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102 Natl Sci Rev, 2017, Vol. 4, No. 1 REVIEW the regime of Li2 S–Li2 S8 and reach a high energy density [88], unlike Li/S flow batteries in traditional configurations that rely only on soluble polysulfides (Li2 S4 –Li2 S8 ) in non-aqueous electrolytes [42,43]. Yang et al. constructed a metal-free, all-organic, semi-solid flow cell. They used polythiophene mi- croparticles as both anolyte and catholyte active couples with a cell potential of 2.5 V. The active polythiophene microparticles were dispersed in 1 M TEABF4–PC solution and circulated through the flow cell. It showed stable charge/discharge perfor- mance, with a high EE of 60.9% at 0.5 mA/cm2 [89]. