KR102461543B1 - Hybrid nanofibers and anodes for lithium ion battery comprising the same - Google Patents

Abstract

Composite nanofibers according to various embodiments of the present invention include: a porous nanofiber matrix; and a structure distributed on the surface of the matrix, wherein the structure includes nano-spheres, wherein the nano-spheres include: a hollow metal oxide; A carbon layer coated on the metal oxide may be included.

Description

Anode material for lithium ion batteries including composite nanofibers and composite nanofibers

The present invention relates to a composite nanofiber and an anode material for a lithium ion battery comprising the composite nanofiber.

With the rapid increase in energy demand, lithium-ion batteries (LIBs) are attracting attention as next-generation energy storage devices and vehicle energy sources. Therefore, it is essential to develop innovative anode materials to improve the performance of LIB. Transition metal oxides (TMOs) have been recognized as suitable anode materials because of their high theoretical capacity. However, the rapid capacitance reduction of TMO due to large volume expansion during cycling prevented its application as a LIB anode material. Therefore, various TMO nanostructures including nanoparticles, nanowalls, nanobelts, nanofibers and nanoflakes have been extensively studied.

An object of the present invention is to provide a composite nanofiber having a novel structure and an anode material for a lithium ion battery including the composite nanofiber.

In order to achieve the object of the present invention as described above,

Composite nanofibers according to various embodiments of the present invention include: a porous nanofiber matrix; and a structure distributed on the surface of the matrix, wherein the structure includes nano-spheres, wherein the nano-spheres include: a hollow metal oxide; A carbon layer coated on the metal oxide may be included.

The composite nanofiber according to various embodiments of the present invention is a novel structure that has not been reported before, has a stable structure, has a large specific surface area, and has excellent electrical conductivity, so that it can be used in various fields such as secondary batteries, biomaterials, catalysts, and sensors. can be used The composite nanofibers of the present invention can exhibit excellent lithium storage properties including excellent speed performance and cycling stability due to their unique nanostructure. The composite nanofiber of the present invention relieves volume expansion in the charging/discharging process, and allows the liquid electrolyte to efficiently penetrate into the electrode. In addition, it can enable fast Li ⁺ ion and electron diffusion, and has excellent rate performance.

The manufacturing method of the composite nanofiber of the present invention can be manufactured through electrospinning and simple one-step heat treatment. In addition, it is possible to mass-produce composite nanofibers having such a novel structure, which has high industrial application potential.

1 is a schematic diagram for explaining a method of manufacturing a composite nanofiber according to an embodiment of the present invention.
2 is a result of observing the morphology of the Co(Ac) ₂ /GO/PVA/PS composite nanofibers formed with ZIF-67 nanocrystals obtained after electrospinning.
3 is a result of observing the morphology of the composite nanofiber after the reduction heat treatment.
4 is an XRD pattern of composite nanofibers according to various embodiments.
5 is a result of observing the morphology of S200 nanofibers.
6 is a result of observing the morphology of S250 nanofibers.
7 is an XPS analysis result for investigating the chemical state and molecular environment of S250 nanofibers.
8 is a result of observing the morphology of S300 nanofibers.
9 is a result of observing the electrochemical properties of composite nanofibers according to various embodiments.
10 is a result of comparing the Li ⁺ ion storage performance of the composite nanofibers of the present invention and other conventional nanostructures.
11 is a result of analyzing the electrochemical dynamics of S250 nanofibers.
12 is a photograph for confirming the structural stability of the nanostructure of the sample obtained after 300 cycles.

Hereinafter, various embodiments of the present document will be described with reference to the accompanying drawings. The examples and terms used therein are not intended to limit the technology described in this document to specific embodiments, and should be understood to cover various modifications, equivalents, and/or substitutions of the embodiments.

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

Composite nanofibers according to various embodiments of the present invention include: a porous nanofiber matrix; and metal-organic frameworks (MOF)-derived structures distributed on the surface of the matrix.

The porous nanofiber matrix may include reduced-grephene oxide (rGO).

The porous rGO matrix of the present invention may include mesopores having a diameter of 5 nm to 50 nm and macropores having a diameter of 50 nm or more. When the composite nanofiber of the present invention is used as a battery electrode through these mesopores and macropores, it is possible to provide an additional contact surface between the active material and the electrolyte to promote rapid ion/electron diffusion, thereby improving the speed characteristics of the electrode.

The structure uniformly distributed on the matrix surface is a structure derived from MOF, and may be formed by being modified from the MOF of at least one of ZIF-8, ZIF-67, ZIF-8, MIL-100, and MOF-5.

