Microspheres for lithium secondary batteries, method for producing same, negative electrode material composition for lithium secondary batteries including same, and lithium secondary batteries including same

Abstract

Microspheres for lithium secondary batteries, according to the present invention, may each include: silicon (Si) nanocrystals; an amorphous compound including a transition metal and a chalcogen; and carbon nanotubes (CNTs). A method for preparing the microspheres for lithium secondary batteries, according to the present invention, may include the steps of: preparing a mixture by mixing a transition metal salt, a carbon source, and silicon (Si) nanopowder; producing droplets through ultrasonic spraying of the mixture; performing spray pyrolysis on the droplets; performing a first heat-treatment on the microspheres obtained through the spray pyrolysis; performing a secondary heat treatment for chalcogenization on the microspheres obtained through the first heat treatment; and performing a third heat treatment for oxidation on the microspheres obtained through the second heat treatment. A negative electrode material composition for lithium secondary batteries, according to the present invention, may include the microspheres. A lithium secondary battery, according to the present invention, may include the microspheres.

Description

Microspheres for lithium secondary batteries, a method for producing the same, a negative electrode composition for lithium secondary batteries containing the same, and a lithium secondary battery containing the same

[0001] The present invention relates to microspheres for lithium secondary batteries, a method for producing the same, a negative electrode material composition for lithium secondary batteries including the same, and a lithium secondary battery including the same.

[0002] Silicon (Si) has a high gravimetric capacity (4200 mA·h·g) when utilized at low discharge potentials. -1 ) and volumetric capacity (2400 mA·h·cm -3 ) is attracting attention as a potential replacement for conventional graphite anodes. The key to Si's high discharge capacity is its unique chemical properties. Si forms bonds with up to four lithium (Li) atoms, resulting in Li 4.4 Si / Li 22 While graphite can form Si5 alloys, only one Li atom can bond to six carbon (C) atoms (LiC6). Also, Si has a low charge-discharge of about 0.2 V vs. Li / Li. + Si has the advantages of low discharge potential, cost-effectiveness, resource abundance, and environmental friendliness. Despite these advantages, the adoption of Si as an anode material poses a problem: its densely packed crystal structure causes significant volume expansion (approximately 400%) during the charge-discharge process. This expansion can have adverse effects, including pulverization and amorphization of the active Si material within the electrode. Consequently, the active material is lost, and the capacity of the battery decreases over time. Furthermore, another problem arises when the electrolyte decomposition at low potentials leads to the formation of a thick, continuously growing solid electrolyte interphase (SEI). This reduces the electrochemical activity of the electrode, further compromising the cycling performance and overall lifespan of the battery.

[0003] Therefore, there is a need to develop a cathode material that can compensate for the volume expansion of Si, has excellent long-term cycling characteristics and rate characteristics, and has excellent structural stability.

[0004] The purpose of the present invention is to provide a microsphere for a lithium secondary battery having a novel structure, a method for producing the same, a negative electrode material composition for a lithium secondary battery including the same, and a lithium secondary battery including the same.

[0005] In order to achieve the above purpose of the present invention,

[0006] The microspheres for a lithium secondary battery of the present invention may include silicon (Si) nanocrystals; an amorphous compound including a transition metal and a chalcogen; and carbon nanotubes (CNTs).

[0007] The method for producing microspheres for a lithium secondary battery of the present invention may include the steps of: preparing a mixture by mixing a transition metal salt, a carbon source, and silicon (Si) nanopowder; preparing droplets by ultrasonic spraying the mixture; spray pyrolyzing the droplets; first heat-treating the microspheres obtained by the spray pyrolysis; second heat-treating the microspheres obtained by the first heat-treatment for chalcogenization; and third heat-treating the microspheres obtained by the second heat-treatment for oxidation.

[0008] The negative electrode material composition for a lithium secondary battery of the present invention may include the microspheres.

[0009] The lithium secondary battery of the present invention may include the microspheres.

[0010] The porous microspheres of the present invention have very high structural stability and high conductivity, so that they can withstand severe volume changes during repeated cycling and provide conductive paths for faster Li ions, resulting in excellent electrochemical performance.

[0011] Therefore, microspheres can be used in various fields such as secondary batteries, medical devices, and catalysts. Preferably, microspheres according to various embodiments of the present invention can be used as a negative electrode material for a lithium secondary battery (LIB).

[0012] The present invention utilizes spray pyrolysis and heat treatment to produce microspheres with a novel structure. Furthermore, these microspheres with a novel structure can be mass-produced, offering significant potential for industrial application.

[0013] FIG. 1 is a schematic diagram illustrating a microsphere and a method for manufacturing the same according to one embodiment of the present invention.

[0014] Figure 2 shows changes in the morphology and crystal structure of microspheres that have undergone primary heat treatment. (a) is an FE-SEM image, (b) and (c) are TEM images, (d) is a high-resolution TEM (HR-TEM) image, (e) is a selected-area electron diffraction (SAED) pattern, (f) is an XRD pattern, and (g) is an elemental mapping image.

[0015] Figure 3 shows changes in the morphology and crystal structure of microspheres that have undergone secondary heat treatment. (a) and (b) are FE-SEM images, (c) and (d) are TEM images, (e) is a high-resolution TEM (HR-TEM) image, (f) is a selected-area electron diffraction (SAED) pattern, (g) is an XRD pattern, (h) is a Raman spectrum, and (i) is an elemental mapping image.

[0016] Figure 4 shows changes in the morphology and crystal structure of microspheres that have undergone a third heat treatment. (a) and (b) are FE-SEM images, (c) and (d) are TEM images, (e) is a high-resolution TEM (HR-TEM) image, (f) is a selected-area electron diffraction (SAED) pattern, (g) is an XRD pattern, (h) is a Raman spectrum, (i) is a TG curve, and (j) is an elemental mapping image.

[0017] Figure 5(a) shows the cyclic voltammetry (CV) curves for Si / FeSeOx@NGC / N-CNT microspheres during the initial 5 cycles, and (b) shows the CV curves at a current density of 0.1 A g -1 Initial constant current discharge-charge profiles for the microspheres of the examples and comparative examples, (c) and (d) at current densities of 0.5 and 3.0 A / g, respectively. -1 (e) is the result of investigating the cycling performance at various current densities.

