KR20200123676A - An anode active material for lithium secondary battery and a method for manufacturing the same - Google Patents

Abstract

An embodiment of the present invention provides: a negative electrode active material for a lithium secondary battery comprising carbon fibers, and metal oxide particles coated on at least a part of the surface of the fibers; a manufacturing method thereof; and a negative electrode comprising the same. According to an aspect of the present invention, it is possible to provide the negative electrode active material for the lithium secondary battery having a large capacity, and having a high capacity even at a high current density.

Description

Anode active material for lithium secondary battery and its manufacturing method {AN ANODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY AND A METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a negative electrode active material for a lithium secondary battery and a method for manufacturing the same, and more particularly, a negative electrode for a lithium secondary battery that can significantly improve the electrochemical properties of a lithium secondary battery by coating metal oxide particles on at least a portion of the carbon fiber surface. It relates to an active material and a method of manufacturing the same.

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

The conventional graphite material used as an anode active material for a lithium secondary battery has a theoretical capacity of only 372 mAh/g, and shows a capacity utilization rate close to 95% of the current theoretical capacity, so a new anode active material is required for additional capacity increase. Metal oxide, which is one of the candidate negative active materials for next-generation lithium secondary batteries, has a larger theoretical capacity than graphite materials and is more environmentally friendly, but there is a problem in that the capacity rapidly decreases as charging and discharging are repeated.

Among transition metal oxides, NiFe ₂ O ₄ (NFO), which has a spinel structure, is one of the major candidate materials capable of realizing inexpensive, high-performance capacitance. Ni-Fe bimetal oxide has an advantage of having superior specific capacity compared to other metal oxides, low toxicity, abundantly present in nature, and relatively low delithiation potential. However, NFO nanoparticles have low electron and ionic conductivity, and are difficult to apply to lithium secondary batteries due to volume expansion during charging and discharging.

In order to solve this problem, a conductive network is prepared by doping or coating a conductive material such as a carbon matrix such as carbon nanotubes, graphene, and carbon fibers on the NFO nanoparticles by various methods such as sol-gel, hydrothermal or coprecipitation. Although the method has been tried, since the nanoparticles easily aggregate due to the high surface energy, practical production and application are difficult.

Accordingly, there is a need for a novel anode active material that is easy to manufacture and has excellent electrochemical properties.

The present invention is to solve the problems of the prior art described above, and an object of the present invention is to provide a negative active material for a lithium secondary battery and a method of manufacturing the same, which has superior capacity characteristics, rate characteristics, and circulation characteristics compared to the conventional negative electrode active material.

One aspect of the present invention, carbon fiber; And metal oxide particles coated on at least a portion of the surface of the fiber. It provides a negative active material for a lithium secondary battery.

In one embodiment, the carbon fiber may be a vapor-grown carbon fiber.

In one embodiment, the metal may be a transition metal.

In one embodiment, the metal may be a mixture of nickel and iron.

In one embodiment, the carbon fiber may have an average diameter of 50 to 150 nm and an average length of 10 to 1,000 μm.

In one embodiment, the particles may have an average particle diameter of 50 to 2,000 nm.

In one embodiment, the negative active material may have a cluster structure.

In one embodiment, the negative electrode active material, the XRD 2θ value using Cu-Kα radiation, a peak of 26 ± 1 °; And at least one peak selected from the group consisting of 30±1°, 35±1°, 37±1°, 43±1°, 53±1°, 57±1°, and 62±1°.

In one embodiment, the negative active material may have an atomic concentration ratio of oxygen, carbon, and metal in a range of 1: 0.1 to 10: 0.1 to 5, respectively.

In one embodiment, the negative active material may have a maximum discharge capacity of 500 mAh/g or more.

In one embodiment, the negative active material may have a coulomb efficiency of 60.0 to 99.9%.

In one embodiment, the negative electrode active material may have a discharge capacity of at least 60% based on the second discharge capacity after 800 charging and discharging times having a current density of 3,000 mA/g.

In one embodiment, the negative electrode active material may have a discharge capacity of 500 mAgh/g or more under conditions of a current density of 100 to 3,000 mA/g.

Another aspect of the present invention, (a) preparing a solution containing a metal precursor, urea and a solvent; (b) adding carbon fiber to the solution and reacting at 100 to 300° C. for 1 to 12 hours; And (c) calcining the product of step (b) at 400 to 600° C. for 1 to 12 hours; containing, a method for producing a negative active material for a lithium secondary battery.

In one embodiment, the molar ratio of the metal precursor and the urea may be 1:1 to 5.