In addition, the structure may include a plurality of nanospheres. The nanospheres may include a hollow metal oxide and a carbon layer coated on the metal oxide. In this case, the metal oxide may be a transition metal oxide. For example, transition metal oxides are Fe ₂ O ₃ , NiO, CoO, Co ₃ O ₄ , SnO ₂ , MnO ₂ , MoO ₃ , CuO, ZnO, TiO ₂ , MgO, V ₂ O ₅ , Cr ₂ O ₃ , Y ₂ O ₃ , CeO ₂ , ZrO ₂ , Al ₂ O ₃ , WO ₃ , CoMoO ₄ , ZnFeO ₄ , NiCo ₂ O ₄ , CoFeO ₄ , ZnCo ₂ O ₄ , NiMoO ₄ , NiFe ₂ O ₄ , and ZnMn ₂ O It may be at least one selected from the group consisting of ₄ . According to an embodiment of the present invention, the nanospheres may be cobalt oxide, and in this case, may include a binary CoO phase and a Co ₃ O ₄ phase.

The carbon layer may be an N-doped graphite carbon layer (N-Doped Graphitic C, NGC). NGC can be formed in several layers on the surface of nanospheres.

The NGC layer coated on these hollow metal oxide nanospheres can provide a primary transport path for electrons, and can improve electrical conductivity and electrochemical reactivity.

The above-described composite nanofiber is a novel structure that has not been previously reported, has a stable structure, has a large specific surface area, and has excellent electrical conductivity, so that it can be used in various fields such as secondary batteries, biomaterials, catalysts, and sensors. Preferably, it can be used as an anode material of a flexible lithium polymer battery. The composite nanofiber of the present invention can maintain durability even in bending, and there is little decrease in capacity.

Hereinafter, a method for manufacturing a composite nanofiber according to various embodiments of the present invention will be described.

The manufacturing method of the composite nanofiber of the present invention comprises the steps of mixing graphene oxide, a metal salt, a carbonizable organic material, and a thermally decomposable polymer particle;

electrospinning the mixture to form a first precursor;

forming a second precursor in which MOF nanocrystals are generated by supporting the first precursor on an organic linker;

forming a third precursor by performing a first heat treatment on the second precursor; and

It may include the step of obtaining a composite nanofiber by a second heat treatment of the third precursor.

Specifically, in the mixing step, the metal salt may be a precursor of various MOFs. For example, it may be a metal salt capable of forming an MOF of at least one of ZIF-8, ZIF-67, ZIF-8, MIL-100, and MOF-5.

\*29The organic material that can be carbonized is sucrose, dextrin, citric acid, ethylen glycol, poly ethylen glycol, PVP (Polyvinylpyrrolidone), PEDOT (Polyethylenedioxythiophene), PAN (Polyacrylonitrile), PAA (polymethylacrylic acid), PVA (polyvinylalcohol), PMMA (polymethylacrylic acid) methacrylate), PVDF (　polyvinylidene fluoride), PVac (polyvinylacetate), PS (polystyrene), PVC (polyvinylchloride), PEI (polyetherimide), PBI (polybenzimidasol), PEO (polyethyleneoxide), PCL (poly e-caprolactone), PA-6 It may include at least one selected from the group consisting of (polyamide-6), polytrimethylenetetraphthalate (PTT), poly D,L-lactic acid (PDLA), polycarbonate, polydioxanone, polyglycolide, and dextran.

The thermally decomposable polymer particles are made of polystyrene, polyvinyl acetate, polymethyl methacrylate, polyvinylpyrrolidone, polyvinyl alcohol, polycarbonate, polyester, polyetherimide, polyethylene, polyethylene oxide, polyurethane, and polyvinyl chloride. It may include at least one selected from the group. These thermally decomposable polymer particles may be decomposed into gas during heat treatment to form pores. By forming pores, gas diffusion can occur smoothly in the next heat treatment step, and composite nanofibers having a desired structure can be obtained.

Next, the first precursor may be prepared by electrospinning this mixture. The prepared mixture can be loaded into a plastic syringe and spun. During electrospinning, the distance between the tip and the collector may be maintained at an interval of 10 cm to 20 cm. In addition, the drum may rotate at a rotation speed of 100 rpm to 200 rpm. The voltage applied between the tip and the current collector may be 10 kV to 30 kV. The first precursor may be prepared through these conditions.

This first precursor may be stabilized at a temperature of 80 °C to 120 °C.

Next, the first precursor may be supported on an organic linker to form a second precursor in which MOF nanocrystals are generated. MOF nanocrystals may be precipitated on the matrix surface of the first precursor through coordination bonding with an organic linker and transition metal ions.

Next, the second precursor may be subjected to a first heat treatment to form a third precursor. The first heat treatment may be performed at 200° C. to 600° C. at a heating rate of 2° C./min to 8° C./min under H ₂ /Ar mixed gas, thereby reducing the second precursor. Through this, as the thermally decomposable polymer particles are decomposed into a gaseous form, pores may be formed in the matrix of the second precursor, and nanospheres including NGC-coated metal particles may be formed on the MOF nanocrystals.

Next, the third precursor may be subjected to a second heat treatment to obtain the composite nanofibers of the present invention. The second heat treatment may be performed at 200° C. to 300° C. in an air atmosphere. Through this, the metal particles may be oxidized to a hollow metal oxide by the Kirkendall diffusion effect.

