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Redox Flow Batteries: Fundamentals and Applications 113 http://dx.doi.org/10.5772/intechopen.68752 was first demonstrated with intercalation materials by Chiang et al. [33], which are typically used for lithium ion batteries. Such semi-solid lithium redox flow batteries combine the merits of high energy density for lithium ion batteries and the decoupled character of conventional redox flow batteries. In order to form a percolation network for charge transfer, several strategies have been proposed: (i) dispersing conductive additive such as carbon into the electrolytes, (ii) adding redox mediators and (iii) inserting a metal wire as a current collec- tor [34]. It has been found that the conductive electrolytes encounter the issue of shunt current between cells in a stack. Energy-dense batteries, based on lithiation chemistry and intercalation chemistry of abundant elements (such as Na, Mg and Al etc.), contribute significantly to the transportable applications of various electronic devices and revolution of our modern societies. The successful develop- ment in these materials raises opportunities in new applications for flow batteries. Li-, Na- and organic molecule-based semi-solid redox flow batteries have been developed recently (Table 5) [33–37]. For a pumping system with solid suspension, the rheological properties of suspension need to be considered. In contrast to the flow batteries with both (de)lithiation and electron transfer reactions occurring inside the electrochemical cells (Figure 2d), a new concept using redox shuttle molecules has been introduced [38], wherein solid active materials are kept statically in the tank and only the shuttle molecules are circulated in the electrochemical cell (Figure 4). Electrochemical redox reactions of the shuttle molecules go on at the electrode inside the cell, whereas chemical (de)lithiation of the active solid materials in the tank occurs through the reactions between the solid materials and the shuttle molecules. Since the active solid materials are not involved in the electrochemical reaction, conductive additives (such as carbon black) are not necessary in such a system. In addition, low concentration shuttle molecules of only several mM are sufficient to induce the (de)lithiation reaction of a large amount of solid materials. Semi-solid flow batteries LiCoO2/Li4Ti5O12 LiCoO2/Li4Ti5O12 P2-type NaxNi0.22Co0.11Mn0.66O2/ NaTi2(PO4)3 Symmetric battery with polythiophene Zn/polyaniline Suspension 26 vol% LiCoO2, 0.8 vol% Ketjen; 25 vol% Li4Ti5O12, 0.8 vol% Ketjen Carbon-free 0.5 vol% LiCoO2, 1 vol% Li4Ti5O12 Active material with 1.3 wt% conductive additive Polythiophene (8.41 g L1), Ketjenblack (2 g L1) 10 wt% polyaniline powder in suspension Remarks Ref. C/3 to C/8 rate, high energy efficiency [33] Low current density from 0.002 to 0.008 mA cm2, [34] low coulombic efficiency of about 11.5% Current density below 0.5 mA cm2, low voltage [35] efficiency of about 40%, energy density of about 9 Wh L1 Low current density (<1 mA cm2), energy efficiency [36] of 60.9% at 0.5 mA cm2 0.28 V overpotential at 20 mA cm2 [37] Table 5. Selected examples for semi-solid redox flow batteries.PDF Image | Redox Flow Batteries Fundamentals and Applications
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