[0018] Figure 6 shows the current density at 1.0 A / g. -1 This is the result of evaluating cycling performance.

[0019] Figure 7(a) is 3.0 A g -1 (a) is the Nyquist impedance plot for a fresh cell during cycling, (b) is the Nyquist impedance plot after the 7th cycle, (c) is the Nyquist impedance plot after the 1300th cycle, and (d) is the real part of the impedance Z' versus ω for the low frequency region after the 1300th cycle. -1 / 2 (ω=2πf is the angular frequency), (e) is the FE-SEM image of the microsphere after the 500th cycle, and (f) is the FE-SEM image of the microsphere after the 1300th cycle.

[0020] Hereinafter, various embodiments of this document are described with reference to the attached drawings. The embodiments and terminology used herein are not intended to limit the technology described in this document to specific embodiments, but should be understood to encompass various modifications, equivalents, and / or alternatives of the embodiments.

[0021] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

[0022]

[0023] Microspheres according to various embodiments of the present invention are novel microspheres having a structure that can be applied to lithium secondary batteries. The microspheres of the present invention may include silicon (Si) nanocrystals; an amorphous compound including a transition metal and a chalcogen; and carbon nanotubes (CNTs).

[0024] Silicon nanocrystals can have diameters ranging from 30 to 90 nm.

[0025] The amorphous compound may include a transition metal and a chalcogenide. Specifically, the amorphous compound may be an oxidized compound of a transition metal chalcogenide. The amorphous compound may be an intermediate between a transition metal chalcogenide and a transition metal oxide. That is, the amorphous compound may form heterointerfaces between the chalcogenide phase and the metal oxide phase. The amorphous compound may have a diameter of 10 to 40 nm. The amorphous compound may improve cycle and rate characteristics.

[0026] The transition metal may be at least one selected from the group consisting of Fe, Ni, Co, Cu, Zn, Mo, Ag, Ti, and Pt. The chalcogen may be at least one selected from the group consisting of S (sulfur), Se (selenium), Po (polonium), and Te (tellurium). For example, the amorphous compound may be FeSeOx (wherein x is an integer greater than or equal to 1).

[0027] A thin carbon coating layer can be placed on the exterior of the amorphous compound. This carbon layer may be nitrogen (N)-doped graphitic carbon (NGC). The carbon layer may be amorphous carbon. The carbon layer acts as an electron transport and conduction path, and the coating can improve overall electron / ionic conductivity and ensure rapid charge transfer during redox processes. Furthermore, by maintaining stress within the microspheres, structural stability can be ensured, and electrochemical performance, such as excellent capacity characteristics, can be enhanced.

[0028] Carbon nanotubes may be nitrogen (N) doped. Carbon nanotubes may have an outer diameter of 20 to 30 nm.

[0029] These carbon layers and carbon nanotubes can prevent volume expansion of Si and improve electrical conductivity.

[0030] The microspheres of the present invention may be porous spherical in shape. The microspheres of the present invention may include mesopores with an average pore diameter of approximately 40 nm. This porous structure not only facilitates efficient electrolyte penetration but also mitigates volume expansion.

[0031] Therefore, the microspheres of the present invention can be used in various fields such as secondary batteries, medical devices, and catalysts. Preferably, the microspheres according to various embodiments of the present invention can be used as a negative electrode material for a lithium secondary battery (LIB).

[0032]

[0033] Hereinafter, a method for manufacturing microspheres for lithium secondary batteries according to various embodiments of the present invention will be described.

[0034] A method for manufacturing microspheres for a lithium secondary battery according to various embodiments of the present invention may include the steps of: preparing a mixture by mixing a transition metal salt, a carbon source, and silicon (Si) nanopowder; preparing droplets by ultrasonic spraying the mixture; spray pyrolyzing the droplets; first heat-treating the microspheres obtained by the spray pyrolysis; second heat-treating the microspheres obtained by the first heat-treatment for chalcogenization; and third heat-treating the microspheres obtained by the second heat-treatment for oxidation.

[0035] The transition metal salt may be at least one salt selected from the group consisting of Fe, Ni, Co, Cu, Zn, Mo, Ag, Ti and Pt.

[0036] The carbon source may include at least one selected from the group consisting of sucrose, dextrin, citric acid, ethylen glycol, poly ethylen glycol, PVP (Polyvinylpyrrolidone), PEDOT (Polyethylenedioxythiophene), PAN (Polyacrylonitrile), PAA (polyacrylic acid), PVA (polyvinylalcohol), PMMA (polymethyl methacrylate), PVDF (polyvinylidene fluoride), PVac (polyvinylacetate), PS (polystyrene), PVC (polyvinylchloride), PEI (polyetherimide), PBI (polybenzimidasol), PEO (polyethyleneoxide), PCL (poly e-caprolactone), PA-6 (polyamide-6), PTT (polytrimethylenetetraphthalate), PDLA (poly D,L-lactic acid), polycarbonate, polydioxanone, polyglycolide, and dextran.

[0037] A mixture can be prepared by mixing a transition metal salt, a carbon source, and silicon (Si) nanopowder, and then the mixture can be ultrasonically sprayed to produce droplets.

[0038] Next, the droplets generated by ultrasonic atomization can be subjected to spray pyrolysis.

[0039] Next, the microspheres obtained after the spray pyrolysis process can be subjected to a primary heat treatment. The primary heat treatment step can expose the microspheres obtained through spray pyrolysis to dicyanodiamide (DCDA) powder. The primary heat treatment step can be performed at 700 to 900°C under a N2 atmosphere in the presence of DCDA, which acts as a carbon source. Through this, high-density N-CNTs intricately intertwined with the microspheres can be formed.

[0040] In the secondary heat treatment step, chalcogenization may be performed using at least one powder from the group consisting of sulfur (S), selenium (Se), polonium (Po), and tellurium (Te). The secondary heat treatment step may be performed at 200 to 400°C under a H2 / Ar mixed gas (5:95 = vol%). Through this, a transition metal chalcogen compound phase may be formed.

