커켄달 효과와 주형법을 통해 합성한 α-Fe₂O₃ 중공입자로 구성된 다공성1차원 구조체의 리튬 이차전지 음극활물질 적용

Application of Porous Nanofibers Comprising Hollow α-Fe₂O₃ Nanospheres Prepared by Applying Both PS Template and Kirkendall Diffusion Effect for Anode Materials in Lithium-ion Batteries

Abstract

본 연구는 ${\alpha}-Fe_2O_3$ 중공입자로 구성된 다공성 1차원 나노구조체를 전기방사 공정 및 두단계의 후 열처리 과정을 통해 주형법과 커켄달 효과를 동시 적용하여 합성했다. 열처리 과정 중, 수 nm의 치밀한 Fe 금속입자는 커켄달 효과에 의해 중공구조를 갖는 ${\alpha}-Fe_2O_3$ 입자로 최종 변환되었다. 또한, 전기방사 용액에 첨가한 PS 나노비드는 첫 열처리 과정 중 분해되어 구조체 내 수많은 기공을 형성, 환원 및 산화를 위한 가스들이 구조체 내부로 원활히 침투될 수 있는 역할을 했다. 최종 생성물인 ${\alpha}-Fe_2O_3$ 중공입자로 구성된 다공성 1차원 구조체를 리튬 이차전지의 음극활물질로 적용한 결과, $1.0A\;g^{-1}$의 높은 전류밀도에도 불구하고 30 사이클 후 $776mA\;h\;g^{-1}$의 높은 방전 용량을 나타냈다. 이와 같은 우수한 리튬 저장특성은 본 구조체를 구성하는 중공형 ${\alpha}-Fe_2O_3$ 입자와 입자들 사이의 나노기공으로부터 기인한 결과이다. 본 연구에서 제안한 중공 입자로 구성된 다공성 1차원 나노구조체 합성 방법은 다양한 전이금속 화합물 조성에 적용 가능하므로 에너지 저장 분야를 포함한 여러 분야에 응용 가능하다.

Porous nanofibers comprising hollow ${\alpha}-Fe_2O_3$ nanospheres were prepared by applying both template method and Kirkendall diffusion effect to electrospinning process. During heat-treatment processes, the solid Fe nano-metals formed by initial heat-treatment in the carbon matrix were converted into the hollow structured ${\alpha}-Fe_2O_3$ nanospheres. In particular, PS nanobeads added in the spinning solution were decomposed and formed numerous channels in the composite, which served as a good pathway for Kirkendall diffusion gas. The resulting porous nanofibers comprising hollow ${\alpha}-Fe_2O_3$ nanospheres were applied as an anode material for lithium-ion batteries. The discharge capacities of the nanofibers for the 30th cycle at a high current density of $1.0A\;g^{-1}$ was $776mA\;h\;g^{-1}$. The good lithium ion storage property was attributed to the synergetic effects of the hollow ${\alpha}-Fe_2O_3$ nanospheres and the interstitial nanovoids between the nanospheres. The synthetic method proposed in this study could be applied to the preparation of porous nanofibers comprising hollow nanospheres with various composition for various applications, including energy storage.

Fig. 1. SEM images and XRD pattern of the as-spun nanofibers: (a) low and (b) high-magnification FE-SEM images and (c) XRD pattern.

Fig. 2. SEM images and XRD pattern of the nanofibers after initial heat-treatment at 500 ℃ for 3 h under 5% H₂/Ar atmosphere: (a) low and (b) high-magnification FE-SEM images and (c) XRD pattern.

Fig. 3. FT-IR spectra of the nanofibers (a) before and (b) after initial heat-treatment at 500 ℃ for 3 h under 5% H₂/Ar atmosphere.

Fig. 4. XRD pattern, Morphologies, SAED pattern, and elemental mapping images of the porous nanofibers comprising hollow α-Fe₂O₃ nanospheres: (a) XRD pattern, (b) FE-SEM image, (c) TEM image, (d,e) HR-TEM images, (f) SAED pattern, and (g) elemental mapping images.

Fig. 5. FE-SEM images of the nanofibers prepared using the solution without PS nanobeads: (a, b) as-spun nanofibers, (c, d) nanofibers obtained after initial heat-treatment at 500 ℃ for 3 h under 5% H₂/Ar atmosphere, and (e, f) nanofibers obtained after second heat-treatment at 500 ℃ for 3 h under air atmosphere.

Fig. 7. Electrochemical performance of the porous nanofibers comprising hollow Fe₂O₃ nanospheres and solid structured α-Fe₂O₃ nanofibers: (a), (b) discharge-charge curves and (c) cycling performances at a current density of 1.0 A g^-1.

Fig. 6. (a) N₂ adsorption and desorption isotherms and (b) BJH desorption pore-size distribution of the porous nanofibers comprising hollow α-Fe₂O₃ nanospheres and solid structured α-Fe₂O₃ nanofibers.