In one embodiment, the molar ratio of the metal precursor and the solvent may be 1: 0.5 to 1.5.

In one embodiment, the reaction of step (b) may be a one-stage hydrothermal synthesis reaction.

In one embodiment, step (c) may be performed in an Ar atmosphere.

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

In one embodiment, the weight ratio of the negative active material, the binder, and the conductive material may be 40 to 100: 1 to 20: 1 to 20, respectively.

According to an aspect of the present invention, it is possible to provide a negative active material for a lithium secondary battery having a large capacity, excellent circulation stability and rate characteristics, and having a high capacity even at a high current density.

The effects of the present invention are not limited to the above effects, and should be understood to include all effects that can be deduced from the configuration of the invention described in the detailed description or claims of the present invention.

1 is a schematic view showing a method of manufacturing an active material according to an embodiment of the present invention.
2 is a diagram illustrating an X-ray diffraction test and an X-ray photoelectron spectroscopy analysis result for comparative analysis of the structure and phase purity of an active material prepared according to an embodiment of the present invention.
3 shows a scanning electron microscope photograph for confirming the shape of a material used in an embodiment of the present invention.
4 shows a transmission electron microscope photograph for confirming the structure and properties of an active material prepared according to an embodiment of the present invention.
5 shows an analysis result of a non-charge/discharge test under a constant current-constant voltage condition to confirm the electrochemical performance of an electrode manufactured according to an embodiment of the present invention.
6 shows an analysis result of the rate characteristic of an electrode manufactured according to an embodiment of the present invention.
7 shows the results of an electrochemical impedance spectroscopy test before and after 100 cycle tests of an electrode manufactured according to an embodiment of the present invention.

Hereinafter, the present invention will be described with reference to the accompanying drawings. However, the present invention may be implemented in various different forms, and therefore is not limited to the embodiments described herein. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are assigned to similar parts throughout the specification.

Throughout the specification, when a part is said to be "connected" to another part, this includes not only "directly connected" but also "indirectly connected" with another member interposed therebetween. . In addition, when a part "includes" a certain component, it means that other components may be further provided, rather than excluding other components unless specifically stated to the contrary.

When a range of numerical values is described herein, the value has the precision of significant figures provided according to the standard rules in chemistry for significant figures, unless a specific range thereof is stated otherwise. For example, 10 includes a range of 5.0 to 14.9, and the number 10.0 includes a range of 9.50 to 10.49.

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

Referring to Figure 1, the negative electrode active material for a lithium secondary battery according to an aspect of the present invention, carbon fiber; And metal oxide particles coated on at least a portion of the surface of the fiber.

Carbon fiber is a fibrous carbon material containing 90% by weight or more of a carbon element, and a fibrous organic precursor material manufactured from polyacrylonitrile, pitch, or rayon is pyrolyzed in an inert atmosphere, or a raw material gas such as hydrocarbon is used as a metal catalyst. It can be prepared by various methods such as pyrolysis in the presence of vapor and growing in a gas phase in the form of a fiber, or by electric discharge of a carbon material.

The carbon fiber may serve as a support for the metal oxide particles and may also serve as a conductive matrix. The carbon fiber may be involved in electron and ion transport to improve rate characteristics of the negative electrode active material.

The carbon fiber may be a vapor-grown carbon fiber. Carbon fibers manufactured by the vapor phase growth method have excellent mechanical strength and electrical conductivity, and can have superior efficiency when applied to electrodes for lithium secondary batteries due to a unique network-type morphology.

The metal may be a transition metal, a mixture of transition metals, or a mixture of nickel and iron, but is not limited thereto.

The negative electrode active material according to an embodiment of the present invention is a hybrid nanocomposite in which metal oxide particles are coated on at least a part of a carbon fiber surface, and the advantages of carbon fiber are excellent stability, conductivity, ionic conductivity and metal oxide. The advantage of high capacity can be implemented at the same time.

Graphite material, which is a generally used negative electrode active material, has a theoretical capacity of only 372 mAh/g, and carbon fiber also has a similar level of capacitance. Although the metal oxide negative active material has a large capacity and is environmentally friendly, there is a problem in that the capacity decreases rapidly during charging and discharging, and the volume expands excessively when ions are captured, thereby deteriorating the stability of the secondary battery. On the other hand, the negative electrode active material according to an embodiment of the present invention can realize a significantly superior capacitance compared to a conventional graphite material or carbon fiber having a capacitance of only about 350 to 360 mAh/g.