[0041] The third heat treatment step can be performed at 200 to 300°C in an air atmosphere. This process can cause the transition metal chalcogenide compound to undergo a phase transformation into an amorphous phase. Furthermore, mesopores can be formed, resulting in a porous structure.

[0042] The present invention utilizes spray pyrolysis and heat treatment to produce microspheres with a novel structure. Furthermore, these microspheres with a novel structure can be mass-produced, offering significant potential for industrial application.

[0043]

[0044] Meanwhile, with reference to FIG. 1, the formation mechanism of microspheres according to one embodiment of the present invention will be described in more detail.

[0045] First, referring to ① of Fig. 1, the precursor colloidal solution consisting of Si nanopowder, Fe nitrate, and PVP was uniformly dropletized by an ultrasonic atomizer. Fe nitrate served as an Fe precursor for the formation of the FeSeOx phase, while the amphiphilic PVP simultaneously served as a carbon source for forming the NGC matrix and a surface functionalizer for the Si nanopowder. The functionalization induced by PVP resulted in uniform dispersion of Si in the solution. The generated precursor droplets were pyrolyzed in a vertical quartz reactor maintained at 700°C using N2 as a carrier gas. During pyrolysis, the N-rich species present in the PVP decomposed into the N-doped AC matrix, while the Fe2O3 nanoparticles were formed by the O-containing nitrate (NO) of the precursor salt. 3- ) was formed due to the presence of anionic groups and carboxyl groups of PVP. As shown in ② of Fig. 1, through this process, microspheres containing Si and Fe2O3 nanoparticles uniformly embedded within the AC matrix (referred to as Si / Fe2O3 / AC microspheres) were formed.

[0046] During the first heat treatment, the Si / Fe2O3 / AC precursor microspheres were exposed to DCDA at 800°C under a N2 atmosphere. This step reduced the Fe2O3 nanoparticles to metallic Fe nanocrystals. The surrounding PVP-derived N-doped AC matrix restricted the particle growth and aggregation of the metallic Fe nanocrystals, while the matrix itself was converted to NGC due to the catalytic effect of the Fe nuclei. The NGC matrix served as the primary pathway for rapid electron transfer. Furthermore, N-CNTs were grown on the microsphere surface using the metallic Fe nanocrystals, which acted as a secondary conductive path, as a catalyst. Referring to ③ in Fig. 1, these N-CNTs were formed through catalytic chemical vapor deposition (CCVD) utilizing CHx and NH3 gases generated from DCDA.

[0047] Referring to ④ of Fig. 1, in the second heat treatment step, a selenization reaction was performed at 300°C under a H2 / Ar (vol=5:95%) atmosphere. Excess Se was also used in a similar manner. In this step, a phase transition from metallic Fe to FeSe2 occurred in the H2Se atmosphere formed by the combination of H2 and Se. Through this process, Si / FeSe2@NGC-AC / N-CNT microspheres composed of two-phase Si-FeSe2 nanocrystals surrounded by a NGC-AC matrix together with well-grown N-CNTs were formed.

[0048] Referring to ⑤ of Fig. 1, the final step involved a third heat treatment at 300°C in an air atmosphere. During the oxidation process, the FeSe2 phase transformed into an amorphous FeSeOx phase. Simultaneously, AC was selectively removed from the carbon matrix, creating mesopores with an average pore diameter of approximately 40 nm, which improved overall electrochemical performance. This porous structure not only facilitates efficient electrolyte penetration but also mitigates volume expansion.

[0049]

[0050] Hereinafter, the present invention will be described in detail by way of examples. However, the following examples are only intended to illustrate the present invention, and the present invention is not limited to the following examples.

[0051]

[0052] Example: Fabrication of Si / FeSeOx@NGC / N-CNT

[0053] Porous microspheres comprising two-phase silicon-amorphous iron selenite (Si / FeSeOx) nanocrystals enclosed within an N-doped graphitic carbon (NGC) matrix with well-grown and highly entangled N-CNTs were prepared via a simple spray pyrolysis technique and a multi-step heat treatment. For the preparation of the spray solution, 2.0 g of polyvinylpyrrolidone (PVP; DAEJUNG, Mw = 40,000) and 0.15 M of iron(II) nitrate enehydrate (Fe(NO3)3·9H2O; KANTO CHEMICAL CO., INC., 98.5%) were dissolved in 200 mL of distilled water and stirred for 1 h. Then, 2.0 g of Si nanopowder (Φ = 40-120 nm, LIOYANG TONGRUN INFO Technology. Co., Ltd., > 99%) was added to the solution, and the mixture was sonicated for 3 h and then stirred vigorously for 24 h. The solution containing Si / Fe-salt / PVP was transferred to an ultrasonic atomizer connected to a vertical quartz reactor. The atomizer generated aqueous droplets that were transported through the quartz reactor preheated to 700 °C, and N2 was supplied at a rate of 10 L min. -1 was used as a carrier gas at a flow rate of . After this, the powder obtained from spray pyrolysis was heated in a N2 atmosphere at 800℃ for 5 hours, 5℃ min -1 The first heat treatment process was performed at a ramp rate of . Through this, highly entangled N-CNTs were grown from the powder, which was placed in an alumina crucible with a small alumina boat filled with dicyanodiamide (DCDA, Sigma-Aldrich, 99%) and covered with a lid. The weight ratio of the prepared powder and the DCDA powder was maintained at 1:10. After the first heat treatment, the resulting powder was heated at 300 ℃ for 6 h (heating rate 5 ℃ min) under a H2 / Ar mixed gas (5:95 = vol%). -1) The formation of FeSe2 phase within the structure was promoted through a second heat treatment step for selenization. Se powder (SAMCHUN, 99.5%) was used as the source of selenium gas during the process. Finally, Si / FeSeOx@NGC / N-CNT microspheres were obtained through a third heat treatment step of oxidation at 250°C for 3 hours in an air atmosphere.

[0054]

[0055] Comparative example: Fabrication of Si / FeSeOx@NGC and Si / AC microspheres

[0056] For comparison, microspheres composed of two-phase silicon-amorphous iron selenite (Si / FeSeOx) nanocrystals enclosed within an NGC matrix but without N-CNTs were also prepared. These microspheres, denoted Si / FeSeOx@NGC, were synthesized using the same spray pyrolysis and heat treatment procedures as in the previous example. The powder obtained after spray pyrolysis was subjected to a two-step heat treatment, i.e., selenization and oxidation under the same conditions as in the previous example. As a result, Si / FeSeOx@NGC microspheres were obtained.