When the second heat treatment temperature is lower than 200 ℃, some amorphous carbon converted from carbonizable organic material remains, which may weaken the electrical conductivity, and when it is higher than 300 ℃, NGC is also thermally decomposed, resulting in sintering and densification of metal oxide nanoparticles. may occur and agglomerate. Preferably, the second heat treatment temperature may be 250 °C.

Hereinafter, a method of manufacturing a composite nanofiber according to an embodiment of the present invention will be described in more detail with reference to FIG. 1 .

First, cobalt acetate tetrahydrate (Co(Ac) ₂ ), graphene oxide (GO), PVA and PS nanobeads were mixed and then electrospun to obtain a first precursor as shown in ① in FIG. 1 (a). can

Next, referring to ② of FIG. 1(a), a second precursor in which ZIF-67 nanocrystals are precipitated on the surface of the GO matrix of the first precursor can be obtained by supporting the first precursor on an organic linker.

Next, referring to ③ of FIG. 1(a), by reducing heat treatment of the second precursor, pores may be formed due to the decomposition of PS nanobeads in the GO matrix, and may be reduced to a porous rGO matrix. In particular, the pores enable efficient penetration of reducing gas into the rGO matrix to form uniformly distributed nano-sized metallic Co particles. ZIF-67 nanocrystals can be converted into composite nanocrystals coated with NGC on the surface of Co metal particles. The metallic Co particles can serve as a catalyst for the graphitization of carbon, as a result, the amorphous carbon around the Co metal was graphitized to become graphitic carbon (GC). In addition, the N-rich organic ligand of the ZIF-67 crystal acts as an N doping source, where N is doped into the GC to form an N-doped GC (NGC). Thus, a third precursor of Porous Co@NGC/rGO can be obtained.

Next, referring to ④ of FIG. 1A , the third precursor may be subjected to oxidative heat treatment to form a hollow cobalt oxide structure through the Kirkendall diffusion mechanism. That is, porous cobalt oxide@NGC/rGO composite nanofibers can be obtained.

Hereinafter, the present invention will be described in detail by way of Examples. However, the following examples are only for illustrating the present invention, and the present invention is not limited by the following examples.

Example

After adding 0.5 g of graphene oxide (GO) and 2.0 g of PS nanobeads dispersed in 20 mL of distilled water to 30 mL of ethyl alcohol (Duksan, 99.9%), cobalt acetate tetrahydrate (Co( CH ₃ COO) ₂ 4H ₂ O, DaeJung Chemicals & Metals Co., Ltd., 98%) and 3.0 g of poly(vinyl alcohol) (PVA, Kanto Chemical Co. Inc., Mw: 2000) were added and stirred overnight. stirred vigorously. 40 nm PS nanobeads serving as pore formers were prepared by emulsion polymerization without an emulsifier. GO was prepared from graphite flakes via a modified Hummers method. The prepared spinning solution was loaded into a plastic syringe equipped with a 21-gauge stainless steel needle. The solution was spun into a drum current collector covered with Al foil at a flow rate of 4 mL h ^-1 . In the electrospinning process, the distance between the tip and the current collector was fixed at 15 cm, and the rotational speed of the drum current collector was 180 rpm. The applied voltage between the current collector and the tip was 20 kV. The spun Co(Ac) ₂ /GO/PVA/PS composite nanofibers were stabilized in an air atmosphere at 100 °C. Then, 0.5 g of stabilized Co(Ac) ₂ /GO/PVA/PS composite nanofibers were added together with 3.0 g of 2-methylimidazole (Wako Pure Chemical Industries, Ltd, 98%) to 50 mL of methanol solution, followed by Co (Ac) ₂ /GO/PVA/PS composite nanofibers were kept at 20 °C for 24 h to form ZIF-67 nanocrystals. After that, the initial heat treatment for reduction was performed at 400 °C for 3 hours at a heating rate of 5 °C min ^-1 under a 5 % H ₂ /Ar mixed gas atmosphere. Secondary oxidation heat treatment was performed at a heating rate of 5 °C min ^-1 in an air atmosphere for 3 h at temperatures of 200, 250, and 300 °C. Nanofibers obtained after secondary heat treatment at 200, 250, and 300 °C are called 'S200', 'S250', and 'S300' nanofibers, respectively.

Meanwhile, bare Co ₃ O ₄ nanofibers with filled structures were also prepared for comparison. In this case, Co(Ac) ₂ /PVA as-spun fibers without GO and PS nanobeads were prepared by electrospinning. After that, after heat treatment in air atmosphere at 600 °C for 3 h, C-free bare Co ₃ O ₄ solid nanofibers were finally obtained.

