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well as safety concerns if/when crossover occurs.Non-aqueous Li-based fl ow batteries utilizing organic redox compounds (e.g., ferrocene-based redox, [ 24,25 ] 2,2,6,6-tetramethylpiperidine-1-oxyl, (TEMPO)-based) [ 8,11 ] as catholytes have been shown an effective approach to increase the power density, [ 24,25 ] cycle life, [ 24,25 ] and energy density of the RFBs. [ 1,6,7,9,24,25 ] However, the low solubility of organic redox compounds has limited further improvement in energy density. Alternatively, a non-aqueous Libased semi-solid fl ow battery was proposed by Duduta et al. [ 26 ] In this concept, insoluble active materials (lithium cobalt oxide (LiCoO 2 ), [ 26 ] lithium iron phosphate (LiFePO 4 ), [ 27 ] silicon [ 28 ] ) are mixed with conducting carbon (e.g. Ketjenblack (KB)) and Li + ion supporting electrolyte to form a suspension, which permits catholyte/anolyte concentration beyond solubility limit and shows much higher achieved volumetric capacity. Along this concept, we have recently reported a sulfur/carbon (S/C) composite semi-solid fl ow battery achieving superior volumetric capacity (294 Ah L −1 catholyte ). [ 29 ] Another important emerging direction in the fi eld is applying redox reactions to mediate and facilitate intercalation or conversion reactions of energy storage materials. [30][31][32] Applying this concept, Jia et al. [ 33 ] have recently demonstrated a high-energy density all redox fl ow lithium battery reaching tank energy density ≈500 Wh L −1 (50% porosity).In this work, we propose a new concept of multiple redox semi-solid-liquid (MRSSL) fl ow battery, which takes advantage of both highly soluble active materials in the liquid phase and high-capacity active materials in the solid phase, to form a biphase MRSSL catholyte. Here, we used liquid LiI electrolyte and solid S/C composite as an example to demonstrate an LiI-S/C MRSSL catholyte, which achieved the highest catholyte volumetric capacity (550 Ah L −1 catholyte ) to date and superior energy density (580 Wh L −1 catholyte+lithium ) with high columbic effi ciency (>95%). We further show that the presence of LiI synergistically facilitates the electrochemical utilization and reduces the viscosity of the catholyte. A continuous fl ow battery system based on the LiI-S/C MRSSL catholyte is demonstrated and the infl uence of fl ow rate and current density on the battery performance will be discussed. The MRSSL concept provides wide-open opportunities for numerous combinations of solid and liquid active materials and offers a new direction for designing next-generation high-energy-density fl ow batteries.

Redox flow batteries are promising technologies for large-scale electricity storage, but have been suffering from low energy density and low volumetric capacity. Here we report a flow cathode that exploits highly concentrated sulphur-impregnated carbon composite, to achieve a catholyte volumetric capacity 294 Ah l À 1 with long cycle life (4100 cycles), high columbic efficiency (490%, 100 cycles) and high energy efficiency (480%, 100 cycles). The demonstrated catholyte volumetric capacity is five times higher than the all-vanadium flow batteries (60 Ah l À 1 ) and 3-6 times higher than the demonstrated lithium-polysulphide approaches (50-117 Ah l À 1 ). Pseudo-in situ impedance and microscopy characterizations reveal superior electrochemical and morphological reversibility of the sulphur redox reactions. Our approach of exploiting sulphur-impregnated carbon composite in the flow cathode creates effective interfaces between the insulating sulphur and conductive carbon-percolating network and offers a promising direction to develop high-energy-density flow batteries.

Organic redox-active materials are promising for redox flow batteries (RFBs) owing to their inherent low-cost, vast abundance, and high structure tunability. However, many organic RFBs suffer from low energy density owing to low solubility. We demonstrate a facile lithium−organic nanocomposite suspension (LIONS) by melting solid organic materials into the void of carbon networks in the semisolid posolyte to achieve high-energy-density Li-RFBs. We demonstrate the first organic-based semisolid Li-RFBs using 10-methylphenothiazine (MPT), delivering a high volumetric capacity (55 Ah L −1 ) and energy density (190 Wh L −1 ) with high Coulombic efficiency (>98%) and cycling stability (>100 cycles). The demonstrated volumetric capacity is 8 fold that of the liquid-MPT RFB, achieving the highest energy density among Li−organic-based RFBs. LIONS is a universal approach to enable the use of lowsolubility organic materials in RFBs, providing a new direction for all insoluble organic active material to achieve low-cost and high-energy-density RFBs.

We propose and demonstrate the concept of a self-mediating redox flow battery that employs inherently present redox shuttles to access charges stored in solid electrode materials without circulating solids. Soluble and inherently present polychalcogenides shuffle electrons between the stack and the tank without circulating sulfur/selenium particles. This strategy promises a high energy density provided by solid materials without circulating solids or using foreign redox mediators, which directly addresses the most critical challenges facing “semisolid” and “redox-targeting” approaches. The full electrochemical reactions are completed via combinations of electrochemical, comproportionation, and disproportionation reactions of inherently present polychalcogenides without circulating solids. A high degree of material utilization (≤99%) and high volumetric capacities (1096–1268 Ah L–1 catholyte) are achieved. This concept can be widely applied to solid redox active materials that naturally exhibit soluble reaction intermediates such as sulfur, selenium, iodine, etc., demonstrating the promising potential of using self-mediation for efficient, high-energy, and low-cost energy storage applications.

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 1.   Q. Huang and Q. Wang, ChemPlusChem, 2014, doi: 10.1002/cplu.201402099. 2.   W. Wang, Q. Luo, B. Li, X. Wei, L. Li and Z. Yang, Adv. Funct. Mater., 2013, 23, 970-986. 3.   C. Menictas and M. Skyllas-Kazacos, J. Appl. Electrochem., 2011, 41, 1223-1232. 4.   Y. Yang, G. Zheng and Y. Cui, Energy Environ. Sci., 2013, 6, 1552-1558. 5.   M. Duduta, B. Ho, V. C. Wood, P. Limthongkul, V. E. Brunini, W. C. Carter and Y.-M. Chiang, Adv. Energy Mater., 2011, 1, 511-516. 6.   H. Chen, Q. Zou, Z. Liang, H. Liu, Q. Li and Y. C. Lu, Nat. Commun., 2014, Accepted. 7.   F. Y. Fan, W. H. Woodford, Z. Li, N. Baram, K. C. Smith, A. Helal, G. H. McKinley, W. C. Carter and Y.-M. Chiang, Nano Lett., 2014, 14, 2210-2218. 8.   Z. F. Deng, Z. A. Zhang, Y. Q. Lai, J. Liu, J. Li and Y. X. Liu, J. Electrochem. Soc., 2013, 160, A553-A558. Figure 1