[0057] Additionally, microspheres composed solely of Si nanoparticles within an AC matrix (i.e., without amorphous FeSeOx, NGC matrix, and N-CNT) were prepared. These microspheres, denoted as Si / AC, were prepared via a one-pot spray pyrolysis process. The spray solution for Si / AC microspheres was prepared by adding 2.0 g of PVP and 2.0 g of Si nanopowder to 200 mL of distilled water. The mixture was sonicated for 3 h and then vigorously stirred for 24 h. The temperature of the thermal reactor and the flow rate of N2 carrier gas were 700 °C and 10 L min, respectively. -1 was maintained. Through this, Si / AC microspheres were obtained.

[0058]

[0059] Experimental Example 1 - Morphology Observation

[0060] The morphology of the microspheres obtained at each step of the example was observed, and the results are as shown in FIGS. 2, 3, and 4.

[0061] First, Si / Fe2O3 / AC microspheres obtained by spray pyrolysis were subjected to a primary heat treatment at 800°C for 5 h under a N2 atmosphere in the presence of DCDA, which served as a carbon source. After this, high-density N-CNTs intricately intertwined with the microspheres were formed. The changes in the morphology and crystal structure of the microspheres subjected to the primary heat treatment can be confirmed in Fig. 2. As can be seen in the FE-SEM image shown in Fig. 2 (a), the spherical shape of the microspheres remained unchanged after the treatment. However, the surface of the microspheres, which were completely covered with well-grafted N-CNTs, was significantly modified due to the reaction of CHx and NH3 gases generated during DCDA decomposition with the metallic Fe catalyst. In particular, the Fe2O3 nanocrystals within the microspheres were reduced to metallic Fe nanoparticles through a carbon reduction process.

[0062] The TEM image in Fig. 2(b) reveals the presence of well-grown N-CNTs. The high-magnification TEM image in Fig. 2(c) shows that the N-CNTs have an outer diameter of approximately 20-30 nm and a length of up to several hundred nanometers. In addition, as can be seen in Fig. 2(c), metallic Fe nanoparticles (Φ = 14 nm) were clearly identified together with Si nanoparticles (Φ = 40 nm) within the CNT wall (thickness of ~2.5 nm).

[0063] The high-resolution TEM (HR-TEM) image in Fig. 2(d) shows lattice fringes with a spacing of 0.21 nm corresponding to the (110) planes of metallic Fe nanoparticles. In addition, an NGC layer with a lattice fringe distance of 0.34 nm corresponding to the (002) planes of GC surrounding the Fe nanoparticles was also observed. Lattice fringes with a spacing of 0.31 nm corresponding to the (111) planes of Si nanocrystals were also found. The NGC layer was generated by graphitization of PVP-derived N-doped carbon species through the catalytic effect of metallic Fe. The N-doped CNTs and GC matrix can enhance the overall electrical conductivity of the nanostructure due to their smaller atomic radius and higher electronegativity compared to carbon. In addition, the N-CNTs well-grafted with the NGC layer can serve as primary and secondary transport pathways for rapid and sustained electron transfer, respectively, thereby supporting rapid redox processes in the electrochemical process.

[0064] The selected-area electron diffraction (SAED) pattern in Fig. 2(e) supports the above results by showing well-resolved diffraction rings of pure Si, metallic Fe, and carbonaceous materials (CNT and NGC) in the nanostructure.

[0065] Similarly, the XRD pattern in Fig. 2(f) shows diffraction peaks associated with pure Si, metallic Fe, and carbon, which are consistent with the HR-TEM and SAED results. Furthermore, the average crystallite sizes of Si and metallic Fe were calculated using the Scherrer equation and were found to be 36 nm and 15 nm, respectively. Meanwhile, low-intensity diffraction peaks of Fe2O3 due to surface oxidation of metallic Fe were also observed.

[0066] The element mapping image in Fig. 2 (g) shows a homogeneous distribution of Si, Fe, O, C, and N elements in the microspheres, confirming the formation of Si / Fe@NGC-AC / N-CNT. Meanwhile, a trace amount of O element was also found, which is due to surface oxidation of the metallic Fe nanoparticles.

[0067] The above Si / Fe@NGC-AC / N-CNT was subjected to an additional selenization process (secondary heat treatment process) at 300°C for 6 hours under a mixed gas of H2 / Ar (vol = 5:95%), and the structural and phase changes of the microspheres after the second heat treatment process are as shown in Fig. 3.

[0068] Referring to Figures 3(a) to (c), the overall structure of the microspheres with well-grafted N-CNTs was maintained even after heat treatment.

[0069] Referring to the enlarged TEM image of Fig. 3(d), metallic Fe nanoparticles (Φ = 20 nm) were observed inside the N-CNT together with Si nanoparticles (Φ = 60 nm). In addition, the FeSe2 phase was successfully formed through the reaction of H2Se gas generated by the combination of H2 and Se precursors during the selenization process with metallic Fe and Fe2O3.

[0070] The HR-TEM image in Fig. 3(e) shows lattice fringes spaced at 0.19 nm, corresponding to the (112) plane of the FeSe2 crystal lattice. Moreover, the NGC layer surrounding the FeSe2 nanoparticles was clearly observed, and lattice fringes spaced at 0.31 nm, corresponding to the (111) plane of the Si nanocrystals, were also found.

[0071] The SAED patterns in Fig. 3(f) and the XRD patterns in Fig. 3(g) support the aforementioned results, as they exhibit diffraction rings and diffraction peaks of FeSe2, Si, and carbon, respectively. In addition, the crystallite size of FeSe2 in the Si / FeSe2@NGC-AC / N-CNT microspheres was calculated using the Scherrer equation and was confirmed to be 32 nm.

