KR102142491B1 - Manufacturing method of anode materials for lithium secondary battery and lithium secondary battery by using the same - Google Patents

Abstract

The present invention relates to a method of manufacturing a negative active material for a lithium secondary battery and a lithium secondary battery using the same, and more particularly, to provide a method of manufacturing a negative active material having high electrical conductivity, rate characteristics, and life characteristics, and a lithium secondary battery using the same. Is to do.
According to various embodiments of the present invention, a metal oxide such as spinel-based manganese-ferrite (MnFe ₂ O ₄ ) may be coated with dopamine to uniformly grow on the surface of a carbon-based material having improved dispersibility and adhesion.
That is, the prepared negative electrode active material can not only obtain high electrical conductivity, but also obtain a high surface area as a one-dimensional coaxial nanocable structure so that charging/discharging reactions can occur at an active site with a lot of lithium during the negative reaction. Therefore, it exhibits a remarkable effect in improving the rate characteristics and life characteristics of the lithium secondary battery.

Description

Manufacturing method of anode materials for lithium secondary battery and lithium secondary battery by using the same}

The present invention relates to a method of manufacturing a negative active material for a lithium secondary battery and a lithium secondary battery using the same, and more particularly, to provide a method of manufacturing a negative active material having high electrical conductivity, rate characteristics, and life characteristics, and a lithium secondary battery using the same. Is to do.

From small electronic devices such as mobile phones and laptops to hybrid electrical vehicles (HEVs) and electric vehicles (EVs), the technology related to energy storage devices as the performance of devices improves. There have been many developments.

In recent years, interest in electrode material technology for realizing high energy density lithium ion batteries has exploded, and in particular, many studies have been conducted for high-capacity anode materials. However, there is still no material that can replace graphite developed by Sony in 1991, and graphite is an obstacle to realizing a high energy density lithium ion battery due to its low theoretical capacity of 372 mAh/g. In addition, alloy-based anode active materials such as Si, Sn, and Sb, which have been discussed as substitutes for graphite, have a high theoretical capacity, but severe volume expansion and non-uniform solid electrolyte interphase (SEI) layer formation during Li ⁺ charge/discharge process. It has been reported that the cycle stability due to is very poor. As another alternative to graphite, transition metal oxides (TMOs) such as NiO, Fe ₂ O ₃ , Fe ₃ O ₄ , TiO ₂ , Co ₃ O ₄ , V ₂ O ₅ etc. are directly alloyed with lithium. Unlike alloy-based anode materials that store lithium through the conversion reaction, a high theoretical capacity (≥500 mAh/g) was exhibited through a conversion reaction that forms a Li ₂ O matrix embedded with a reduced metal phase. Show characteristics. However, this monobranous metal oxide exhibits a volume change during the conversion reaction process, which has a problem in that it leads to low life characteristics and rate characteristics.

Therefore, in recent years, metal ferrites, which are a combination of two metal oxides, have begun to attract attention as anode materials for lithium ion batteries. Such metal ferrite may exhibit improved electrical conductivity through a change in electronic structure as different types of metal elements are introduced, as well as relatively high electrochemical properties and chemical stability compared to the prior art. Nevertheless, metal ferrite exhibits low rate characteristics due to its low electrical conductivity properties, and lithium conduction due to the volume change of particles and aggregation between nanoparticles having high surface energy during charging/discharging due to the conversion reaction. There is a problem that the life characteristics are deteriorated because of the interference.

In order to solve this problem, research has been conducted to improve the rate characteristics by increasing the reaction zone with lithium using a large surface area during charging/discharging and shortening the polaron hopping distance of lithium ions through nanoparticleization of particles. , Fragmentary solutions such as nanoparticles still have a problem in that they do not solve the problem of aggregation between nanoparticles due to high surface energy during the firing process.

In general, in order to prevent the aggregation of nanoparticles, many attempts have been made to coat carbon by surrounding the surface of each nanoparticle from a solution containing sucrose or glucose or a polymer precursor, but unfortunately, the carbon coating method is used in a reducing atmosphere. Due to the additional heat treatment, it is difficult to apply because the crystal structure of the metal oxide is collapsed or impurities such as the metal phase that are additionally reduced are generated. In addition, the coating layer thus formed is mostly made of amorphous carbon, so the electrical conductivity is relatively low.

For example, Kollu et al. synthesized a composite in which metal oxide nanoparticles were dispersed on the rGO surface using ultrasonic and hydrothermal synthesis, which is a lithium ion battery negative electrode material, which is the volume change of metal oxide during the conversion reaction. And showed improved electrical conductivity. Carbon nanotubes, another carbon-based material, have been applied to improve the physical properties of materials in various fields due to their excellent electrical conductivity as a one-dimensional structure. However, there was no specific surfactant added to the complexing process with the carbon-based material having electrical conductivity of the metal oxide nanoparticles, and there was no clear identification of the driving force that causes the nanoparticles to adhere to the surface of the carbon-based material. .

In addition, carbon-based materials such as carbon nanotubes (CNTs) tend to become entangled due to strong inner-tube Van der Waals interactions, and are hydrophobic to aqueous solutions, so they are not well dispersed. Does not. Therefore, it is difficult to form a one-dimensional coaxial nanocable in which metal oxide nanoparticles grow uniformly on the surface of the carbon-based material in the dispersion in which the carbon-based material is dispersed. Conventionally, in many cases, an aqueous nitric acid solution has been used to solve the problem related to the low dispersion degree of carbon nanotubes, which are carbon-based materials. For example, Zhang et al. synthesized a one-dimensional MnO ₂ /CNT nanocomposite in which MnO ₂ nanosheets were uniformly formed on the surface of carbon nanotubes through oxidized carbon nanotubes prepared through nitric acid treatment. However, such acid treatment forms a carboxylic group on the surface, which rather interferes with the adhesion of oxide particles, and creates defects in the carbon structure on the surface of the carbon nanotubes, thereby lowering the physical properties of the carbon nanotubes. In particular, it reduces electrical conductivity and thermal stability.