Experimental Example 1 - Co(Ac) with ZIF-67 nanocrystals obtained after electrospinning ₂ Observation of the morphology of /GO/PVA/PS composite nanofibers

To form ZIF-67 nanocrystals on nanofibers, Co(Ac) ₂ /GO/PVA/PS composite nanofibers were reacted with 2-methylimidazole, and the morphology of the resulting nanofibers is shown in FIG. shown. Referring to (a) and (b) of Figure 2, it can be seen that the nanofiber maintains a 1D structure. Referring to (C), (d) and (e) of Fig. 2, it was found that a number of rhombic dodecahedron-shaped ZIF-67 nanocrystals having an average size of 40 nm were uniformly generated on the nanofiber. Able to know. Referring to the XRD pattern of FIG. 2(f), it can be seen that the newly formed nanocrystals have a typical diffraction peak of the ZIF-67 phase. Due to the strong coordination of 2-methylimidazole as an organic linker to the cobalt ion, the metal node of the Co(Ac) ₂ /GO/PVA/PS composite nanofibers in methanol, uniform ZIF-67 nanocrystals grow on the surface. did. The element mapping data shown in (g) of FIG. 2 shows a uniform distribution of elements Co, O, C and N throughout the structure, and it can be seen that ZIF-67 nanocrystals were uniformly formed during the process.

Experimental Example 2 - Observation of the shape of composite nanofibers after reduction heat treatment

Co(Ac) ₂ /GO/PVA/PS composite nanofibers formed with ZIF-67 nanocrystals were heat treated at 400 °C in a 5% H ₂ /Ar mixed gas atmosphere, and the morphology after heat treatment is shown in FIG. 3 . Referring to (a) and (b) of Figure 3, the overall fiber structure of the nanofibers and ZIF-67 nanocrystals in the rhombic dodecahedron shape were well maintained even after heat treatment. On the other hand, PS nanobeads uniformly inserted between the GO sheets of the composite structure are decomposed into gaseous products to form numerous meso and macropores between the GO sheets. That is, referring to FIG. 3C , a well-developed porous structure can be clearly identified through the FE-SEM image. The pores can serve as passages to efficiently permeate the reducing gas inside the structure. Therefore, referring to Fig. 3(d) and (e), many metallic Co nanocrystals with narrow size distribution can be embedded in carbon frameworks modified from GO sheets and ZIF-67. Referring to FIG. 3(f), it can be seen through the HR-TEM image that the 20 nm metallic Co nanocrystals were coated with an N-doped GC layer (NGC). During the reduction process, PVA was initially decomposed into amorphous C (amorphous C) and converted to graphite C (graphitic C) due to the catalytic function of Co metal even at a relatively low temperature of 400 °C. Furthermore, the N-rich organic ligand of ZIF-67 acted as an N-doped source, and the N-doped source doped with the GC structure forms NGC. In general, N-doped C can improve electrical conductivity because of its smaller atomic radius and higher electronegativity compared to carbon. Referring to (f) of FIG. 3 , an NGC layer having a lattice pattern with an interval of 0.34 nm with respect to a (002) crystal plane can be observed. In addition, the lattice pattern at intervals of 0.20 nm corresponds to the (110) crystal plane of the cubic metal Co phase. Referring to (g) of FIG. 3 , selected-area electron diffraction (SAED) indicates the formation of metallic Co nanocrystals. As revealed in the XRD pattern, the broad and low-intensity Co peaks show that ultrafine Co nanocrystals were successfully formed during low-temperature reduction. The additional CoO peak was due to partial oxidation of Co nanocrystals exposed to air. Through the element mapping image shown in Fig. 3(h), the formation of Co metal uniformly coated with NGC in the rGO matrix nanofiber structure can be further confirmed.

Experimental Example 3 - Observation of properties of composite nanofibers after oxidation heat treatment

To apply the nanoscale Kirkendall diffusion mechanism, secondary heat treatment was performed at 200, 250, and 300 °C under an air atmosphere. Referring to FIG. 4 , it can be seen that the conversion of the metal Co to cobalt oxide occurred regardless of the oxidation temperature. At relatively low oxidation temperatures of 200 °C and 250 °C, binary CoO and Co ₃ O ₄ phases were formed. However, as the temperature increased to 300 °C, only Co ₃ O ₄ single phase was identified.

Meanwhile, FIG. 5 is a view of observing the morphology of S200 nanofibers obtained after low-temperature heat treatment at 200 °C. Similar to the nanofiber before oxidation (FIG. 3), referring to FIGS. 5A to 5D, the entire porous nanofiber structure can be observed. However, referring to FIG. 5(e), it can be seen that the composite fiber is composed of many hollow nanospheres formed by the nanoscale Kirkendall diffusion process in the HR-TEM image. Referring to Fig. 5(f), well-defined hollow cobalt oxide nanospheres modified from solid Co nanospheres are observed. In addition, it can be confirmed that the NGC layer formed using the Co catalyst during the reduction process is well coated on the nanospheres. The inset image of FIG. 5(f) shows a clear lattice pattern (0.24 nm) for the (311) crystal plane of the cubic Co ₃ O ₄ phase and a lattice pattern (0.21 nm) for the (200) plane of the CoO phase. The SAED pattern shown in Fig. 5(g) shows that the S200 nanofibers are composed of binary cobalt oxide phase, which is also in good agreement with the XRD result of Fig. 4 . The elemental mapping image shown in Fig. 5(h) shows that the Co particles were transformed into cobalt oxide, which was uniformly distributed in the rGO matrix porous nanofibers.