[0072] Raman spectroscopy was used to analyze the crystal properties of the carbonaceous material in Si / FeSe2@NGC-AC / N-CNT microspheres. Referring to Fig. 3(h), the peaks at 187, 240, and 292 cm corresponding to Se-Se bonds -1 506 cm , which represents the peak of Si-Si bonding. -1 A very strong peak appeared at 1348 cm. Additionally, -1 and 1586 cm -1 Characteristic peaks were observed in , which are assigned to the D-band and G-band of carbonaceous materials, respectively. The relative intensity ratio (RIR) of the D and G-bands (I D / I G ) is used as an indicator of the crystallinity of the carbonaceous matrix. A high I of 1.1 for Si / FeSe2@NGC-AC / N-CNT microspheres D / I G The value indicates an increase in disorder, which is mainly due to the low crystallinity of the nanostructures due to the presence of abundant N atom doping sites in the carbonaceous matrix.

[0073] Consistent with the previous results, the elemental mapping image in Fig. 3(i) shows a uniform dispersion of Si, Fe, Se, C, and N elements in Si / FeSe2@NGC-AC / N-CNT, indicating the formation of selenide together with the highly conductive N-doped carbon matrix. In addition, the absence of O element indicates a complete phase transformation from Fe2O3 to FeSe2 during the selenization process, which is consistent with the XRD results in Fig. 3(g).

[0074] The Si / FeSeOx@NGC / N-CNT microspheres were synthesized through an oxidation process (third heat treatment process) at 250°C for 3 hours under an air atmosphere. By proceeding at a temperature of 250°C, FeSeO x Selective removal of AC from the phase formation and NGC matrix can be achieved.

[0075] Figure 4 shows the results of physical property analysis of the final manufactured Si / FeSeOx@NGC / N-CNT microspheres. Figures 4 (a) and (b) show that the spherical shape with highly entangled N-CNTs is maintained even after the multi-step heat treatment process including oxidation.

[0076] TEM images of Figs. 4(c) and (d) show FeSeOx nanoparticles (Φ = 20 nm) resulting from the oxidation of FeSe2, and these nanoparticles were located on each N-CNT, which is consistent with the results confirmed previously in Figs. 2(c) and 3(c).

[0077] Additionally, the enlarged TEM image in Fig. 4(d) reveals the presence of Si nanoparticles with an average size of 80 nm. Meanwhile, no lattice patterns corresponding to FeSeOx nanoparticles were observed, suggesting an amorphous phase. This suggests that an amorphous FeSeOx phase was formed as an intermediate between FeSe2 and Fe2O3.

[0078] The HR-TEM image in Fig. 4(e) shows lattice fringes of 0.31 nm corresponding to the (111) plane of Si. In addition, lattice fringes corresponding to the NGC layer surrounding the FeSeOx nanoparticles can also be clearly identified. Referring to the SAED pattern in Fig. 4(f) and the XRD pattern in Fig. 4(g), only the diffraction patterns related to Si and NGC are shown. The XRD pattern was different from that of the Si / FeSe2@NGC-AC / N-CNT microspheres in Fig. 3(g), mainly due to the absence of the FeSe2 diffraction peak and the reduced intensity of the Si peak reflecting the presence of the amorphous FeSeOx phase.

[0079] Referring to Fig. 4(h), the presence of amorphous FeSeOx within the nanostructure was further confirmed by Raman spectroscopy, which corresponds to Se-Se and Fe-O bonds at 248 and 296 cm -1 It exhibited characteristic peaks of low intensity at 518, 1344, and 1579 cm -1 The prominent peaks are attributed to the Si-Si bonds of Si nanoparticles and the D-band and G-band of carbonaceous materials, respectively. Compared with Si / FeSe2@NGC-AC / N-CNT microspheres, I D / I G As can be seen from the slight decrease in value to 1.0, it was confirmed that AC was selectively removed within the carbonaceous matrix through the optimized oxidation process.

[0080] Meanwhile, to further confirm the combustion of AC and the formation of FeSeOx phase, the TG curve was observed as shown in (i) of Fig. 4. Referring to Fig. 4(i), the TG curve showed a weight loss up to 150°C due to the evaporation of water within the nanostructure, and a stable period continued until 310°C. This suggests the formation of an amorphous FeSeOx phase during oxidation. In addition, the weight loss region between 310°C and 610°C (Δm = -51 wt%) is due to the sublimation of SeO2 and the combustion of the N-CNT and NGC matrix.

[0081] Meanwhile, Table 1 below shows the elemental analysis (EA) results of Si / FeSeOx@NGC / N-CNT microspheres according to the examples and Si / FeSeOx@NGC and Si / AC microspheres according to the comparative examples.

[0082] SampleCarbon(wt%)Nitrogen(wt%)Si / FeSeOx@NGC / N-CNT442.9Si / FeSeOx@NGC1.70.3Si / AC5.91.3

[0083] *

[0084] According to Table 1 above, the estimated C and N contents for the Si / FeSeOx@NGC / N-CNT microspheres were confirmed to be 44 and 2.9 wt%, respectively, which further supports the TG results of the preceding Fig. 4(i).

[0085] Figure 4(j) shows the element mapping results of Si / FeSeOx@NGC / N-CNT microspheres, showing a homogeneous distribution of Si, Fe, Se, O, C, and N elements, which confirms the successful formation of Si and FeSeOx within the N-doped carbon matrix.

[0086]

[0087] Experimental Example 2 - Observation of Electrochemical Characteristics

[0088] The Si / FeSeOx@NGC / N-CNT microspheres according to the examples were assembled as anodes inside CR2032 coin cells, and a series of electrochemical property analyses were performed. Meanwhile, for comparison, cells using Si / FeSeOx@NGC and Si / AC microspheres according to the comparative examples as anodes were also prepared.

[0089] Figure 5(a) shows 0.1 mVs within the voltage range of 0.001 to 3.0 V. -1 Cyclic voltammetry (CV) curves for the Si / FeSeOx@NGC / N-CNT microspheres during the first five cycles obtained at a scan rate of . In the first cathodic sweep, the weak and broad region at 2.01 V corresponds to the insertion of Li ions into the amorphous iron selenide of FeSeOx to form lithium iron selenide as an intermediate. The subsequent broad peak at 1.36 V indicates that lithium iron selenide is converted to metallic Fe and Li2Se with the activation of the intermediate iron oxide. The broad reduction peak at 0.63 V is due to the conversion of iron oxide to metallic Fe and Li2O and the formation of a stable SEI layer through electrolyte decomposition. In addition, the sharp peak at 0.01 V is attributed to the amorphous Li 15 This means the formation of Si4 alloy phase and the insertion of Li ions into the NGC matrix.