Therefore, in the present invention, metal oxide nanoparticles produced by a coprecipitation reaction using the hydrophilicity and adhesion of dopamine are uniformly grown on the surface of the dopamine-coated carbon-based material to be a single beaker at low temperature. ) Process to provide a negative active material for lithium secondary batteries.

Nanoscale Res Lett. 2012; 7(1): 33

An object of the present invention is to uniformly grow metal oxide nanoparticles produced by a coprecipitation reaction on the surface of a carbon-based material coated with dopamine by using the hydrophilicity and adhesion properties of dopamine to produce a single beaker at low temperature. ) Process to provide a negative active material for lithium secondary batteries.

Another object of the present invention is to provide a lithium secondary battery including the negative active material.

According to an exemplary aspect of the present invention, a metal oxide core; And a carbon-based material shell coated with dopamine surrounding the core, wherein the method comprises:

(A) preparing a carbon-based material coated with dopamine; And

(B) mixing and reacting the dopamine-coated carbon-based material with a metal oxide precursor; it relates to a method for producing a negative active material for a lithium secondary battery comprising a.

The (A) step includes the steps of (A1) injecting and dispersing a carbon-based material into a dopamine precursor solution; And

(A2) adjusting the pH of the dispersed dispersion to 7.5 to 9.5 and then stirring for 1 to 20 hours to perform oxidation and self-polymerization reaction; it is preferable to include.

In the step (A1), it is preferable to mix the carbon-based material and the dopamine precursor in a weight ratio of 1:0.01 to 0.5.

The step (B) includes (B1) mixing the dopamine-coated carbon-based material and a metal oxide precursor;

(B2) adjusting the pH to 9 to 11 by raising the temperature of the mixed mixture to a temperature of 60 to 80 ℃; And

(B3) reacting the pH-adjusted mixture at a temperature of 80 to 100° C. for 1 to 5 hours; it is preferable to include.

The carbon-based material is preferably any one or more selected from multi-walled carbon nanotubes, carbon nanofibers, graphene, graphene oxide, and reduced graphene oxide.

The metal oxide is MFe ₂ O ₄ ,

The M is preferably any one selected from manganese, cobalt, nickel, zinc, copper, magnesium and calcium.

The nanoparticle size of the metal oxide is 10 to 20 nm in diameter,

It is preferable to include the metal oxide in an amount of 85 to 90 wt% based on the total weight of the negative active material.

The metal oxide precursor is Mn(NO ₃ ) ₂ ·4H ₂ O, Fe(NO ₃ ) ₃ ·9H ₂ O, MnSO ₄ ·H ₂ O, FeSO ₄ ·7H ₂ O, Mn(CH ₃ CO ₂ ) ₂ · 4H ₂ O and Fe (CH ₃ CO ₂ ) ₂ It is preferably any one or more selected from.

The dopamine precursor is preferably Dopamine hydrochloride, Dopamine-1,1,2,2-d4 hydrochloride, or a mixture thereof.

The dopamine is preferably coated on the carbon-based material with a thickness of 1 to 5 nm on the surface of the carbon-based material.

According to another exemplary aspect of the present invention, it relates to a negative active material for a lithium secondary battery manufactured through the above manufacturing method.

According to another exemplary aspect of the present invention, it relates to a lithium secondary battery including a negative active material manufactured through the manufacturing method.

According to various embodiments of the present invention, a metal oxide such as spinel-based manganese-ferrite (MnFe ₂ O ₄ ) may be coated with dopamine to uniformly grow on the surface of a carbon-based material having improved dispersibility and adhesion.

That is, the prepared negative electrode active material can not only obtain high electrical conductivity, but also obtain a high surface area as a one-dimensional coaxial nanocable structure so that charging/discharging reactions can occur at an active site with a lot of lithium during the negative reaction. Therefore, it exhibits a remarkable effect in improving the rate characteristics and life characteristics of the lithium secondary battery.

1 is a diagram showing the synthesis process of the MnFe ₂ O ₄ @PDA-coated MWCNT coaxial nanocable structure of Example 2.
2 is a Fourier transform infrared graph for confirming the presence or absence of a polydopamine (PDA) coating layer coated in Example 1. FIG.
FIG. 3 is a graph showing the results of analysis by thermogravimetric analysis (TGA) for calculating a PDA coating amount and calculating a composite for Examples 1 and 2 and Comparative Example 1. FIG.
4 is a graph showing the results of analysis by X-ray diffraction analysis for comparative analysis of the crystal phase of the final products prepared in Example 2 and Comparative Examples 1 and 2. FIG.
5 is an image showing the results of analysis with a field-emission scanning electron microscopy (FE-SEM) for observing particle shape after synthesis of Example 2 and Comparative Examples 1 and 2, (a) Shows Comparative Example 1, (b) shows Comparative Example 2, and (c) shows Example 2.
6 shows the results of analyzing the microstructure observation for Example 2 and Comparative Example 2, (a), (b) and (c) are transmission electron microscopy (TEM) analysis of Example 2 Image, (d) is a SAED (selected aperture electron diffraction) pattern image of Example 2, (e) is an EDS (Energy Dispersive X-ray Spectroscopy) mapping composition distribution image of Example 2, (f), (g) and (h) shows a high-resolution transmission electron microscope (HR-TEM) analysis image of Comparative Example 2.
7 is a graph showing the results of evaluation by the cyclic voltammogram (CV) for Example 2 and Comparative Examples 1 and 2, (a) is Comparative Example 1, (b) is Comparative Example 2, ( c) shows Example 2.
8 is a graph showing the results of evaluating the initial charge/discharge cycles of Example 2 and Comparative Examples 1 and 2, (a) is Comparative Example 1, (b) is Comparative Example 2, (c) is Example 2 Represents.
9 is a graph showing the results of evaluation of the rate characteristics and long life cycle characteristics of Example 2 and Comparative Examples 1 and 2.

