface to form a carbon layer on the surface. For this purpose, a two-stage heat treatment is preferably performed, and a first heat treatment is performed in the range of 450 to 550 ° C and a second heat treatment is performed in the range of 600 to 900 ° C.

The lithium-iron-metal oxide-carbon composite nanofiber cathode active material produced in the above step is in the form of monoaxial or two to three-axis multiaxial nanofiber and has an average diameter of 50 to 800 nm. The thickness of the carbon layer coated on the surface of the nanofiber in the firing step is 50% or less of the diameter of the nanofiber.

In the process of forming LiFe _1-x M _x PO ₄ phase in the firing step, shrinkage may occur and the inside of the fiber shape may be hollow.

The cathode active material of the present invention having such a structure maintains a nanofiber shape having a large surface area even after being subjected to a sintering process and also has a carbon layer coated on the surface thereof to provide a low working voltage and a slow ion conductivity The problem is solved.

In order to investigate the characteristics of the cathode active material prepared according to the present invention, a cathode active material having a composite carbon layer on the surface of nanofibers of LiFePO ₄ , LiFe _0.9 Mn _0.1 PO ₄ and LiFe _0.7 Mn _0.3 PO ₄ was prepared.

First, as raw materials, lithium nitrate (LiNO ₃ ), manganese nitrate tetrahydrate (MnNO ₃ .4H ₂ O), iron nitrate 9 hydrate (FeNO ₃ .9H ₂ O), ammonium phosphate (NH ₄ H ₂ PO ₄ ) , And polyvinylpyrrolidine (PVP) was used as a chelating agent.

A viscous solution having a viscosity of 0.1 to 1.5 Pa · s was prepared by mixing the raw materials and the chelating agent according to the chemical stoichiometry and using a mixed solvent of distilled water, nitric acid and methanol.

In the third step, the spun nanofiber precursor was dried in a vacuum oven at 100 ° C. for about 8 hours or more.

Finally, in the fourth step, the dried nanofiber precursor is subjected to a first heat treatment at 500 ° C. for 10 hours in a gas atmosphere containing one or more gases of nitrogen, argon and oxygen, A cathode active material coated with a carbon layer on the surface was prepared by heat treatment.

FIG. 1 is a result of X-ray diffraction analysis of the cathode active material prepared according to this embodiment. X-ray diffraction analysis was performed using Cu-Ka wavelengths in the range of 2θ = 10 ° to 70 °.

According to FIG. 1, the cathode active material produced according to this embodiment has an orthoromic structure in which the space group belongs to Pnma or Pnmb, and a change in structure due to an increase in the ratio of Mn is not observed And it can be confirmed that they are all the same olivine series composite metal oxides.

From this, it can be confirmed that there is no structural change due to the addition of Mn.

FIGS. 2 to 4 are field scanning electron microscope (FE-SEM) photographs of the cathode active material prepared according to this embodiment. FIG. 2 is a photograph of LiFePO ₄ with X = 0, FIG. 3 is a photograph of LiFe _0.9 Mn _0.1 PO ₄ with X = 0.1, and FIG. 4 is a photograph of LiFe _0.7 Mn _0.3 PO ₄ with X = 0.3.

It can be confirmed that the fiber has a uniform and uniform diameter irrespective of the composition at a low magnification of 5,000 times that of all the compositions.

Also, even at a high magnification of 100,000 times that of all the compositions, no aggregation was observed in the fiber regardless of the composition, and the average diameter was about 190 nm.

FIG. 5 is a result of performing Fourier-transform infrared spectroscopy (FT-IR) on the cathode active material manufactured according to the present embodiment.

The peak at about 1750 cm ^{-1 in the} graph is a peak generated from the double bond of carbon and oxygen due to the induction effect of -PO _4. This indicates that carbon exists on the surface of the nanofiber-type cathode active material prepared according to this embodiment Respectively.

6 to 8 show the results of EDS analysis of the cathode active material prepared according to this embodiment. FIG. 6 shows the analysis results for LiFePO ₄ with X = 0, FIG. 7 shows the results for LiFe _0.9 Mn _0.1 PO ₄ with X = 0.1, and FIG. 8 shows the analysis for LiFe _0.7 Mn _0.3 PO ₄ with X = Results.

