A Sulfur-Impregnated Flow Cathode for High-Energy Lithium Flow Batteries
Authors: Chen Hongning , Zou Qingli , Liang Zhuojian , Liu Hao , Li Quan , Lu Yi-Chun 
Published: February 2020

Introduction

   Redox flow batteries are promising energy storage technologies but have been suffering from low energy density and low volumetric capacity1, 2. Increasing the energy density of RFBs has been one of the major research efforts for RFBs, which can significantly increases its competitiveness for both stationary and transportation applications3-5.

In this work,6we employ sulfur-impregnated carbon (S/C) composite as a flow cathode to achieve high-energy lithium-flow batteries. Pseudo-in situ electrochemical impedance spectroscopy (EIS) are used to reveal superior electrochemical reversibility of the sulfur redox reactions. Our approach of exploiting sulfur-impregnated carbon composite in the flow cathode offers a promising direction to develop high-energy flow batteries.

Results and Discussion

Figure 1 shows the first discharge/charge profiles and the scanning electron microscope/energy-dispersive X-ray spectroscopy (SEM/EDX) images of  a mechanically-mixed sulfur-carbon suspension with 5.0 vol% sulfur-12.0 vol% Ketjen black (5S-12KB-MM, 3.2 M [S]) and a sulfur-impregnated S/C composite suspensions with 5.0 vol% sulfur-12.0 vol% Ketjen black (5S-12KB, 3.2 M [S]). The first gravimetric discharge capacity of the mechanically-mixed 5S-12KB-MM catholyte (700 mAh/gS, 71 Ah/L) is lower than that of the S/C composite 5S-12KB catholyte (1235 mAh/gS, 128 Ah/L) by ~50%. This suggests that the utilization of sulfur in the 5S-12KB catholyte is enhanced by uniformly intermixing of S and C using sulfur impregnation. The SEM/EDX image of the 5S-12KB catholyte shows that the S and C atoms are evenly distributed and overlapped across the composite suspension. On the other hand, the S and C atoms are separated in the mechanically mixed 5S-12KB-MM suspension.6 The influences of the concentration of carbon and the concentration of sulfur on the electrochemical behavior/performance of the S/C catholyte in Li-suspension cells will be discussed.   

To examine the reversibility of various electrochemical processes of the S/C catholyte, we employed pseudo-in situ EIS measurement as the reaction proceeds. Figure 2 shows the discharge and charge steps of the 20.0 vol% sulpfur-26.0 vol% Ketjen black (20S-26KB, 12.9 M [S]) Li-suspension cells  inserted with five EIS measurements (D1 – D5) during discharge and  four EIS measurements (C1 – C4) during charging.7, 8. The high-frequency-intercept has been attributed to the cell ohmic resistance, the middle-frequency semi-circle has been attributed to the interfacial resistance/capacitance of the catholyte and the low-frequency semi-circle has been attributed to the charge-transfer-resistance/pseudo capacitance of the catholyte. First, the ohmic resistance of the catholyte is the smallest and is insensitive to the any steps. Second, the interfacial resistance first decreased (D1–D4) and dramatically increased upon full discharge (D5). This suggests that the interfacial resistance decreases by transforming the insulating solid sulfur to soluble polysulfides, which improves the contacts between sulfur species and carbon, but significantly increases due to the formation of insulating film-like Li2S. The interfacial resistance decreased after charging to polysulfide phase (C1-C3) and remained small at the end of the charging. Finally, no significant increase in the charge-transfer resistance was noticed at early stage of discharge (D1-D3) until the later stage of discharge (D4-D5), which is attributed to the formation of insulating Li2S solids, which blocks the charge transfer. Reversibly, the charge-transfer resistance decreases upon charging, which is attributed to the formation of polysulfides from decomposing the insulating Li2S solid.6 Further investigations on the electrochemical reversibility, energy efficiency, cycle life, single cell design, and flow cell performance at various flow rates will be discussed.

Acknowledgments

   The work described was substantially supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China, under Theme-based Research Scheme through Project No. T23-407/13-N, and partially supported by project RNEp1-13 of the Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong.

References

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Figure 1