Conventional methods of manufacturing a negative active material for lithium secondary batteries increase the dispersibility of carbon-based materials through nitric acid treatment that can cause defects on the surface of the carbon-based materials, but the present invention uses dopamine to increase the dispersibility of carbon-based materials. It is intended to provide a method of manufacturing a negative active material for a lithium secondary battery capable of improving dispersibility in an aqueous solution and also improving adhesion to metal oxide nanopowder without causing damage to the surface of.

Hereinafter, various aspects and various embodiments of the present invention will be described in more detail.

According to an aspect of the present invention, a metal oxide core; And a carbon-based material shell coated with dopamine surrounding the core, wherein the method comprises:

(A) preparing a carbon-based material coated with dopamine; And

(B) mixing and reacting the dopamine-coated carbon-based material with a metal oxide precursor; it provides a method for producing a negative active material for a lithium secondary battery comprising a.

The step (A) is a step of preparing a dopamine-coated carbon-based material, comprising: (A1) injecting and dispersing the carbon-based material into a dopamine precursor solution; And (A2) adjusting the pH of the dispersed dispersion to 7.5 to 9.5 and then stirring for 1 to 20 hours to perform oxidation and self-polymerization reactions.

More specifically, in the step (A1), it is preferable that the carbon-based material is added to the dopamine precursor solution and dispersed for 1 to 100 minutes using ultrasonic waves or the like. It is preferable to mix the carbon-based material and the dopamine precursor in a weight ratio of 1:0.01 to 0.5, but if it is outside the above range, a large amount of unreacted material may be present, which is not preferable. More preferably, it is a weight ratio of 1:0.1 to 0.3.

The carbon-based material is preferably any one or more selected from multi-walled carbon nanotubes, carbon nanofibers, graphene, graphene oxide, and reduced graphene oxide. More preferably, it is a multi-walled carbon nanotube.

The carbon nanotubes exhibit excellent electrical conductivity as a one-dimensional structure, and are effective in improving the electrochemical properties of a negative electrode material of a lithium secondary battery. Conventionally, the synthesis of a one-dimensional coaxial nanocable in a method of enclosing a central carbon nanotube with nanoparticles using a one-dimensional tubular structure and a high Young's modulus of carbon nanotubes has been reported, but carbon nanotubes There was no special surfactant added for this complexing process of and metal oxide nanoparticles, and there is no clear identification of the driving force that causes the nanoparticles to adhere to the surface of the carbon nanotubes. A technique for coating dopamine was introduced.

The dopamine is a material having a strong surface adhesion ability of mussels, and can be effectively applied to lithium secondary batteries because it can utilize characteristics of environmental stability, biocompatibility, and particularly high dispersibility in water.

The dopamine precursor is preferably Dopamine hydrochloride, Dopamine-1,1,2,2-d4 hydrochloride, or a mixture thereof.

In the step (A2), it is preferable to adjust the pH of the dispersed dispersion to 7.5 to 9.5 using a buffer solution or the like. Then, the mixture is stirred for 1 to 20 hours to prepare a dopamine-coated carbon-based material through oxidation and self polymerization. The reaction product after the reaction is completed may be washed and dried as necessary to obtain a final product.

The step (B) is a step of reacting by mixing a carbon-based material coated with dopamine and a metal oxide precursor through the step (A).

The step (B) includes (B1) mixing the dopamine-coated carbon-based material and a metal oxide precursor; (B2) adjusting the pH by raising the temperature of the mixed mixture to a temperature of 60 to 80 ℃; And (B3) reacting the pH-adjusted mixture at a temperature of 80 to 100° C. for 1 to 5 hours. It is preferable to include.

In more detail, after mixing the dopamine-coated carbon-based material and the metal oxide precursor, it is preferable to adjust the pH of the mixture to 10 to 12. At this time, a precipitate corresponding to the metal hydroxide is formed while the pH is adjusted. After that, the metal oxide is obtained through an oxidation reaction at a temperature of 80 to 100° C. for 1 to 5 hours while stirring is continued. Conventionally, there has been a problem that it takes more than 12 hours of synthesis time and an electric furnace to maintain the high temperature by performing hydrothermal synthesis at a high temperature of 180° C. It is characterized in that the metal oxide is produced through an oxidation reaction. If the reaction temperature is less than or the reaction time is less than 1 hour, a large amount of unreacted product is generated, which is not preferable.

The metal oxide is MFe ₂ O ₄ , and M is preferably any one selected from manganese, cobalt, nickel, zinc, copper, magnesium and calcium.

More preferably, the metal hydroxide precursor is Mn(NO ₃ ) ₃ ·4H ₂ O, Fe(NO ₃ ) ₃ ·9H ₂ O, MnSO ₄ ·H ₂ O, FeSO ₄ ·7H ₂ O, Mn(CH ₃ CO ₂ ) ₂ · 4H ₂ O and Fe(CH ₃ CO ₂ ) _{2 It} is any one or more selected from.