6 is a view of observing the morphology of S250 nanofibers. The porous rGO matrix nanofibers and hollow nanospheres have a morphology similar to that of S200 nanofibers. However, the S250 nanofiber has a relatively high heat treatment temperature and low carbon content. Most of the AC formed in the reduction process was selectively removed at the high oxidation temperature of 250 °C, while the AC was maintained at the low temperature of 200 °C. Therefore, the C materials of NGC and rGO together with cobalt oxide constituted S250 nanofibers. On the other hand, the carbon content of S250 nanofibers calculated through TGA and EA is 33 wt %, which is lower than the carbon content of S200 nanofibers of 43 wt %. On the other hand, the GC-doped N content of S250 nanofibers estimated from the EA results is 6.1 wt %, which is similar to 6.4 wt % of the GC-doped N content of S200 nanofibers. Referring to FIG. 6(f), the NGC layer coated on the hollow cobalt oxide nanospheres was well observed. Referring to (g) of FIG. 6 , the SAED pattern represents binary CoO and Co ₃ O ₄ phases. The elemental mapping image in Fig. 6(h) shows the presence of cobalt oxide together with NGC, and it can be seen that they are uniformly distributed in the rGO matrix porous nanofibers.

Meanwhile, FIG. 7 is an XPS analysis result for investigating the chemical state and molecular environment of S250 nanofibers. Referring to Fig. 7(a), the high-resolution Co 2p XPS spectrum shows two major peaks (780.4 for Co 2p _3/2 and 780.4 and Co 2p _1/2 for Co 2p 3/2) along with two shakeup satellites (labeled “Sat.”). 795.9 eV) was clearly shown. Through this, the formation of Co ₃ O ₄ can be clearly seen. CO ²⁺ was detected due to the presence of the CoO phase. Referring to Figure 7 (b), in the C 1s spectrum of the S250 nanofiber, peaks corresponding to sp 2 bonded carbon (C - C), NC / sp ³ bonded C - C and C = O are 284.2, 285.1 and 288.0 eV. Referring to (C) of Figure 7, in the N 1s spectrum, pyridinic N (398.4 EV), pyrrolic N (400.0 EV), and graphitic N (400.9 EV) three peaks were observed. In general, n-doped carbon improves electrical conductivity and electrochemical reactivity. In addition, defects formed in the n-doped carbon provide additional lithium insertion sites. Referring to (d) of FIG. 7 , the four resonance peaks at 186.9, 466.2, 506.4 and 670.1 cm ⁻¹ coincided with typical Co-O frequencies for cobalt oxide in the low range region. In addition, D-(1345 cm ^-1 ) and G-band (1,593 cm cm ^-1 ) related to the crystallinity of graphite were also identified. The relative intensity ratio of D to G band of S250 (I _D /I _G ) was 0.91, suggesting that the nanofibers were mainly composed of graphite C with high electrical conductivity. Referring to (e) of FIG. 7 , the BET surface area of the S250 nanofiber was 77 m ² g ⁻¹ . Referring to FIG. 7(f), the Barrett-Joyner-Halenda pore size distribution shows the presence of hollow and macropores in the nanofiber. A broad distribution of mesopores at 3 nm and 50 nm and macropores above 50 nm was formed between the rGO sheets, which is due to PS nanobead decomposition during the reduction step.

On the other hand, Figure 8 shows the morphology of the S300 nanofiber. S300 nanofibers have more macropores compared to S200 and S250 nanofibers because both rGO and NGC are burned at a high temperature of 300 °C. Also, referring to FIG. 8( d ), it can be seen that the cobalt oxide nanospheres constituting the fiber exhibited a hard structure due to densification and sintering during processing and agglomerated with each other. Referring to (e) of FIG. 8 , a lattice pattern with an interval of 0.46 nm corresponding to the (111) crystal plane of the Co ₃ O ₄ phase can be confirmed. Referring to (f) of FIG. 8 , a pure Co ₃ O ₄ phase having a large crystal size can be confirmed. The elemental mapping image shown in (g) of FIG. 8 shows the formation of carbon-free Co ₃ O ₄ porous nanofibers.

Experimental Example 4 - Application of composite nanofibers to lithium batteries

Examples prepared at different temperatures by investigating the effect of the morphological characteristics of hierarchically porous rGO nanofibers containing NGC-coated hollow cobalt oxide nanospheres on the electrochemical performance as LIB anodes (S200 nanospheres) fibers, S250 nanofibers, and S300 nanofibers) and bare solid Co ₃ O ₄ nanofibers were compared.