[0090]

[0091] *During the first anodic scan, a weak and broad oxidation region of 0.19–0.50 V was observed for Li 15This indicates that Si4 is phase transformed into amorphous Si and Li ions are extracted from the NGC matrix. The broad peak at 1.64 V signifies the electrochemical reaction of Li2O and metallic Fe to form Fe2O3 phase. The subsequent peaks at 1.87 and 2.51 V signify the formation of FeSe intermediate from metallic Fe and Li2Se, followed by the formation of FeSe2 and Li ions from the FeSe intermediate, respectively. From the second cycle onwards, a slightly different redox process was observed as an amorphous FeSe2 / Fe2O3 phase was formed, which is distinct from the amorphous FeSeOx heterointerfacial phase. The broad peaks at 1.95 V and 1.22 V represent the reaction of FeSe2 and Li ions to form metallic Fe and Li2Se, whereas the tight peaks at 0.86 and 0.70 V correspond to the reaction of Fe2O3 and Li ions. The corresponding anodic peaks were also observed during the first anodic sweep. Additionally, the broad and unclear nature of the peaks associated with the Fe compound may be due to the amorphous nature of this compound. The cathodic and anodic peaks of Si at 0.16 / 0.01 and 0.34 / 0.53 V, respectively, became more pronounced and increased in intensity due to the gradual activation of Si during subsequent cycles.

[0092] Based on the above description, the overall reaction mechanism related to the discharge and charge processes of Si / FeSeOx@NGC / N-CNT microspheres can be summarized as follows.

[0093]

[0094] FeSe2+ xLi + + xe - ↔ Li x FeSe2(1)

[0095] Li x FeSe2+ (2-x)Li + + (2-x)e - ↔ FeSe + Li2Se (2)

[0096] FeSe + Li + + 2e -↔ Fe + Li2Se (3)

[0097] Fe2O3+ 6Li + + 6e - ↔ 2Fe + 3Li2O (4)

[0098] Si + xLi + + xe - ↔ Li x Si (5)

[0099] 4Li x Si + (14-4x)Li + + (14-4x)e - ↔ Li 15 Si4(6)

[0100]

[0101] To verify the CV results, as shown in Fig. 5(b), the current density was 0.1 A g -1 The initial constant current discharge-charge profiles for the microspheres of the examples and comparative examples were confirmed.

[0102] The plateaus where the electrochemical reactions occurred were consistent with the peak positions recorded in the CV curves. For Si / FeSeOx@NGC / N-CNT microspheres, ambiguous charge and discharge voltage profiles were obtained at different voltage potentials, which were associated with the multistep reactions of the amorphous FeSeOx phase and Li ions. The initial discharge / charge capacities of Si / FeSeOx@NGC / N-CNT, Si / FeSeOx@NGC, and Si / AC microspheres were 1954 / 1125, 1854 / 1209, and 3571 / 3001 mA·h·g, respectively. -1, and the initial Coulombic efficiencies (ICEs) were 58, 65, and 84%, respectively. Compared with the ICE of Si / AC microspheres, Si / FeSeOx@NGC / N-CNT and Si / FeSeOx@NGC showed slightly lower ICE values, which was attributed to the high initial irreversible capacity loss due to the presence of the conversion-based anode material (FeSeOx). The ICE for the Si / FeSeOx@NGC / N-CNT anode was low during the first cycle but immediately increased to 96% after the fourth cycle of activation and maintained a high Coulombic efficiency (CE) value (>99%) throughout the cycles. This indicates the formation of a stable SEI layer. Overall, the CV and initial discharge-charge voltage profiles suggest that the structural advantages induced by the Si / FeSeOx@NGC / N-CNT microspheres enhanced the lithium ion redox kinetics and improved the battery performance.

[0103] To verify the CV and initial voltage profile results, the cycling and rate performances of Si / FeSeOx@NGC / N-CNT, Si / FeSeOx@NGC, and Si / AC microspheres were evaluated. Referring to Figs. 5(c), (d), and 6, the cycling performances of the cells with each microsphere applied were measured at current densities of 0.5, 1.0, and 3.0 A / g. -1 was investigated in 0.5 A g -1 Before evaluating the cycling characteristics, the activation step was 0.1 A g for two consecutive cycles. -1 As observed, the Si / FeSeOx@NGC / N-CNT anode exhibited stable cycling performance up to 400 cycles, with a high CE of 99.8% and a peak current of 954 mA hg -1 It exhibited a reversible discharge capacity of 85% with an average decay rate of 0.038% per cycle.

[0104] The excellent cycling performance of the cell using the Si / FeSeOx@NGC / N-CNT anode is attributed to the synergistic effect among the various components. For example, after the initial discharge / charge process, FeOx / FeSex nanoparticles with heterointerfaces were formed from the FeSeOx nanoparticles. Thereafter, the Si and FeOx / FeSex nanoparticles showed stable cycling performance by mitigating each other's volume expansion due to different redox reaction voltages during the charge / discharge process. Additionally, the NGC matrix enhanced the structural integrity of the nanostructure by suppressing the volume expansion of Si and Fe species. The anode containing Si can still be separated or delaminated from the Cu current collector due to the volume stress during the discharge / charge process. Therefore, to prevent the complete separation of a part of the damaged structure, in the present invention, N-CNT was introduced onto the surface of the Si / FeSeOx@NGC microspheres, thereby enabling stable and sustained electrochemical reactions. However, the discharge capacity of Si / FeSeOx@NGC microspheres decreased rapidly until the 50th cycle because of the absence of N-CNTs and the lack of an NGC matrix to suppress the volume expansion of electroactive materials such as Si and Fe species.

