Nickel Hollow Spheres Concatenated by Nitrogen‐Doped Carbon Fibers for Enhancing Electrochemical Kinetics of Sodium–Sulfur Batteries

Abstract The high energy density of room temperature (RT) sodium–sulfur batteries (Na‐S) usually rely on the efficient conversion of polysulfide to sodium sulfide during discharging and sulfur recovery during charging, which is the rate‐determining step in the electrochemical reaction process of Na‐S batteries. In this work, a 3D network (Ni‐NCFs) host composed by nitrogen‐doped carbon fibers (NCFs) and Ni hollow spheres is synthesized by electrospinning. In this novel design, each Ni hollow unit not only can buffer the volume fluctuation of S during cycling, but also can improve the conductivity of the cathode along the carbon fibers. Meanwhile, the result reveals that a small amount of Ni is polarized during the sulfur‐loading process forming a polar Ni—S bond. Furthermore, combining with the nitrogen‐doped carbon fibers, the Ni‐NCFs composite can effectively adsorb soluble polysulfide intermediate, which further facilitates the catalysis of the Ni unit for the redox of sodium polysulfide. In addition, the in situ Raman is employed to supervise the variation of polysulfide during the charging and discharging process. As expected, the freestanding S@Ni‐NCFs cathode exhibits outstanding rate capability and excellent cycle performance.

2 be prepared in the same way for comparing with the same mass loading of sulfur. Finally, in order to detect the sodium storage property of nickel sulfide in the S@Ni-NCFs composite, sulfur was removed from S@Ni-NCFs by using CS 2 solution. The products were labeled as Ni-NCFs/S. Preparation of Na 2 S 6 solution. 320 mg of S and 156 mg of sodium sulfide (Na 2 S) were mixed and added into 10 mL tetraglyme (TEGDME) solvent. Then, the suspension was stirred at 80 °C for 6 h to get a reddish-brown Na 2 S 6 solution.

Material Characterization
The morphologies of the samples were investigated by field-emission scanning microscope (FESEM, JSM-7800F, Japan) and transmission electron microscopy (TEM, JEM-2100, Japan). The EDS spectroscopy attached to FESEM was employed to record the elemental distribution. The crystal structures were detected through powder X-ray diffraction (XRD, MAXima-X XRD-7000) with Cu Kα radiation (λ = 1.5406 Å). The sulfur and carbon contents in the prepared composites were determined by Thermogravimetric analyzer (TGA, Q50, USA). The BET specific surface area and pore structure were tested by Brunauer-Emmett-Teller method (BET, Quantachrome Instruments, USA). The Thermo Scientific ESCALAB 250Xi electron spectrometer was applied to collect the X-ray photoelectron spectroscopy (XPS) spectra. In addition, ex-situ Raman was recorded by using Invia Refl (Renishaw, UK) from 100 to 3000 cm -1 , while the in-situ Raman was recorded by Lab-RAM HR Evolution (Horiba, EL-CELL in Germany) Raman microscope with a computer controller (CHI 660D).

Assembly and measurement of Na-S batteries
All electrochemical measurements were studied at room temperature by assembling CR2032 coin cells in an argon glove-box. The S@Ni-NCFs and S@NCFs wafers with an area load of about 0.5 ~ 0.7 mg cm -2 were directly used as working electrodes without adding any conductive additives or PVDF binder. In this system, the sodium foil was served as both the counter and reference electrode, and the glass fiber membrane (Whatman GF/A) was acted as the separator. Meanwhile, 1 M NaClO 4 dissolved in tetraethylene glycol dimethyl ether (TEGDME) was used as the electrolyte and the dosage was 90 µL. After stewing for 8 h, the coin cells started galvanostatically charging and discharging within the voltage ranging from 0.5 to 2.8 V on a Land cycler (1C = 1675 mAh g -1 , Wuhan Kingnuo Electronic Co, China).
Cyclic voltammograms (CV) curves were performed at a scan rate of 0.1 mV s -1 using Arbin Instruments. Electrochemical impedance spectroscopy (EIS) was tested by Zahner electrochemical workstation.

Assembly and measurement of symmetrical batteries
Two identical disk electrodes (Ni-NCFs or NCFs) without sulfur load were both used as anode and cathode for assembling into a standard CR2032 coin cell. Among it, the glass fiber membrane was employed as separator, then 40 μL of Na 2 S 6 (0.2 mol L -1 ) and 50 μL of blank electrolyte (1 M NaClO 4 in TEGDME) were added. The CV curves were performed at a scan rate of 50 mV s -1 between -0.8 V and 0.8 V, while the EIS were measured on Zahner electrochemical workstation.

Calculated diffusion coefficient of sodium ions (D Na+ )
The diffusion coefficients of sodium ions (D Na+ ) in S@Ni-NCFs and S@NCFs electrodes can be calculated according to the following equations: In equation (1), R, T, n, F, A, and C represent gas constant, absolute temperature, the number of electrons transferred per mole during oxidation, Faraday constant, effective area of work electrode, and Na+ concentration in cathode material, respectively. While in equation (2), R is a resistance parameter representing the combination of solution resistance and charge transfer resistance. From equation (1) and equation (2), it can be found that the D Na+ is only associated with the value of σ, which can be obtained by plotting Z' vs. ω -1/2 and the slop is σ.        [9] 21.5 % 1 M NaClO 4 +PC+FEC 100 mA g -1 800 / S@iMCHS [14] 46 % 1.0 M NaClO 4 + PC/EC + 5 wt % FEC 100 mA g -1 200 0.056 % micoporous carbon/sulfur MCPS [15] 47 % 1 M NaClO 4 +EC/DEC +SiO 2 -IL-ClO 4 0.5 C 100 0.31 %