The carbon fiber may have an average diameter of 50 nm or more, 60 nm or more, 70 nm or more, 80 nm or more, or 90 nm or more, and an average diameter of 150 nm or less, 140 nm or less, 130 nm or less, 120 nm or less, or 110 nm or less However, it is not limited thereto.

The carbon fibers may have an average length of 10 µm or more, 20 µm or more, 30 µm or more, 40 µm or more, or 50 µm or more, and an average length of 1,000 µm or less, 800 µm or less, 600 µm or less, 400 µm or less, or 200 µm It may be the following, but is not limited thereto.

The metal oxide particles may have an average particle diameter of 50 nm or more, 60 nm or more, 70 nm or more, 80 nm or more, 90 nm or more, or 100 nm or more, and the particles have an average particle diameter of 2,000 nm or less, 1,900 nm or less, 1,800 nm Hereinafter, 1,700 nm or less, 1,600 nm or less, 1,500 nm or less, 1,400 nm or less, 1,300 nm or less, 1,200 nm or less, 1,100 nm or less, or 1,000 nm or less, but is not limited thereto.

If the sizes of the fibers and particles are out of the above range, the diffusion time of electrons or ions increases, thereby reducing conductivity, or it may be difficult to secure a space for trapping ions.

The negative active material may have a cluster structure. Referring to FIG. 1, the cluster-type structure may mean a structure in which the carbon fibers having a one-dimensional structure in which the particles are coated on at least a portion of the surface are formed in constant or random proximity.

The negative electrode active material has a peak of 26±1° XRD 2θ using Cu-Kα radiation; And at least one peak selected from the group consisting of 30±1°, 35±1°, 37±1°, 43±1°, 53±1°, 57±1°, and 62±1°. The peak of 26±1° may mean the carbon fiber, and 30±1°, 35±1°, 37±1°, 43±1°, 53±1°, 57±1° and 62±1 At least one peak selected from the group consisting of ° may mean the metal oxide particle.

The negative electrode active material may have an atomic concentration ratio of oxygen, carbon, and the metal of 1: 0.1 to 10: 0.1 to 5, respectively. The atomic concentration ratio of the metal means the concentration ratio of all metal atoms included in the negative electrode active material.

The negative electrode active material has a maximum discharge capacity of 500 mAh/g or more, 600 mAh/g or more, 700 mAh/g or more, 800 mAh/g or more, 900 mAh/g or more, or 1,000 mAh/g or more, and 2,000 mAh/g Hereinafter, it may be 1,900 mAh/g or less, 1,800 mAh/g or less, or 1,700 mAh/g or less, but is not limited thereto. Compared to a conventional graphite material having a theoretical capacity of only 372 mAh/g, the negative electrode active material may have a significantly superior capacity.

The negative active material may have a coulomb efficiency of 60.0 to 99.9%. The coulomb efficiency refers to the ratio of the charging capacity to the discharge capacity.

The negative electrode active material may have a discharge capacity of 60% or more, 65% or more, or 70% or more based on the secondary discharge capacity after 800 charging and discharging times having a current density of 3,000 mA/g. The anode active material has remarkably excellent circulation stability compared to the prior art, and thus can maintain excellent discharge capacity even under a high current density condition of 3,000 mA/g.

The negative electrode active material may have a discharge capacity of 500 mAgh/g or more, 550 mAgh/g or more, or 600 mAgh/g or more under conditions of a current density of 100 to 3,000 mA/g. Since the negative active material has excellent rate characteristics, the discharge capacity may satisfy the above range even under variable current density conditions.

A method of preparing a negative electrode active material for a lithium secondary battery according to another aspect of the present invention includes the steps of: (a) preparing a solution including a metal precursor, a urea, and a solvent; (b) adding carbon fiber to the solution and reacting at 100 to 300° C. for 1 to 12 hours; And (c) calcining the product of step (b) at 400 to 600° C. for 1 to 12 hours.

The molar ratio of the metal precursor and the urea may be 1: 1 to 5, and the molar ratio of the metal precursor and the solvent may be 1: 0.5 to 1.5, but is not limited thereto.

The reaction of step (b) may be a one-stage hydrothermal synthesis reaction, and step (c) may be performed in an Ar atmosphere, so that the above-described negative electrode active material for a lithium secondary battery may be prepared in a simple and economical manner.

In the negative electrode for a lithium secondary battery according to another aspect of the present invention, a slurry including the negative electrode active material, a binder, and a conductive material may be applied to a current collector, and a weight ratio of the negative electrode active material, the binder, and the conductive material is each 40 to 100: 1 to 20: 1 to 20, but is not limited thereto.