1 is a diagram illustrating a synthesis process according to an embodiment of the method for manufacturing a negative active material for a lithium secondary battery, and will be described in more detail based on FIG. 1. First, step 1 shows the effect of surface treatment with dopamine (PDA) as a dispersant for multi-walled carbon nanotubes (MWCNT). In general, when MWCNT is put in water, it does not disperse well even after ultrasonic dispersion due to the strong inner-tube Van der Waals interaction, and sinks to the bottom of the beaker. On the contrary, the dopamine-coated MWCNT maintains a stable dispersion state in an aqueous solution due to a coating layer formed after 10 hours stirring in a dopamine precursor (Dopamine hydrochloride) solution. During this stirring process, the dopamine monomer is oxidized and self-polymerized at the same time. It is coated on MWCNTs by a strong π-π stacking interaction between the dopamine rings and the π clouds on the outer wall of the MWCNT. As a result, MWCNT coated with dopamine by self-polymerization has high dispersibility in aqueous solution through many hydrophilic functional groups corresponding to -OH and NH ₂ contained in dopamine. Step 2 shows a dopamine-coated MWCNT (PDA-coated MWCNT) uniformly dispersed in an aqueous solution in which Mn ^{2 +} and Fe ^{2 +} ions are dissolved. During continuous stirring at 60 °C, the transition metal ions are attracted by the -OH catechol group contained in the dopamine coating layer of MWCNT. As shown in Step 3, the co-precipitation reaction once at pH 11 When this starts, MnFe ₂ (OH) ₆ precipitates are preferentially formed on the MWCNT surface by transition metal ions concentrated on the MWCNT surface. As a strong binder, dopamine maintains the adhesion of the precipitated hydroxide and MWCNT well even during continuous stirring. The additional reaction at 98° C. converts the transition metal hydroxide into an oxide crystal phase in an air atmosphere. In other words, MnFe ₂ (OH) ₆ coprecipitated on PDA-coated MWCNT is completely crystallized to become MnFe ₂ O ₄ nanoparticles, and finally, the nanocable nanostructure assembled in the form of the MWCNT central axis and the MnFe ₂ O ₄ shell. It becomes MnFe ₂ O ₄ @PDA-coated MWCNT.

In the above manufacturing method, (i) (A1) a carbon-based material and a dopamine precursor are mixed in a weight ratio of 1: 0.1 to 0.3, and (ii) (B2) the mixture is heated to a temperature of 70 to 80° C. The pH is adjusted to 10 to 12, and (iii) (B3) the pH-adjusted mixture is reacted at a temperature of 90 to 100° C. for 1 to 3 hours, and (iv) the metal oxide precursor is Mn(NO ₃ ) ₃ ·4H ₂ O and Fe(NO ₃ ) ₃ ·9H ₂ O, and the carbon-based material is most preferably a multi-walled carbon nanotube. If all of the above conditions (i) to (iv) are satisfied, lithium In terms of the reversible capacity of the secondary battery, it is possible to secure more than 50% of the initial capacity after 350 cycles, as well as a uniform coaxial nanocable because the aggregation of metal oxide nanoparticles on the surface of carbon-based materials does not occur at all. Can be formed. However, if any of the above conditions (i) to (iv) were not satisfied, it was confirmed that the reversible capacity was rapidly reduced, the durability was deteriorated, and the aggregation of the metal oxide nanoparticles occurred.

In addition, according to another aspect of the present invention, there is provided a negative active material for a lithium secondary battery manufactured through the method for preparing a negative active material for a lithium secondary battery described above.

The negative active material may include a metal oxide core; And a carbon-based material shell coated with dopamine surrounding the core.

The nanoparticle size of the metal oxide is preferably 10 to 20 nm in diameter. If it is outside the above range, the performance of the lithium secondary battery may be degraded, which is not preferable.

In addition, it was confirmed that the metal oxide represented 85 to 90 wt% based on the total weight of the negative electrode active material, so that the metal oxide nanoparticles covered the surface of the carbon-based material up to 90 to 100% and significantly improved the rate characteristics of the lithium secondary battery. .

The dopamine is coated on a carbon-based material with a thickness of 1 to 5 nm, and is preferably coated with 1 to 2 wt% based on the total weight of the negative active material. Even if a small amount of dopamine is coated, an initial charge of 700 mAh/g or more It was confirmed that the discharge capacity was shown.

Hereinafter, the present invention will be described in more detail through examples and the like, but the scope and content of the present invention may be reduced or limited by the following examples. In addition, if it is based on the disclosure of the present invention including the following examples, it is obvious that a person skilled in the art can easily implement the present invention, in which experimental results are not specifically presented, and patents to which such modifications and corrections are attached Naturally, it is within the scope of the claims.

In addition, the experimental results presented below are only representative of the experimental results of the above Examples and Comparative Examples, and each effect according to various embodiments of the present invention not explicitly presented below will be specifically described in the corresponding section. .

Example One: MWCNT for Polydopamine (PDA) coating method

0.2 g of Multi-walled carbon nanotuve (MWCNT) was dispersed in 40 mL of distilled water with 0.04 g of Dopamine hydrochloride through ultrasonication for about 30 minutes. Then, while continuously stirring, add 10 mL of 10 mM Tris-buffer solution to adjust the pH to 8.5. Finally, the mixture is recovered while stirring for about 10 hours, washed with a centrifuge using distilled water and ethyl alcohol, dried in an oven at 80° C. for 12 hours, and recovered.