[0105] The cell using Si / FeSeOx@NGC anode had a discharge capacity of 80 mA·h·g at the end of the 400th cycle. -1 , and the CE showed 99.7%, resulting in a capacity retention of only 8% at an average decay rate of 0.23% per cycle. Similarly, Si / AC microspheres exhibited a high theoretical capacity of Si (4200 mA hg -1) showed the highest capacity initially. However, the Si / AC microspheres showed poor cycling performance because they lacked carbonaceous materials and FeSeOx nanoparticles that could accommodate the volume stress caused by Si nanoparticles. As a result, the Si / AC microsphere anode showed a capacity of 430 mA·h·g at the end of the 400th cycle (99.0% CE). -1 It showed a reversible capacity of . The capacity retention rate was only 18% with an average decay rate of 0.21% per cycle.

[0106] As can be seen in Fig. 6, 1.0Ag -1 Similar capacity trends were observed at high current densities of 1.0 A g -1 To evaluate the cycling performance at current densities of 0.1, 0.2 and 0.5 A g -1 Multiple activation steps were performed for two cycles each. After the 700th cycle, the Si / FeSeOx@NGC / N-CNT anode was operated at 615 mA·h·g -1 The discharge capacity and CE of 99.5% and capacity retention of 99% were maintained, and the average capacity decay rate was only 0.0021%. In contrast, the Si / FeSeOx@NGC and Si / AC anodes maintained a discharge current of 68 and 4 mA hg, respectively, after the 700th cycle. -1 It showed discharge capacity of 15% and capacity retention rate of 0.2%.

[0107] Also, referring to Fig. 5(d), Si / FeSeOx@NGC / N-CNT microspheres have a 3.0Ag -1 It exhibits excellent cycling performance even at high current densities of 0.1, 0.5 and 1.0 A g. -1 After the activation step, the cells using Si / FeSeOx@NGC / N-CNT, Si / FeSeOx@NGC and Si / AC anodes showed good performance at 806, 81 and 5 mA hg after the 1500th cycle. -1The reversible discharge capacity of Si / FeSeOx@NGC / N-CNT microspheres was shown. The initial capacity of Si / FeSeOx@NGC / N-CNT microspheres was particularly high at 3.0 A g -1 It increased during the first 50 cycles, induced by the activation process of the electrode material at high current densities of 0.5 and 1.0 A g -1 Compared with the cycling performance of Si / AC anode, Si / AC anode exhibited lower discharge capacity despite the high theoretical capacity of Si. This is due to the low intrinsic electrical conductivity of Si, especially at 3.0 A g -1 This is because the redox kinetics are poor at high current densities. During cycling at different current densities, the Si / FeSeOx@NGC / N-CNT anode exhibited a gradual increase in discharge capacity with repeated cycling. This is due to the expansion of the heterointerface in the form of FeOx / FeSex crystal structure during the insertion and deintercalation of lithium ions. As cycling progresses, this expansion promotes the insertion and deintercalation of Li ions, resulting in a higher discharge capacity. In addition, the expanded heterointerface of the electroactive materials induces the formation of a stable polymer gel-like film in the form of a reversible SEI layer, which led to fast redox kinetics and increased capacity.

[0108] Referring to Figure 5(e), to further verify the structural advantage, the speed performance of the microspheres was tested from 0.1 to 30 Ag. -1 The Si / FeSeOx@NGC / N-CNT microspheres were evaluated at various current densities of 0.1, 0.2, 0.5, 1.0, 2.0, 3.0, 5.0, 7.0, 10, 15, 20, 25, and 30 A g -1 During the 10th cycle at current densities of 1155, 1029, 885, 747, 600, 524, 459, 416, 356, 277, 233, 203, and 179 mA hg, respectively. -1 It showed a discharge capacity of . The current density was 0.1 A g -1When returned to the capacity is 1006 mA hg -1 (87% retention). In contrast, the cells using Si / FeSeOx@NGC and Si / AC anodes showed good performance at 0.1, 0.2, 0.5, 1.0, 2.0, and 3.0 A g -1 At current densities of 486 / 2390, 189 / 2153, 90 / 1552, 57 / 849, 33 / 208 and 24 / 8 mA hg -1 It showed poor rate characteristics with a discharge capacity of 3.0 A g -1 At higher current densities, Si / FeSeOx@NGC and Si / AC anodes exhibited nearly zero capacity due to structural instability and slow redox reaction kinetics. Moreover, for Si / AC microspheres, the current density was 0.1 A g -1 Even when returned to the original state, the capacity is not recovered, indicating that the structure has completely collapsed, especially at high current densities.

[0109] The superior rate performance of Si / FeSeOx@NGC / N-CNT microspheres compared to Si / FeSeOx@NGC and Si / AC microspheres originates from the synergistic effect of the highly entangled N-CNT and NGC matrix. This effect promotes efficient and rapid charge transfer by improving the electrical contact between the FeOx / FeSex active sites and Si nanoparticles within the electrode. In addition, the microporous structure and heterointerface due to the amorphous FeSeOx facilitated the penetration of the electrolyte and enhanced the diffusion rate of Li ions, thereby enhancing the redox reaction rate.

[0110] To verify the excellent Li ion transport properties of Si / FeSeOx@NGC / N-CNT microspheres, electrochemical impedance spectroscopy (EIS) measurements were performed on each sample, and the results are shown in Fig. 7. Nyquist plots were obtained before and after cycling in a fully charged state.

[0111] Referring to Fig. 7(a), the Nyquist plot for the fresh cells showed slightly higher solution resistance (Rs) values ​​(45-47Ω) for Si / FeSeOx@NGC and Si / AC microspheres, indicating subtle differences in the electrode-electrolyte reactions. In addition, the charge transfer resistance (Rct) values ​​for Si / FeSeOx@NGC / N-CNT, Si / FeSeOx@NGC, and Si / AC microspheres were 199, 495, and 346Ω, respectively. The Si / FeSeOx@NGC / N-CNT microspheres showed a lower Rct value compared to the other samples due to the higher electrical conductivity of N-CNT and NGC.