Hereinafter, embodiments of the present invention will be described in more detail. However, the following experimental results are only representative of the above examples, and cannot be interpreted as the scope and content of the present invention are reduced or limited by examples. Effects of each of the various embodiments of the present invention not explicitly presented below will be specifically described in the corresponding section.

Chemical

Vapor grown carbon fiber (VGCF), nickel chloride (NiCl ₂ 6H ₂ O, 99.0%), iron chloride (FeCl ₃ 6H ₂ O, 98.0%) and urea (urea, CH ₄ N ₂ O, 95%) was purchased from Sigma Aldrich and used without a separate purification process. All kinds of solutions were prepared using DI water.

1 schematically illustrates a method of manufacturing a negative electrode active material according to an embodiment of the present invention and its characteristics. Referring to FIG. 1, VGCF, nickel chloride, iron chloride, and urea are hydrothermally synthesized in a single stage to prepare a cluster type VGCF/NFO. Raw VGCF (bare VGCF) exists in a clustered structure by coagulation of one-dimensional rod-shaped VGCFs, and Ni ions and Fe ions generate crystal nuclei on the surface of each of these aggregated VGCF rods, which are calcined under Ar atmosphere ( calcination) to prepare a clustered VGCF/NFO hybrid nanocomposite whose surface is coated with NFO.

Example

A cluster-type VGCF/NFO hybrid nanocomposite electrode material was prepared using the following one-stage hydrothermal synthesis method.

A solution was prepared by dissolving 25 mmol of nickel chloride, 50 mmol of iron chloride, and 100 mmol of urea in 75 ml of distilled water. While the solution was stirred, 0.50 g of vapor-grown carbon fiber was added, and added to a 100 ml of Teflon-lined stainless steel high pressure reactor. The high-pressure reactor was reacted while maintaining at 180°C for 6 hours. After the reaction was completed, the high-pressure reactor was cooled to room temperature and washed to remove impurities. The product obtained by raising the temperature at a rate of 5° C./min in an Ar atmosphere was calcined at 550° C. for 5 hours to prepare VGCF/NFO coated with NFO on the VGCF surface.

Comparative example

A solution was prepared by dissolving 25 mmol of nickel chloride, 50 mmol of iron chloride, and 100 mmol of urea in 75 ml of distilled water. The solution was stirred and introduced into a 100 ml Teflon-lined stainless steel high pressure reactor. The high-pressure reactor was reacted while maintaining at 180°C for 6 hours. After the reaction was completed, the high-pressure reactor was cooled to room temperature and washed to remove impurities. The product obtained by raising the temperature at a rate of 5° C./min in an Ar atmosphere was calcined at 550° C. for 5 hours to prepare NFO.

Experimental Example 1

Structure and phase purity of the materials prepared in the Comparative Examples and Examples by using an X-ray diffraction (XRD, D8 Advance) manufactured by Bruker Company equipped with a Cu-K _α (λ=1.5418 Å) filter Was analyzed.

The stoichiometric ratio and powder surface of the material prepared in the above Example were analyzed using an X-ray photoelectron spectroscopy (XPS, K-Alpha) of Thermo Electron.

According to the analysis result, the XRD pattern (a) of pure NFO prepared in the comparative example and the XRD pattern (b) of VGCF/NFO prepared in the above example, Ni 2p(c), Fe 2p(d), O 1s The XPS spectra of (e) and C 1s(f) are shown in FIG. 2.

Referring to Figure 2 (a), the pure NFO exhibited a sharp diffraction pattern of 2θ values at 30.32°, 35.81°, 37.27°, 43.45°, 53.89°, 57.41° and 62.96°. This was attributed to the crystal planes of (220), (311), (222), (400), (422), (511) and (440), respectively. The diffraction peak information of this pure NFO is described in JCPDS card 74-2081.

According to the XRD analysis results, diffraction peaks of other impurities such as iron oxide (Fe ₂ O ₃ ) or nickel oxide (NiO) other than NFO were not found. A sharp and narrow diffraction peak means that the manufactured material is of high purity and has a higher order of crystallinity, and in particular, high level ferrite grains exist in the material from the sharp peak. Can be seen.

Referring to (b) of FIG. 2, except for the characteristic peak of VGCF present at 26.3°, the XRD pattern of the VGCF/NFO hybrid nanocomposite prepared according to the above example exactly matches the NFO.

Referring to FIGS. 2C to 2F, the electronic structure, chemical element valence state, and surface composition of the VGCF/NFO analyzed according to XPS can be confirmed. The formation of clustered VGCF/NFO nanocomposites can be confirmed according to the analysis spectra of Ni 2p, Fe, 2p, O 1s and C 1s, and Si 2p peaks were formed due to the substrate used for this analysis. All spectra were fitted using the Gaussian-Lorenchian function.