As shown in FIG. 6, only a peak of Fe is shown on the right side, but it can be seen that the peak of Mn is increased in FIG. 7 and FIG. From this, it can be confirmed that the compositional ratios of Mn were precisely controlled to 0, 0.1, and 0.3, respectively, in the cathode active material produced according to this example.

While the present invention has been particularly shown and described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Those skilled in the art will understand. Therefore, the scope of protection of the present invention should be construed not only in the specific embodiments but also in the scope of claims, and all technical ideas within the scope of the same shall be construed as being included in the scope of the present invention.

Claims (15)

The raw material and the chelating agent, which are determined by the chemical stoichiometric ratio according to the composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, A first step of preparing a viscous solution by mixing;
A second step of electrospinning the viscous solution to produce a nanofiber precursor;
A third step of drying the nanofiber precursor; And
And a fourth step of baking the dried nanofiber precursor to form a cathode active material in the form of a nanofiber and having a carbon coating on its surface,
Wherein the viscosity of the viscous solution prepared in the first step is in the range of 0.1 to 1.5 Pa · s, and the third step is a mixed solvent in which distilled water, an acid and an alcohol are mixed, Wherein the lithium-iron-metal complex oxide-carbon composite nanofiber cathode active material is carried out for at least 8 hours.
The method according to claim 1,
The chelating agent may be selected from the group consisting of polyvinylpyrrolidine (PVP), polyvinylacetate (PVAc), polyvinyl alcohol (PVA), polyethylene (PE), polypropylene Wherein the cathode active material is at least one selected from the group consisting of poly (3,4-ethylenedioxythiophene) and PEDOT. 4. The method of claim 1, wherein the lithium-iron-metal oxide-carbon composite nanofiber cathode active material is electrospun.
The method according to claim 1,
Characterized in that the raw material is lithium nitrate (LiNO ₃ ), manganese nitrate tetrahydrate (MnNO ₃ .4H ₂ O), iron nitrate tetrahydrate (FeNO ₃ .9H ₂ O), ammonium phosphate (NH ₄ H ₂ PO ₄ ) Wherein the lithium-iron-metal oxide-carbon composite nanofiber cathode active material is prepared by electrospinning.
delete delete The method according to claim 1,
Wherein the electrospinning is carried out in a voltage range of 10 to 30 kV in the second step. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
The method according to claim 1,
Wherein the electrospinning in the second step is carried out in a tip-to-collector distance (TCD) range of 5 to 20 cm. The method of manufacturing a cathode active material for lithium-iron-metal oxide-carbon composite nanofibers using electrospinning .
delete The method according to claim 1,
Wherein the fourth step comprises a first heat treatment in a range of 450 to 550 ° C and a second heat treatment in a range of 600 to 900 ° C. (Method for producing composite nanofiber cathode active material).
The method according to claim 1,
Wherein the fourth step is performed in a gas atmosphere including at least one inert gas and at least one gas in oxygen. The method of manufacturing a lithium-iron-metal oxide-carbon composite nanofiber cathode active material according to claim 1,
A cathode active material produced by one of claims 1 to 3, 6 to 7, and 9 to 10,
A nanofiber having a composition formula of LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti, or Zr); And
And a carbon layer coated on the surface of the nanofibers,
Wherein the carbon of the carbon layer is supplied from a chelating agent. The lithium-iron-metal oxide-carbon composite nanofiber cathode active material according to claim 1,
The method of claim 11,
Wherein the cathode active material has an average diameter of 50 to 800 nm, and the thickness of the carbon layer coated on the surface is 50% or less of the total diameter of the lithium-iron-metal oxide-carbon composite nanofiber cathode active material.
The method of claim 11,
Wherein the cathode active material is one of a monoaxial, a biaxial, and a triaxial type oxide-carbon composite nanofiber cathode active material.
The method of claim 11,
Wherein the cathode active material has a hollow shape in the center of the lithium-iron-metal composite oxide.
A process for producing a lithium ion secondary battery comprising a positive electrode plate, a negative electrode plate and an electrolyte,
Wherein the composition formula is LiFe _1-x M _x PO ₄ (0 <X <1, M = Ni, Mn, Co, V, Cr, Cu, Ti or Zr) on the surface of a nanofiber; And
And applying the positive electrode active material to a current collector to produce a positive electrode plate.

Images