Figure 9 (a) relates to the initial discharge and charge curves of the samples at a constant current density of 1.0 Ag ^-1 . The ballast corresponding to the discharge/charge process accurately reflects the peak position obtained from the CV curve. During charging, the cells show a slightly different profile. The bare solid Co ₃ O ₄ and S300 nanofibers appear to have a charge plateau at 2.04 V, whereas the S200 and S250 nanofibers exhibit a very steep charge plateau. This indicates a slightly different reaction mechanism during charging. The initial discharge capacities of bare solid Co ₃ O ₄ , S200, S250 and S300 nanofibers were 1133, 1520, 1631 and 1382 mAh g ^-1 , and their coulombic efficiency (CE) values were 71%, 49%, and 65%, respectively. and 75%. The initial discharge capacity of all samples is much higher than the theoretical capacity of 890 mAh g ^-1 of the anode material, Co ₃ O ₄ . The additional capacity may be due to the formation of a polymeric film through decomposition of the alkyl carbonate-based electrolyte. The relatively low CE of the S200 and S250 nanofibers is due to the high carbon material content of the initial irreversible capacity loss. The difference in voltage hysteresis mainly indicates the polarization potential, and the S250 nanofiber exhibited the lowest voltage hysteresis ( ΔV = 0.480 V) among all samples. This shows that S250 nanofibers promote better ion and electron conduction and thus have high application potential for high-speed LIB anodes.

Figure 9 (b) shows the cycling performance of the nanofiber at a constant current density of 1.0 Ag ^-1 . The bare solid Co ₃ O ₄ nanofibers showed a continuous decrease in capacity to 266 mAh g ⁻¹ after 800 cycles because the solid Co ₃ O ₄ collapsed by large volume changes during repeated cycles. However, the S200, S250 and S300 nanofibers showed relatively good cycle characteristics regardless of the heat treatment temperature. After 800 cycles, reversible discharge capacities of 760, 995, and 1109 mAh g ^-1 were respectively exhibited, and the corresponding CEs were 99.4%, 99.5%, and 99.9%, respectively. The initial capacity increase of S200 nanofibers was observed during the initial 50 cycles. The capacity increase during cycling through the formation of a polymer gel-like film on the active material is well known for most transition metal oxide anode materials. The formation of polymer gel-like films on transition metal oxides is particularly facilitated by small particles and particle sizes, such as S200 nanofibers. In addition, the initial capacity of S300 nanofibers increases up to 130 cycles, which is due to the pulverization of the solid Co ₃ O ₄ nanospheres constituting the fibers, which results in the creation of a new metal surface in each cathodic process and continuous A reversible solid electrolyte interphase (SEI) layer was formed. Fig. 9(C) relates to the cycling performance at a very high current density of 3.0 A g ^-1 , which is a more severe condition. Compared with the performance of 1.0 A g ^-1 , the S250 nanofiber shows excellent cycling performance with the highest reversible capacity of 823 mAh g ^-1 at the 500th cycle even at a very high current density of 3.0 A g ^-1 . The capacity reduction rate was the lowest for S250 nanofibers at 0.092%, 0.144% for bare solid Co ₃ O ₄ nanofibers, 0.118% for S200 nanofibers, and 0.100% for S300 nanofibers. In particular, comparing the cycle characteristics at high current densities of 3 Ag ^-1 or higher, S250 provides superior capacity compared to other samples by effectively solving the dynamic problems related to diffusion of Li ⁺ ions and movement of electrons during repeated charge/discharge processes.

Fig. 9(d) relates to the speed performance of the nanofiber, and the current density increased stepwise from 0.5A g ^-1 to 100 A g ^-1 and then decreased to 0.5 Ag ^-1 . At low current densities of less than 1.5 A g ^-1 , S300 nanofibers exhibit a slightly higher discharge capacity among the samples, but at high current densities between 3.0 and 100 A g ^-1 , the discharge capacity is almost non-existent above 25 A g ^-1 . drops sharply to 0, which is similar to S200 and bare solid Co ₃ O ₄ nanofibers. Conversely, S250 nanofibers showed very high discharge capacity up to 100 A g ^-1 even at very high current densities. At current densities of 0.5, 1.5, 3, 5, 7, 10, 12, 15, 18, 20, 25, 30, 40, 50, 80, 100 A g ^-1 , the final discharge capacity of S250 nanofibers was 1035, respectively, 929, 847, 787, 747, 703, 672, 650, 625, 610, 570, 537, 475, 422, 294 and 222 mAh g -1 were identified. The high-conductivity hierarchical porous structure and NGC layer coated on the hollow cobalt oxide nanospheres act as the primary transport path for electrons, whereas the rGO-nano network surrounding the nanospheres is continuous and provides a secondary electron transport path. It has excellent rate performance by enabling fast Li ⁺ ion and electron diffusion. In addition, when the current density was reversed to 0.5 Ag ⁻¹ , the ^S250 nanofiber was The capacity was recovered, indicating the high structural stability of the S250 nanofibers.

The Li ⁺ ion storage performance of S250 nanofibers was compared with those of other conventional nanostructured cobalt oxide materials and carbon composite materials, and the results are shown in Table 1 and FIG. 10 below. The S250 nanofibers of the present invention showed the best reversible capacity at the highest current density and stable cycling performance compared to nanostructured cobalt oxide materials and carbon composite structures with various morphologies.