2C shows the deconvoluted spectrum of the Ni core level, and the spectrum of the Ni core level is composed of two spin orbit two-stage peaks, Ni 2p3/2 and Ni2p1/3. . The broad peak of 854.5 eV means Ni 2p3/2, the smaller peak of 871.6 eV means Ni 2p 1/3, and the two satellite peaks of 860.2 and 877.7 eV mean Ni ³⁺ and Ni ²⁺ , respectively.

Figure 2 (d) shows the XPS spectrum of Fe 2p including two peaks of Fe 2p3/2 and Fe 2p1/2, the peak of Fe 2p3/2 due to the (ji) coupling of the spin orbit Fe It can be seen that it is larger than the peak of 2p1/2. It is known in a number of studies that for Fe 2p, 708.5 eV means Fe 2p3/2, and 722.3 eV means Fe 2p1/2.

Referring to Figure 2 (e), 528.3 eV and 531.0 eV O 1s inner level spectrum is a number of hydroxy groups, metal-oxygen bonds, and low oxygen coordination in the small NiFe ₂ O ₄ species. It is consistent with that of a typical oxygen present at water defect sites.

Fig. 2(f) shows an element spectrum of C 1s, and the peaks of 284.7 eV, 287.1 eV and 283.2 eV correspond to C-C, C=O and C-H, respectively.

The morphology of the material prepared in the above Example was analyzed using a field emission scanning electron microscope (FE-SEM, LEO SUPRA 55) of Carl Zeiss.

(I) to (iii) of FIG. 3(a) show FE-SEM images of individual VGCFs, and the VGCF has a one-dimensional fibrous morphology having a diameter of 100 nm and a length of 50 to 200 μm. This is schematically illustrated in (iv) of FIG. 3(a).

(I) to (iii) of FIG. 3(b) show typical low-magnification and high-magnification FE-SEM images of pure NFO, and the NFO has a clustered morphology aggregated in a size of 100 nm to 1 μm. This is schematically illustrated in (iv) of FIG. 3(b).

(I) to (iii) of FIG. 3(c) show images taken at various magnifications of the VGCF/NFO hybrid nanocomposite.The VGCF/NFO hybrid nanocomposite is a morphology in which NFO nanoclusters are randomly incorporated on VGCF Have. This is schematically illustrated in (iv) of FIG. 3(c). The ordered interconnection network of such a clustered VGCF/NFO hybrid nanocomposite can improve electronic conductivity and reduce stress and volume expansion/contraction during charging and discharging.

Manufactured in the above example using a high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2010) equipped with an energy dispersive X-ray spectroscopy (EDS) The internal microstructures, crystallographic intrinsic properties, and elemental analysis of the material were performed.

HR-TEM was photographed to confirm the specific structure and intrinsic properties of the clustered VHCF/NFO hybrid nanocomposite, and the results are shown in FIG. 4.

4A and 4B are HR-TEM images taken at different magnifications of NFO-coated VGCF, and it can be seen that the average diameter of VGCF is about 100 nm.

Referring to FIG. 4C, a diffused ring having a dot pattern of a selected area electron diffraction (SAED) exhibits a characteristic of a polycrystalline material clearly revealing the properties of a nanophase material.

Referring to (d) of FIG. 4, grid stripes having interplanar spacing of 0.269 nm and 0.231 nm similar to planes (311) and (440) can be confirmed.

Referring to (i) to (iv) of FIG. 4(f), the presence and stoichiometric amounts of Ni, Fe, O and C can be confirmed from the energy dispersion spectrum. From the plotted EDS results, it can be seen that Ni, Fe, O and C were homogeneously distributed.

Referring to the above results, the electrochemical performance may be improved by growing NFO on the VGCF. This seems to be because VGCF, which has excellent conductivity, allows direct electrical contact between active material particles.

Manufacturing example

VGCF/NFO prepared in the above example, carbon black (Super P) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed in a weight ratio of 80:10:10, respectively, and N-methyl- A slurry was prepared by mixing with 2-pyrrolidone (N-methyl-2-pyrolidone, NMP). The slurry was applied on a Cu foil by a doctor blade method and dried at room temperature for 24 hours. In order to remove residual moisture, it was dried in an oven at 100° C. for 5 hours and vacuum dried at 120° C. for 5 hours to prepare a working electrode.

