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Synthesis by Electrospinning and Electrochemical Properties of Na2 Fe2 (SO4 )3 Nanofibers as a Cathode Material for Sodium-Ion Batteries Yun-Chae Nam, Seon-Jin Lee and Jong-Tae Son∗ Department of Nano-Polymer Science & Engineering, Korea National University of Transportation, Chungju 27469, Korea (Received 9 December 2019; accepted 21 December 2019) Alluaudite-type Na2+2x Fe2−x (SO4 )3 is a promising cathode material for sodium-ion batteries because of its high voltage, low cost, and feasible synthesis at low temperature. However, it has a low ionic conductivity (1 × 10−7 S/cm) and electrical conductivity (2 × 10−9 S/cm). In this study, we improved the ionic conductivity by using electrospinning to activate ion diffusion. We also improved the electrical conductivity by using poly(vinylpyrrolidone) to form a carbon coating. Scanning electron microscopy revealed that the synthesized material consisted of nanofibers with sizes of approximately 400–800 nm. The surface area was measured by using atomic force microscopy. An X-ray Diffraction analysis was carried out to determine the structure of the Na2 Fe2 (SO4 )3 nanofibers. Additionally, the initial charge-discharge capacities of Na2 Fe2 (SO4 )3 were found to be 22.27 mAh/g and 19.39 mAh/g, respectively, at a current density of 0.1 C-rate. Keywords: Na-ion batteries, Cathode material, Na2+2x Fe2−x (SO4 )3 , Electrospinning DOI: 10.3938/jkps.77.836

I. INTRODUCTION

Lithium-ion batteries (LIBs) are a major component of electronic devices, electrical/hybrid vehicles, and electric energy storage systems. However, lithium resources are limited, which raises lithium’s cost and lowers its availability. Thus, finding an alternative to lithium and developing sustainable secondary batteries are imperative [1–3]. Sodium-ion batteries (SIBs) should be an excellent alternative because of sodium’s abundance and low cost. Furthermore, the electropositive property of Na is close to that of Li. Therefore, SIBs, owing to their cost, safety and sustainability, can replace LIBs [2–4]. High-voltage cathode materials are mainly comprised of layered oxides and polyanion compounds (phosphates, sulfates, pyrophosphates, mixed poly-anions, fluorophosphates, etc.). Among them, sulfate materials are very promising as high voltage cathodes in SIBs, especially when combined with Fe (also an abundant and inexpensive metal) [5,6]. The alluaudite-type Na2+2x Fe2−x (SO4 )3 has been noted for its high potential of Fe3+ /Fe2+ redox couples (3.8 V) and moderate capacity (≈100 mAh/g) since it was first reported by Yamada et al. in 2014. A significant advantage of Na2+2x Fe2−x (SO4 )3 is its low temperature synthesis (≈ 350 ◦ C) [5,7]. Nonetheless, the low ionic conductivity (1 × 10−7 ∗ E-mail:

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pISSN:0374-4884/eISSN:1976-8524

S/cm) and electronic conductivity (2 × 10−9 ) of Na2+2x Fe2−x (SO4 )3 greatly limits its electrochemical performance [8]. Several strategies for improving its ionic and electronic conductivities have been proposed, most of which are based on solid-state, ionothermal, and simu