Materials Voltage
Range
(V) Current
Rate
[A g ^-1 ] Initial
C _dis /C _cha
[mA hg ^-1 ] Discharge
Capacity
[mA hg ^-1 ]
/cycle no Current Density
/Rate Capability
/Cycle no. Ref Porous rGO nanof ibers comprising NGC coated cobalt oxide hollow nanospheres 0.001-3.0 1.0
3.0 1632/1060
1525/942 995/800
823/500 3/848/10
5/787/10
7/747/10
10/703/10
25/570/10
30/537/10
40/475/10
50/422/10
80/ 294/10
100/222/10 In
this
work C-doped Co ₃ O ₄ hollow nanofibers 0.01-3.0 0.2 1385/1230 1121/100 3/607/10 [S1] Porous Co ₃ O ₄ nanotubes 0.01-3.6 0.05 1918/1343 1131/20 - [S2] Co ₃ O ₄ Nanosheetassembled
multishelled
hollow spheres 0.01-3.0 0.178 1013/~749 866/50 1.78/501/- [S3] Co ₃ O ₄ yolk-s hell
powder with multishells 0.01-3.0 0.5 1331/1005 958/100 3/770/10 [S4] Nanosized Co ₃ O ₄ 0. 01-3.0 0.22 1411/931 913/20 - [S5] Co ₃ O ₄ hollow
microsphere 0.01-3.0 0.02-0.06 1241/1061 633/25 - [S6] Meso- and macroporous
CoO-carbon
composite powders 0.01-3.0 1.4 ~1273/~970 855/100 5/698/5 [S7] Mesoporous Co ₃ O ₄
cubes 0.01-3.0 0.089 1298/986 980/60 8.9/130/5 [S8] Co ₃ O ₄ @graphene
nanocomposite 0.01-3.0 0.05 1097/~753 ~935/30 0.5/484/10 [S9] Co ₃ O ₄ -nanobu bbledecorated
rGO spheres 0.001-3.0 2.0 1471/1133 1156/200 10/842/10 [S10] Multishelled Co ₃ O ₄
hollow microspheres
(triple-shelled) 0.05-3.0 0.05 2064/~1600 1616/30 2/1117/5 [S11] Co ₃ O ₄ nanoparticle 0.01-3.0 0.178 1237/835 ~150/100 - [S12] Co ₃ O ₄ nanowire 0.01 -3.0 0.11 1027/755 611/50 1.1/396/5 [S13] Hollow Co ₃ O ₄ @ C
sphere nanoparticles 0.01-3.0 0.074 ~1050/732 750/20 0.744/500/- [S14] Co ₃ O ₄ nanoparticles 0.01-3.0 0.1 1334/~1070 407/100 - [S15] Plate-like Co3O4 0.02-3.0 0.178 1228/870 618/30 1.78/>500/10 [S16] Single-crystal Co ₃ O ₄
nanoneedles 0.01-3.0 0.15 1290/1031 1079/50 - [S17] Foam-like freestanding
Co ₃ O ₄ nanosheets 0.01-2.5 0.15 1852/1028 631/50 - [S18] Nanorod-assembled
Co ₃ O ₄ hexapods 0.01-3.0 0.1 1540/1100 800/40 0.5/440/40 [S19] MOF-derived porous
hollow Co ₃ O ₄
parallelepipeds 0.01-3.0 0.1 1608/1080 1100/50 1/738/10 [S20] 1-D porous Co ₃ O ₄
nanofibers 0.01-3.0 0.1 1368/1031 916/100 2/584/10 [S21] Co ₃ O ₄ hollow
nanopowder aggregate 0.01-3.0 1.0 1099/824 702/300 10/581/10 [S22] Hollow Co ₃ O ₄
nanosphere
aggregate/N-doped
graphitic carbon 0.001-3.0 1.0 1268/913 1030/250 10/738/10 [8] Co ₃ O ₄ nanoca ge 0.05-3.0 0.05 1150/846 970/30 2/252/5 [21]

11 is a result of analyzing the electrochemical dynamics of S250 nanofibers. Referring to Fig. 11(a), in order to understand the excellent rate performance, the electrochemical dynamics of the cell were investigated through CV in a voltage window of 0.001-3.0 V at different scan rates. Then, to distinguish the capacity control and diffusion control processes in the CV curve, a graph was plotted between the peak current (i) and the scan rate (v) during reduction/oxidation according to the following power law relationship.

i = av ^b (1)

log(i) = b log(v) + log(a) (2)

where a and b are variables and their values determine whether the process is capacitive or diffusion controlled. For example, if b approaches 1.0, the electrochemical reaction process is mainly controlled by the capacity contribution. However, if b converges to 0.5, the diffusion process dominates the charge/discharge process. The b values are determined using the slopes of the log(i) vs log(v) plots at different cathodic and bipolar peaks. The calculated b values at different redox peaks for the samples are shown in Fig. 11(b). The b values of Peaks 1, 2, 3 and 4 of the S250 nanofiber were calculated as 0.85, 0.82, 0.72, and 0.83, respectively, which are close to 1.0. This suggests a dominant capacitive behavior during the redox process. In general, the capacitive effect of the electrode material is closely related to the reaction rate. That is, the higher the ratio of the capacity control process, the better the transmission speed and the better the speed characteristics. Therefore, to quantitatively analyze the capacitive contribution to the current response, the total charge stored in the electrode material was separated into capacitive control and diffusion limiting processes using the following equations.