In a glove box filled with Ar, a CR2032 type lithium secondary battery was constructed using the following materials or materials as the working electrode, counter electrode, electrolyte, and separator, respectively.

-Counter electrode: lithium foil

-Electrolyte: 1 M LiPF ₆ in EC(ethylene carbonate)/DEC(diethyl carbonate) = 50/50(v/v)

-Separation membrane: polyethylene/polypropylene/polyethylene separator

Comparative Production Example

A lithium secondary battery was manufactured in the same manner as in Preparation Example, except that the NFO prepared in Comparative Example was used instead of the VGCF/NFO prepared in Example.

Experimental Example 2

A specific charge/discharge test was performed under a constant current-constant voltage (CC-CV) condition under a voltage range of 0.01 to 3.0 V and a variable current density using an electrochemical circulator (ETH cycler).

The differential charge and differential voltage (dq/dv) curves in the voltage range of 0.01 to 3.0 V are shown in FIG. 5 in order to check the lithium charge/discharge characteristics of the electrodes manufactured using the NFO of the comparative example and the VGCF/NFO of the example. Shown in.

5A and 5B show the first three dq/dv curves of Comparative Preparation Example and Preparation Example, respectively. Referring to this, it can be seen that both Comparative Preparation Examples and Preparation Examples exhibit the characteristics of general transition metal oxides. During the first cycle starting from the cathodic scan, the two reduction peaks of 0.8 V and 1.45 V corresponded to the reduction from Fe ₂ O ₃ and NiO to Fe and Ni metals, respectively. In the cathode test, a peak of 0.8 V indicates the formation of a solid electrode surface layer on the surface of the active material and decomposition of the electrolyte. In the anodic scan, the broad oxidation peak measured at 1.7 V corresponded to the oxidation of Fe ⁰ and Ni ⁰ to Fe ³⁺ and Ni ²⁺ . In the subsequent cycle, a stable overlapping peak was shown. In addition, VGCF/NFO of Preparation Example was excellent in electrochemical reversibility and circulation stability during the cycling test by continuously maintaining the peak intensity and area.

Referring to FIG. 5A, it can be seen that there is a large difference between the first cycle and the subsequent cycle, so that the initial spinel structure is not recovered. As previously known from subsequent cycles, a cathode peak was observed at 0.8 V. The reversible electrochemical reaction can be expressed as the following equation.

^{_{NiFe 2 O 4 + 8e - +}} 8Li + ↔ Ni 0 + 2Fe 0 + 4Li 2 O

_{Ni + Li 2 O ↔ NiO +} 2Li + + 2e -

_{2Fe + 3Li 2 O ↔ Fe 2} O 3 + 6Li + + 6e -

Figure 5 (d) and (e) show the results of measuring the constant current charging and discharging characteristics of the NFO and VGCF/NFO at a voltage range of 0.01 to 3.0 V and a current density of 100 mA/g, respectively. Referring to this, VGCF/NFO has a discharge/charge capacity of 1,653 mAh/g, respectively, in the first cycle, It has a coulomb efficiency of 72.7% at 1,202 mAh/g, and the discharge/charge capacity of the first cycle of NFO has a coulomb efficiency of 67.9% at 1,163 mAh/g and 790 mAh/g, respectively. The capacity loss in the first cycle is mainly due to irreversible processes such as the initial formation of Li ₂ O, the capture of some lithium in the lattice, the formation of the inevitable solid electrolyte interface (SEI) layer, and the decomposition of the electrolyte. This is a common phenomenon in oxide anode active materials. VGCF/NFO has higher initial discharge capacity and improved coulomb efficiency compared to NFO, which is due to the interaction between VGCF and NFO. The discharge capacity of VGCF/NFO in the 2nd and 3rd circulation is 1,261 mAh/g and 1,193 mAh/g, respectively, the charging capacity is 1,150 mAh/g and 1,111 mAh/g, respectively, and the coulomb efficiency is 91.1% and 93.1, respectively. %to be. On the other hand, in the second and third cycles of NFO, the discharge capacity is 864 mAh/g and 826 mAh/g, respectively, and the charging capacities are 775 mAh/g and 752 mAh/g, respectively, and the coulomb efficiency is 89.6% and 91.0, respectively. %to be.

5 (g) and (h) show the results of measuring the circulation characteristics of NFO and VGCF/NFO, respectively. Referring to this, the initial discharge capacities of NFO and VGCF/NFO are 1,163 mAh/g and 1,653 mAh/g, respectively, and the coulomb efficiency is 72.7% and 67.9%. VGCF/NFO has a higher specific discharge capacity by forming a complex with the carbon matrix, and has a high specific capacity of about 1,352 mAh/g and a coulomb efficiency of 99.8% even after 100 cycles. After about 30 cycles, the capacitance increases, because the diffusivity of the electrolyte inside the VGCF/NFO is improved by the interaction between the carbon matrix and the NFO.