Example 2: MnFe ₂ O ₄ @PDA-coated MWCNT Coaxial Synthesis of nanocable structures

Mn(NO ₃ ) ₂ ·4H ₂ O and Fe(NO ₃ ) ₃ ·9H ₂ O corresponding to the stoichiometric amount are dissolved in 200 mL of distilled water, and then in the process of stirring, it corresponds to 10 wt% of the total weight of the complex. After adding the PDA-coated MWCNT, it is dispersed for 10 minutes using ultrasonic waves. After raising the temperature to 70° C., about 50 mL of NaOH aqueous solution corresponding to 2 M is added until the pH becomes 11. At this time, a precipitate corresponding to MnFe ₂ (OH) ₆ is formed and the temperature is raised to 98° C. while stirring is continued. After holding at 98° C. for 2 hours, 20 mL of NaOH aqueous solution was added to complete the reaction. The obtained MnFe ₂ O ₄ @PDA-coated MWCNT was washed in a centrifuge with distilled water and ethyl alcohol, dried in an oven at 80° C. for 12 hours, and recovered to obtain PDA-coated MnFe ₂ O ₄ .

Comparative example One : Spinel Forming a single crystal structure MnFe ₂ O ₄ Cathode oxide synthesis

After dissolving Mn(NO ₃ ) ₂ ·4H ₂ O and Fe(NO ₃ ) ₃ ·9H ₂ O corresponding to stoichiometric amounts in 200 mL of distilled water, the temperature was raised to 70 °C in the process of stirring and then added to 2 M. About 50 mL of the corresponding aqueous NaOH solution is added until the pH reaches 11. At this time, a precipitate corresponding to MnFe ₂ (OH) ₆ is formed and the temperature is raised to 98° C. while stirring is continued. After holding at 98° C. for 2 hours, 20 mL of NaOH aqueous solution was added to complete the reaction. The obtained MnFe ₂ O ₄ nanopowder was washed with distilled water and ethyl alcohol in a centrifuge, dried in an oven at 80° C. for 12 hours, and recovered to prepare a negative active material.

Comparative example 2: MnFe ₂ O ₄ Synthesis of @MWCNT complex

Mn(NO ₃ ) ₂ ·4H ₂ O and Fe(NO ₃ ) ₃ ·9H ₂ O corresponding to the stoichiometric amount are dissolved in 200 mL of distilled water, and in the process of stirring, to 10 wt.% of the total weight of the complex. After the corresponding MWCNT is added, it is dispersed for 10 minutes using ultrasonic waves. After raising the temperature to 70° C., about 50 mL of NaOH aqueous solution corresponding to 2 M is added until the pH becomes 11. At this time, a precipitate corresponding to MnFe ₂ (OH) ₆ is formed and the temperature is raised to 98° C. while stirring is continued. After holding at 98° C. for 2 hours, 20 mL of NaOH aqueous solution was added to complete the reaction. The obtained MnFe ₂ O ₄ @MWCNT was washed with distilled water and ethyl alcohol in a centrifuge, dried in an oven at 80° C. for 12 hours, and recovered.

Experimental Example 1: Fourier transform infrared spectra (FT-IR) analysis

2 shows the FT-IR spectra of Pristine MWCNT and PDA-coated MWCNT. The spectrum of pristine MWCNTs does not observe any apparent absorption peaks in all wavelength ranges. Meanwhile, the spectrum of PDA-coated MWCNT has a strong absorption peak found at 3370 cm ^-1 , which corresponds to the stretching vibration of the -OH catechol group in the aromatic ring of PDA,- The stretching vibration of the NH amine group is also superimposed on this broad peak. Furthermore, the peak at 1600 cm ^-1 corresponds to the C=C resonance vibration of the aromatic ring. Additionally, the peak at 1511 cm ^-1 is the -NH shearing vibration of the amine group of PDA. In general, the -OH catechol group included in PDA plays a decisive role in enhancing the adhesion to the inorganic surface, which in turn played an important role in increasing the adhesion ability between MWCNTs and metal oxide nanoparticles.

Experimental Example 2: Thermogravimetric analysis (TGA)

In FIG. 3, TGA analysis was performed on pristine MWCNT, PDA-coated MWCNT, and MnFe ₂ O ₄ @PDA-coated MWCNT to determine the content of PDA coated on MWCNT. As shown in the TGA curve, evaporation of water molecules occurs below about 200 °C in all samples (~3.99%), and pristine MWCNTs show a constant weight up to around 520 °C without significant decrease. On the contrary, PDA-coated MWCNT and MnFe ₂ O ₄ @PDA-coated MWCNT decrease in weight from about 330 ℃, and PDA-coated MWCNT gradually decreases in weight as shown in the inserted figure, and about 520~ just before MWCNT is decomposed. It can be seen that 88.2% of the total weight remains around 550 ℃. For this reason, we can see that PDA is about 7.79% coated on MWCNT. In the case of MnFe ₂ O ₄ @PDA-coated MWCNT, the TGA curve also decreased to 88.3% of the total weight after 550°C. Therefore, it can be seen that the weight ratio of the oxide of MnFe ₂ O ₄ @PDA-coated MWCNT and MWCNT is about 9:1. Finally, since we applied PDA only as an adhesive and dispersant for MnFe ₂ O ₄ nanoparticles, it is important to precisely control this so that the amount of PDA coated on MWCNT is not excessive enough to affect the electrochemical performance. When the amount of PDA coated in the synthesis process of the present invention is calculated again based on the above TGA result, it becomes 0.78 wt% of the total nanocable structure. Therefore, it can be seen that a very small amount of PDA, which is less than 1 wt%, was coated on MWCNT to help the synthesis.