i = k ₁ v + k ₂ v ^1/2

where k ₁ v and k ₂ v ^1/2 represent the contribution of the capacitive effect and diffusion control process, respectively. i(V)/v _1/2 vs. By plotting v _1/2 , we can determine the constants k1 and k2 from the slope and intercept, respectively. Fig. 11(C) shows the capacitive contribution of S250 nanofibers (k ₁ v , indicated by the red shaded area) at a scan rate of 10 mV s ⁻¹ . S250 nanofibers exhibit the highest proportion for capacitive processes, i.e. 82%, with high b values close to unity, whereas S200, S300 and bare solid Co ₃ O ₄ nanofibers have lower proportions for capacitive processes (e.g. S200 68% for S300, 44% for S300 and 14% for bare solid Co ₃ O ₄ nanofibers). The capacitive contribution of S250 nanofibers was calculated at various scan rates as shown in FIG. 11(d). S2 50 nanofibers showed 45%, 46%, 48%, 51%, 56%, 64% and 82% contributions in the capacitive effect at 1.0, 1.2, 1.6, 2.0, 3.0, 5.0 and 10.0 mV s ⁻¹ . to provide. The highest proportion of the capacitive control contribution of S250 nanofibers indicates the relatively fast transport kinetics of Li ⁺ ions, which improves the rate performance of the electrode. Hollow cobalt oxide nanospheres coated with rGO-network matrix and NGC promoted fast electron transport by improving the electrical contact between the active sites and electrodes of cobalt oxide nanospheres.

Referring to FIG. 11(e), the excellent Li + ion storage properties of S250 nanofibers are fully charged using deconvolution using a Randle-type equivalent-circuit model. was further confirmed by EIS measurements of the cells after 300 cycles in The depressed semicircle in the high intermediate frequency region represents the charge transfer resistance (R _ct ) of the cell. After the 300th cycle, the R _ct values of S200 and S250 nanofibers were 23 and 25 Ω, respectively, with lower impedance values compared to S300 nanofibers (36 Ω) and bare solid Co ₃ O ₄ nanofibers (37 Ω) ( 23 and 25 Ω, respectively). These results demonstrate the high electrical conductivity and structural stability of S200 and S250 nanofibers during repeated lithiation and desorption processes. The porous rGO matrix structure and NGC-coated hollow cobalt oxide nanospheres improve the electrical conductivity and structural stability of the sample.

11(f) shows the relationship between Z _re and ω ^-2/1 in the low-frequency region (where ω is the angular frequency in the low-frequency region (ω=2πf)) S200 and S250 nanofibers shows a less steep slope at low frequencies, indicating that the Li ⁺ ion diffusivity is higher than that of S300 nanofibers and bare solid Co ₃ O ₄ nanofibers.

Figure 12 shows the structural stability of the nanostructures of the samples obtained after 300 cycles. Referring to (a) and (b) of Figure 12, the S200 and S250 nanofibers well maintained the porous nanofiber structure due to high structural stability even after repeated charge/discharge processes at a very high current density of 3.0 Ag ^-1 did. On the other hand, referring to (C) and (d) of Figure 12, S300 nanofibers and bare solid Co ₃ O ₄ nanofibers could not withstand the internal stress due to volume change, and were separated into several pieces after 300 cycles and aggregated. . These results show that S250 nanofibers exhibit excellent lithium storage properties, including excellent speed performance and cycling stability, due to their unique nanostructure. The unique rGO matrix porous structure and hollow cobalt oxide nanospheres relieve volume expansion during the charge/discharge process and allow the liquid electrolyte to penetrate the electrode efficiently. In addition, the NGC layer coated on the hollow cobalt oxide nanospheres serves as the primary transport pathway for electrons, and the rGO network surrounding the nanospheres provides continuous and secondary electron transport pathways, synergistically fast Li ⁺ It can enable ion and electron diffusion and has excellent rate performance.

So far, with respect to the present invention, the preferred embodiments have been looked at. Those of ordinary skill in the art to which the present invention pertains will understand that the present invention may be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments are to be considered in an illustrative rather than a restrictive sense. The scope of the present invention is indicated in the claims rather than the foregoing description, and all differences within the scope equivalent thereto should be construed as being included in the present invention.

Claims (5)

porous nanofiber matrix; and
comprising a structure distributed on the surface of the matrix,
The structure includes nanospheres,
The nano spheres,
a hollow metal oxide;
A composite nanofiber comprising a carbon layer coated on the metal oxide. According to claim 1,
The carbon layer is a composite nanofiber, characterized in that the N-doped graphite carbon layer (N-Doped Graphitic C, NGC). According to claim 1,
The porous nanofiber matrix,
Composite nanofibers containing reduced-grephene oxide (rGO). According to claim 1,
The metal oxide is a composite nanofiber, characterized in that the transition metal oxide. A negative electrode material for a lithium ion battery comprising the composite nanofiber of any one of claims 1 to 4.

Images