FIG. 5(c) shows a schematic diagram of ion and electron transfer in a VGCF/NFO electrode, and FIG. 5(f) shows a corresponding FE-SEM image. Referring to this, the gradual increase in circulation characteristics is because (1) the one-dimensional VGCF improves the electrical conductivity and ion diffusion properties of the anode active material, and (2) the volume expansion and structural stability of the anode active material are maintained during charging and discharging.

The rate capability performance of NFO and VGCF/NFO was measured in a current density range of 100 to 3,000 mA/g and shown in FIG. 6.

6(a) shows the rate characteristics of VGCF/NFO and NFO, and VGCF/NFO is 1,657, 1,099, 958, 813, 692 at current densities 100, 500, 1,000, 1,500, 2,000, and 3,000 mA/g, respectively. And a discharge capacity of 638 mAh/g. After the high current density test of 3,000 mA/g, when the current density was reduced to 100 mA/g, it showed a capacitance of about 1,183 mAh/g and a coulomb efficiency of 98%. After that, about 87% of the second cycle discharge capacity was maintained.

On the other hand, NFO exhibited discharge capacities of 1,163, 732, 561, 373, 230 and 136 mAh/g respectively at current densities of 100, 500, 1,000, 1,500, 2,000 and 3,000 mA/g. After the high current density test of 3,000 mA/g, when the current density was reduced to 100 mA/g, it showed a capacitance of about 608 mAh/g and a capacity retention rate of 58%. From this, it can be seen that the NFO electrode material is strongly affected by high current density.

On the other hand, referring to (b) of FIG. 6, the VGCF/NFO exhibited a capacity retention rate of 87%, indicating excellent structural integrity even at a high current density. In addition, referring to FIG. 6(c) showing the results of a long-term cycling performance of VGCF/NFO, it can be seen that VGCF/NFO exhibits excellent cycling characteristics even at a high current density of 3,000 mAh/g. . The discharge/charge capacity at this time was about 789/731 mAh/g, indicating a coulomb efficiency of 91.37%. In the 800th cycle, the discharge/charge capacity was about 574/571 mAh/g, and the coulomb efficiency was about 99.4%. Through such a long-term cycling test at a high current density, it can be confirmed that the cluster-type VGCF/NFO hybrid nanocomposite electrode material has excellent circulation stability even at a high current density. This is due to the structural integrity of the electrode material and the effect that the carbon matrix inside the composite reduces the load during the capture and release of lithium. In addition, such a carbon matrix solves the volume expansion, which is a problem of transition metal oxide, so that the electrode material can have excellent circulation characteristics and rate characteristics.

Electrochemical impedance spectrometry (EIS) was performed with an electrochemical analyzer (Iviumstat) of Ivium Technologies.

7A and 7B show the EIS analysis results before and after the 100th cycle test of VGCF/NFO and NFO. From this EIS analysis, the electrochemical properties of the electrode material were confirmed. The Nyquist spectrum derived from this consists of two domains.

(1) A semicircle in the high frequency region showing the interfacial properties and the corresponding charge transfer resistance (R _ct )

(2) A straight line in the low-frequency region associated with Warburg impedance and distributed by solid-state diffusion and migration of Li ions from the electrolyte to the electrode surface.

As shown in (a) of FIG. 7, the values of charge transfer resistance (R _ct ) of VGCF/NFO and NFO before the cycling test are different from each other, and the minimum R _ct value of VGCF/NFO is good adhesion between the electrode and the electrolyte. Means. In addition, this is consistent with the rate characteristics of VGCF/NFO.

On the other hand, referring to (b) of FIG. 7, after 100 cycles, the semicircle diameter of the NFO increased compared to that of the VGCF/NFO. Therefore, VGCF/NFO has superior ionic and electronic conductivity compared to NFO. In addition, the steep slope of VGCF/NFO means that Li ⁺ ions move more rapidly from the electrolyte to the electrode. From the Warburg impedance (Z _w ), it can be seen that the porous VGCF/NFO has superior performance compared to the NFO.

The lower part of the EIS spectrum belongs to the bounded diffusion of ions within the particle, which is influenced by the particle geometry and distribution. Figure 7 (c) shows the Warburg impedance spectrum of Z _re with respect to ω ^-0.5 after 100 cycles of cycling tests. From this, it can be seen that the VGCF/NFO is easier to diffuse ion than the NFO during the constant current charging and discharging process.