Experimental Example 3: X-ray diffraction analysis

In FIG. 3, X-ray diffraction patterns were obtained for each of the samples prepared in advance to check the degree of crystallization of the synthesized samples, and for MnFe ₂ O ₄ @PDA-coated MWCNT, 2θ = 20~30˚ low angle. In the region, a broad peak corresponding to MWCNT appears first, and as in pristine MnFe ₂ O ₄ , 2θ = 29.8˚, 35.2˚, 36.8˚, 42.7˚, 53.0˚, 56.6˚, and 61.9˚ clearly without impurity peaks (200 ), (311), (222), (400), (422), (511), (440) peaks appear. All peaks can be seen that the space group Fd3m consistent with the JCPDS card # 73-1964 _{and, MnFe 2 O 4 @ PDA-} coated MWCNT of MnFe ₂ O ₄ spinel phase hayeoteum prove at a low temperature of 98 ℃ growth completely in a short time do.

Experimental Example 4: Field-emission scanning electron microscopy (FE-SEM) analysis

In Figure 5, the shape of each sample prepared with FE-SEM was observed. The FE-SEM image in (a) shows that pristine MnFe ₂ O ₄ nanoparticles synthesized by co-precipitation at 98 °C were synthesized with a size of ≤20 nm. However, such small-sized nanoparticles have a high probability of interparticle aggregation triggered by a driving force of high surface energy. For this reason, as in (b), when the precipitation reaction is performed in a nitrate solution containing MWCNT without PDA coating, the resulting complex shows a large non-uniformity, and the lumped MWCNT and MnFe ₂ O ₄ nanoparticles are aggregated. Each is observed. As the solution is continuously stirred during the synthesis process, the Van der Waals interaction of pristine MWCNT and the high surface energy of MnFe ₂ O ₄ nanoparticles adversely affect the formation of nanostructures having a uniform coaxial nanocable shape. It can be seen as crazy. In contrast, when the precipitation reaction of MnFe ₂ O ₄ is carried out in a uniform PDA-coated MWCNT dispersion as shown in (c), the nanoparticles are uniformly arranged well along the MWCNT, and large aggregation can be seen to disappear. As a result, the coaxial nanocable structure of MnFe ₂ O ₄ @PDA-coated MWCNT is excellent. This well demonstrates the role of MWCNT as a dispersant for high dispersibility.

Experimental Example 5: Transmission electron microscopy (TEM)

In FIG. 6, TEM analysis was performed as in (a)-(c) to confirm the microstructure of the MnFe ₂ O ₄ @PDA-coated MWCNT coaxial nanocable. In (a), it can be seen that the shape of the erected one-dimensional coaxial nanocable nanostructure without aggregation or lumped MWCNT between large MnFe ₂ O ₄ nanoparticles is well distinguished. In (b), it can be clearly observed that the nanoparticles are uniformly attached to the center of the MWCNT through a high-resolution-TEM image. (c) shows a more magnified high-resolution-TEM image, showing that with the help of PDA, MnFe ₂ O ₄ nanoparticles are densely surrounding the multiple walls of MWCNT. In addition, the SAED (selected aperture electron diffraction) pattern of MnFe ₂ O ₄ @PDA-coated MWCNT in (d) appears as a cyclic pattern from various crystal directions of MnFe ₂ O ₄ on the nanocrystalline phase, respectively (400) and (440). , Clearly observed with the (444) plane. This proves that the small nanoparticles on the surface of MWCNT were completely crystallized in a low temperature process. (e) shows the composition distribution through EDS mapping of MnFe ₂ O ₄ @PDA-coated MWCNT. Mn, Fe, O element signals corresponding to MnFe ₂ O ₄ around C signals corresponding to the MWCNT center It can be seen that they uniformly surround the PDA-coated MWCNT surface. On the other hand, in the case of MnFe ₂ O ₄ @MWCNT of MWCNT without PDA surface treatment of (f)-(h), we showed a large aggregate of MnFe ₂ O ₄ and MWCNT as shown in the FE-SEM picture of FIG. The surface was found, and the HR-TEM image of (f)-(g) also proves the inhomogeneity of MnFe ₂ O ₄ @MWCNT. In addition, (h) shows the EDS mapping of MnFe ₂ O ₄ @MWCNT in the area indicated by the red circle of the dotted line in (f), showing the MWCNT surface without surrounding MnFe ₂ O ₄ nanoparticles, which is the center of the MWCNT The weak adhesion between the and metal oxide nanoparticles is demonstrated. For this reason, without a PDA coating layer, MWCNTs create a non-uniform dispersion due to Van der Waals interaction and hydrophobic surface, and lack of driving force for this in relation to the adhesion between MnFe ₂ O ₄ and MWCNT. As a result, the MnFe ₂ O ₄ nanoparticles are not attracted to the surface of the MWCNT, but aggregated to each other, making it difficult to form a uniform coaxial nanocable.