[0112] Referring to Fig. 7(b), after the 7th cycle, the Rct values ​​of all samples decreased due to the formation of the SEI layer and the activation of Si particles after cycling, which improved the charge transfer kinetics. Referring to Fig. 7(c), after the 1300th cycle, the Rct values ​​of Si / FeSeOx@NGC / N-CNT, Si / FeSeOx@NGC, and Si / AC microspheres were 20, 37, and 112 Ω, respectively. In the case of Si / FeSeOx@NGC / N-CNT microspheres, the presence of N-CNT and NGC matrix helped to relieve the stress caused by the volume expansion of the microspheres, and the structure was maintained robustly even after the 1300th cycle. The higher Rct value of Si / AC microspheres compared to the other samples is confirmed to be due to the structural instability of the microspheres caused by the volume expansion of Si particles.

[0113] Referring to Fig. 7(d), to further analyze the Nyquist plot, the real part of the impedance Z' for the low frequency region after the 1300th cycle for all samples was plotted as ω -1 / 2(ω = 2πf is the angular frequency) was plotted. As a result, Si / FeSeOx@NGC / N-CNT microspheres showed a gentle slope at low frequency, which indicates a higher Li ion diffusion coefficient of the electrode than Si / FeSeOx@NGC and Si / AC microspheres. Li ion diffusion coefficient (D Li+ ) was calculated from the plot shown in Fig. 7(d) using the following equation.

[0114]

[0115] D Li+ = 0.5R 2 T 2 / A 2 F 2 C 2 σ w 2

[0116]

[0117] D here Li+ is the lithium ion diffusion coefficient, R is the gas constant, T is the temperature, A is the electrode area, C is the lithium ion concentration, F is the Faraday constant, σ w is the Warburg impedance factor. D of Si / FeSeOx@NGC / N-CNT microspheres Li+ The value is 3.3 × 10 -12 cm 2 s -1 , which is Si / FeSeOx@NGC ( 1.6 × 10 -13 cm 2 s -1 ) and Si / AC microspheres (1.5 × 10 -13 cm 2 s -1 ) was higher than that of the conventional lithium-ion diffusion process. This is because not only did the high electrical conductivity enhance the electron transfer dynamics, but also the lithium-ion diffusion process was improved even after extended cycling due to various synergistic effects that maintained the structural robustness.

[0118] 3.0 A g -1After cycling, the FE-SEM micrographs of the cycled electrodes were observed. As a result, referring to Fig. 7(e), after the 500th cycle, the Si / FeSeOx@NGC / N-CNT microspheres maintained their original spherical shape. In particular, the presence of highly entangled N-CNTs in the microspheres was also clearly confirmed. The N-CNT and NGC matrix effectively accommodated the volume expansion during cycling, preventing structural collapse. Meanwhile, referring to Fig. 7(f), significant changes were observed after 1,300 consecutive cycles. For example, dense, large crystals were observed without obvious N-CNTs due to the significant volume expansion of the active material. In addition, the spherical shape of the microspheres was still maintained, confirming excellent structural robustness.

[0119]

[0120] The present invention has been described above, focusing on preferred embodiments thereof. Those skilled in the art will appreciate that the present invention can be implemented in modified forms without departing from its essential characteristics. Therefore, the disclosed embodiments should be considered illustrative rather than limiting. The scope of the present invention is set forth in the claims, not the foregoing description, and all differences within the scope equivalent thereto should be construed as being encompassed by the present invention.

[0121] The porous microspheres of the present invention have very high structural stability and high conductivity, and thus can be used in various fields such as secondary batteries, medical devices, and catalysts.

[0122] The method for manufacturing porous microspheres of the present invention can manufacture microspheres of a novel structure through spray pyrolysis and heat treatment, and can mass-produce microspheres of such novel structures, so it has high potential for industrial application.

Claims

1. Silicon (Si) nanocrystals; Amorphous compounds containing transition metals and chalcogens; and Microspheres for lithium secondary batteries containing carbon nanotubes (CNTs).

2. In paragraph 1, Microspheres for lithium secondary batteries, characterized in that they further include a carbon layer surrounding the amorphous compound.

3. In paragraph 1, Microspheres for lithium secondary batteries, characterized in that they further include a nitrogen (N)-doped carbon layer surrounding the amorphous compound.

4. In paragraph 1, The above transition metal is at least one selected from the group consisting of Fe, Ni, Co, Cu, Zn, Mo, Ag, Ti and Pt, Microspheres for a lithium secondary battery, characterized in that the chalcogen is at least one selected from the group consisting of S (sulfur), Se (selenium), Po (polonium), and Te (tellurium).

5. In paragraph 1, Microspheres for lithium secondary batteries, characterized in that the amorphous compound is FeSeOx.

6. In paragraph 1, The above carbon nanotube is a microsphere for a lithium secondary battery, characterized in that it is doped with nitrogen (N).

7. In paragraph 1, Microspheres for lithium secondary batteries, characterized in that the above microspheres are porous.

8. A step of preparing a mixture by mixing a transition metal salt, a carbon source, and silicon (Si) nanopowder; A step of producing droplets by ultrasonic spraying the above mixture; A step of spray pyrolysis of the above droplets; A step of first heat treating the microspheres obtained by the above spray pyrolysis; A step of performing a second heat treatment for chalcogenization of the microspheres obtained through the first heat treatment; and A method for manufacturing microspheres for a lithium secondary battery, comprising a step of performing a third heat treatment for oxidation on the microspheres obtained through the above-mentioned second heat treatment.

9. In paragraph 8, A method for producing microspheres for a lithium secondary battery, characterized in that the above transition metal salt is at least one salt selected from the group consisting of Fe, Ni, Co, Cu, Zn, Mo, Ag, Ti, and Pt.

10. In paragraph 8, A method for producing microspheres for a lithium secondary battery, characterized in that the first heat treatment step comprises exposing microspheres obtained by spray pyrolysis to dicyanodiamide (DCDA) powder.

11. In paragraph 8, A method for manufacturing microspheres for a lithium secondary battery, characterized in that the secondary heat treatment step is performed by chalcogenization using at least one powder from the group consisting of S (sulfur), Se (selenium), Po (polonium), and Te (tellurium).

12. In paragraph 11, A method for manufacturing microspheres for a lithium secondary battery, characterized in that the above third heat treatment step comprises oxidation in an air atmosphere.

13. A negative electrode composition for a lithium secondary battery comprising microspheres according to any one of claims 1 to 7.

14. A lithium secondary battery comprising microspheres according to any one of claims 1 to 7.

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