The remarkably excellent rate characteristics and cyclic stability of the clustered VGCF/NFO hybrid nanocomposite may be attributed to the unique form of the hybrid nanocomposite. VGCF plays an important role in the electrical conductivity related to electron and Li ⁺ ion diffusion, and the self-supporting and stress releasing properties of carbon fiber or VGCF are the VGCF/NFO hybrid nanoparticles in the charge and discharge process. The load on the composite can be avoided, and the VGCF/NFO can continue to form the SEI layer during circulation to maintain the coulomb efficiency. In addition, VGCF raw materials and other precursors are economical and readily obtainable, so that a VGCF/NFO electrode material can be easily manufactured therefrom.

The above description of the present invention is for illustrative purposes only, and those of ordinary skill in the art to which the present invention pertains will be able to understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. will be. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not limiting. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as being distributed may also be implemented in a combined form.

The scope of the present invention is indicated by the claims to be described later, and all changes or modified forms derived from the meaning and scope of the claims and the concept of equivalents thereof should be construed as being included in the scope of the present invention.

Claims (20)

Carbon fiber; And
Metal oxide particles coated on at least a portion of the surface of the fiber; containing, a negative electrode active material for a lithium secondary battery. The method of claim 1,
The carbon fiber is a vapor-grown carbon fiber, an anode active material for a lithium secondary battery. The method of claim 1,
The metal is a transition metal, a negative electrode active material for a lithium secondary battery. The method of claim 1,
The metal is a mixture of nickel and iron, a negative electrode active material for a lithium secondary battery. The method of claim 1,
The carbon fiber has an average diameter of 50 to 150 ㎚, an average length of 10 to 1,000 ㎛, a negative electrode active material for a lithium secondary battery. The method of claim 1,
The particles have an average particle diameter of 50 to 2,000 nm, an anode active material for a lithium secondary battery. The method of claim 1,
The negative electrode active material has a cluster-type structure, a negative electrode active material for a lithium secondary battery. The method of claim 1,
The negative active material,
XRD 2θ value using Cu-K _α radiation,
A peak of 26±1°; And
For lithium secondary batteries having at least one peak selected from the group consisting of 30±1°, 35±1°, 37±1°, 43±1°, 53±1°, 57±1°, and 62±1° Negative active material. The method of claim 1,
The negative electrode active material has an atomic concentration ratio of oxygen, carbon and the metal of 1: 0.1 to 10: 0.1 to 5, respectively, a negative active material for a lithium secondary battery. The method of claim 1,
The negative electrode active material has a maximum discharge capacity of 500 mAh/g Above, negative active material for lithium secondary batteries. The method of claim 1,
The negative electrode active material has a coulomb efficiency of 60.0 to 99.9%, a negative electrode active material for a lithium secondary battery. The method of claim 1,
The negative electrode active material has a discharge capacity of at least 60% based on the second discharge capacity after 800 charging and discharging with a current density of 3,000 mA/g. The method of claim 1,
The negative electrode active material has a discharge capacity of 500 mAh/g or more under the conditions of a current density of 100 to 3,000 mA/g, a negative electrode active material for a lithium secondary battery. (a) preparing a solution comprising a metal precursor, urea and a solvent;
(b) adding carbon fiber to the solution and reacting at 100 to 300° C. for 1 to 12 hours; And
(c) calcining the product of step (b) at 400 to 600° C. for 1 to 12 hours; containing, a method for producing a negative active material for a lithium secondary battery. The method of claim 14,
The molar ratio of the metal precursor and the urea is 1: 1 to 5, a method of manufacturing a negative electrode active material for a lithium secondary battery. The method of claim 14,
The molar ratio of the metal precursor and the solvent is 1: 0.5 to 1.5, a method of manufacturing a negative electrode active material for a lithium secondary battery. The method of claim 14,
The reaction of step (b) is a one-stage hydrothermal synthesis reaction, a method for producing a negative active material for a lithium secondary battery. The method of claim 14,
The step (c) is carried out in an Ar atmosphere, a method for producing a negative active material for a lithium secondary battery. A negative electrode for a lithium secondary battery in which the slurry including the negative electrode active material, a binder, and a conductive material according to any one of claims 1 to 13 is applied to a current collector. The method of claim 19,
The negative electrode active material, the binder, and the weight ratio of the conductive material is 40 to 100: 1 to 20: 1 to 20, respectively, a negative electrode for a lithium secondary battery.

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