Experimental Example 6: Cyclic voltammogram (CV) evaluation

In FIG. 7, in order to evaluate the electrochemical performance according to the shape of each of the synthesized samples, a half-cell was constructed using lithium foil as a counter electrode together with a working electrode prepared on a copper electrode plate. And, as shown in (a)-(c), CV measurements were performed for each sample in a voltage range of 0 to 3.0 V at a scanning speed of 0.2 mVs ^-1 . The CV curves of all samples show a pair of oxidation peaks/reduction peaks during lithium charging/discharging of MnFe ₂ O ₄ in a similar voltage range. (a) the pristine MnFe Reduction 0.55 V cathode peak (cathodic peak) of a related (reduction reaction), the first discharge process ₂ O ₄ is a conversion of MnFe ₂ O ₄ (conversion reaction) MnFe ₂ O ₄ + 8Li ^{+ +} 8e ^- → reduction reaction (reduction reaction) of the Fe ³⁺ and Mn ²⁺ according to the Mn + 2Fe + 4Li ₂ O this takes place. According to the above reaction equation, during discharge, Li ions are stored in the form of Li ₂ O, and a metallic phase of Fe and Mn appears. The reason why this peak appears relatively large is because it is an overlapping peak for the two discharge reactions of MnFe ₂ O ₄ , where Fe ₂ O ₃ and MnO are reduced to Fe and Mn at 0.7 V and 0.5 V, respectively. The two reactions come together. Subsequently, in the reversible discharge process, a corresponding peak appears at 0.75 V, and a reversible discharge reaction between phases formed after a large irreversible reaction occurs. Of 1.70 V on a lithium charging process, the anode peak (anodic peak) is _{Mn + Li 2 O → MnO +} 2Li + + 2e - and the reaction ^of-the _{3Fe + 4Li 2 O → Fe 3} O 4 + 8Li + + 8e , A peak shift occurs to 1.80 V in the subsequent cycle due to the polarization effect, and a peak for the reversible charging process of Mn + 3Fe + 5Li ₂ O → MnO + Fe ₃ O ₄ + 10Li appears. In the case of pristine MnFe ₂ O ₄ in (a), an electrochemical reaction occurs well at each voltage in the initial cycle, but in subsequent cycles, both the anode and cathode peak currents (anodic, cathodic peak current) continuously decrease, and the integral of the peak as a whole. As the area is greatly reduced, the capacity tends to decrease significantly. This decrease in peak intensity appears to be due to the inherently low electrical conductivity of MnFe ₂ O ₄ . In contrast, the MnFe ₂ O ₄ @MWCNT in (b) shows a relatively more stable CV curve for 10 cycles, which is because the electrical conductivity of MnFe ₂ O ₄ provides an electron conduction path to the cathode, thereby improving the electrical conductivity. Tells you that you can. Interestingly, the reversibility of the electrochemical reaction in (c) MnFe ₂ O ₄ @PDA-coated MWCNT is similar to that of MnFe ₂ O ₄ @MWCNT and stable CV curves, but in all charging/discharging processes, MnFe ₂ O ₄ @MWCNT showed a much higher peak current continuously, and showed a higher reversible capacity when comparing the peak integral area. In general, in CV measurements, this larger peak area is related to a higher response size, and MnFe ₂ O ₄ @PDA-coated MWCNTs have a large peak area for 10 cycles, especially a large peak shift or peak due to polarization. It shows a stable cycle with no decrease in strength.

Experimental Example 7: Initial charge/discharge cycle evaluation

In FIG. 8, initial charge/discharge cycling evaluation was performed for the reversible capacity at each voltage. (a)-(c) show the initial discharge curve of each sample, the formation cycle of 0.1 C rate up to the first 3 cycles, and the 4th cycle shows the cycle result at 1 C rate. In the case of pristine MnFe ₂ O ₄ in the 1st cycle of (a), the charge/discharge capacity of 984/1405 mAhg ^-1 is shown. It shows a relatively low Coulomb efficiency in the first cycle, which is a phenomenon commonly seen in cathode materials driven by a conversion reaction such as MnFe ₂ O ₄ as confirmed in the CV reformation of the first cycle in FIG. 7 and the SEI layer is formed. This is because a process is involved, and after the metal phase is made, they are not completely oxidized during the charging process, and some metal phases remain irreversible. In the 4th cycle at 1 C, MnFe ₂ O ₄ essentially shows 222/235 mAhg ^-1 due to its low electrical conductivity, which shows a significantly lower capacity than the 1005/1049 mAhg ^-1 shown in the third cycle. The MnFe ₂ O ₄ @MWCNT of (b) shows a lower initial capacity than the pristine MnFe ₂ O ₄ of 778/1356 mAhg ^-1 , but the reversible capacity of 624/639 mAhg ^-1 is much higher in the subsequent cycle at 1C. . Above all, MnFe ₂ O ₄ @PDA-coated MWCNT shows significantly higher capacity values of 989/1479, 968/1050 and 769/774 mAhg ^-1 in all cycles than MnFe ₂ O ₄ @MWCNT. As discussed above, PDA-coated MWCNTs not only provide electron conduction paths to MnFe ₂ O ₄ nanoparticles, but also effectively create a one-dimensional coaxial nanocable structure of MnFe ₂ O ₄ @PDA-coated MWCNTs having a high surface area. The obtained high surface area increases the active site, and increases the amount of Li that can be reversibly accommodated in the same amount of active material, thereby maximizing the theoretical capacity of the MnFe ₂ O ₄ negative electrode.

Experimental Example 8: Rate characteristic evaluation and long life cycle evaluation FIG. 9

9 shows the rate characteristics evaluation results for each sample driven by 10 cycles each at 0.1 ~ 10 C in (a), the charge/discharge capacity at 0.1 C is the lowest charge/discharge in case of pristine MnFe ₂ O ₄ At a speed of 0.1 C, a significant decrease in capacity occurs after a few cycles, and shows ~100 mAhg ^-1 which hardly contributes to Li storage in battery operation after ¹ C. In the case of MnFe ₂ O ₄ @MWCNT, it shows better rate characteristics and improved cycling performance at all C-rates, which is derived from the increase in electrical conductivity by MWCNT. This is the case in the 0.1 C cycle shown in the 125/129 229/232 mAhg ^-1 mAhg ^-1 and at 10 C 5 C, Finally, the recovery capacity of the pristine MnFe ₂ O ₄ 282/295 by severe electrode degradation mAhg ^It can be seen that only ^-1 is recovered, but MnFe ₂ O ₄ @MWCNT recovers back to 1008/1052 mAhg ^-1 . In particular, MnFe ₂ O ₄ @PDA-coated MWCNT shows the highest capacity at all C-rates, and the highest improvement of 779/787, 628/631, 367/371 mA h g-1 at 1C, 2C, and 5C. It represents the rate characteristic, even in the 10 C shows a 145.2 / 149.9 mAhg ^-1, is recovered by 1208/1237 mAhg ^-1 at 0.1 C cycle. Therefore, MnFe ₂ O ₄ @PDA-coated MWCNT shows superior high-speed charge/discharge capability and effectively reduces electrode deterioration in repeated conversion reactions as a one-dimensional coaxial nanostructure. MnFe ₂ O ₄ @PDA-coated MWCNT also shows excellent long life performance as shown in (b), pristine MnFe ₂ O ₄ shows severe capacity fading at the 100th cycle, and MnFe ₂ O ₄ @ for MWCNT may notice that, while showing a reversible capacity of more than 350 after the second cycle time ^{800 mAhg -1, 1C (= 917} mAg -1) maintained well over 1000 mAhg ^-1 reversible capacity during the 350 cycle at. Interestingly, the reversible capacity of MnFe ₂ O ₄ @PDA-coated MWCNT gradually increased to 1185/1192 mAhg ^-1 by the 350th cycle (53% compared to the initial capacity). In addition, this phenomenon is also found in the recovery cycling at the last 0.1 C when evaluating the rate characteristics in (a). This phenomenon has already been confirmed through previous reports on oxide-based anode materials, and the causes are as follows. 1) Very small MnFe ₂ O ₄ nanoparticles of less than 20 nm are rearranged during the activation process corresponding to the initial conversion reaction, thereby continuously increasing the active site of Li. 2) During the conversion reaction, a polymer/gel-like layer is easily formed on the surface of MnFe ₂ O _{4 due to} the electrolytic decomposition reaction, which rather increases the mechanical adhesion between active materials and MWCNT without affecting ion conduction. Finally, 3) an additional lithium storage site is generated due to a "pseudo-capacitance" phenomenon resulting from the structure of the generated SEI layer.

Claims (12)

Poly-dopamine-coated multi-walled carbon nanotube central axis; In the method for producing a negative electrode active material for a lithium secondary battery having a coaxial nanocable structure comprising; and a metal oxide assembled in the form of an outer shell of the central axis,
(A) preparing a multi-walled carbon nanotube coated with polydopamine; And
(B) mixing and reacting the multi-walled carbon nanotubes coated with the polydopamine and a metal oxide precursor, thereby assembling a metal oxide on the shell of the multi-walled carbon nanotubes; including,
Step (A)
(A1) adding and dispersing multi-walled carbon nanotubes into a dopamine precursor solution to prepare a dispersion; And
(A2) adjusting the pH of the dispersion to 7.5 to 9.5 and then stirring for 1 to 20 hours to coat polydopamine on the multi-walled carbon nanotubes according to oxidation and self-polymerization reactions; characterized in that it comprises: Method for producing a negative active material for a lithium secondary battery.
delete The method of claim 1,
The step (A1) is a method for producing a negative active material for a lithium secondary battery, characterized in that mixing the multi-walled carbon nanotubes and the dopamine precursor in a weight ratio of 1: 0.01 to 0.5.
The method of claim 1,
Step (B)
(B1) mixing the multi-walled carbon nanotubes coated with the polydopamine and a metal oxide precursor;
(B2) adjusting the pH to 9 to 11 by raising the temperature of the mixed mixture to a temperature of 60 to 80 ℃; And
(B3) reacting the pH-adjusted mixture at a temperature of 80 to 100° C. for 1 to 5 hours; a method for producing a negative active material for a lithium secondary battery comprising:
delete The method of claim 1,
The metal oxide is MFe ₂ O ₄ ,
The M is manganese, cobalt, nickel, zinc, copper, magnesium and calcium, characterized in that any one selected from the method for producing a negative active material for a lithium secondary battery.
The method of claim 1,
The nanoparticle size of the metal oxide is 10 to 20 nm in diameter,
The method of manufacturing a negative active material for a lithium secondary battery, wherein the metal oxide is included in an amount of 85 to 90 wt% based on the total weight of the negative active material.
The method of claim 1,
The metal oxide precursor is Mn(NO ₃ ) ₂ ·4H ₂ O, Fe(NO ₃ ) ₃ ·9H ₂ O, MnSO ₄ ·H ₂ O, FeSO ₄ ·7H ₂ O, Mn(CH ₃ CO ₂ ) ₂ · 4H ₂ O and Fe (CH ₃ CO ₂ ) ₂ Method for producing a negative active material for a lithium secondary battery, characterized in that at least one selected from.
The method of claim 1,
The dopamine precursor is a method for producing a negative active material for a lithium secondary battery, characterized in that Dopamine hydrochloride, Dopamine-1,1,2,2-d4 hydrochloride, or a mixture thereof.
The method of claim 1,
The polydopamine is a method for producing a negative active material for a lithium secondary battery, characterized in that the coating to a thickness of 1 to 5 nm on the surface of the multi-walled carbon nanotube.
A negative active material for a lithium secondary battery prepared through the manufacturing method of any one of claims 1, 3, 4, and 6 to 10.
A lithium secondary battery comprising a negative active material prepared through the manufacturing method of any one of claims 1, 3, 4, and 6 to 10.

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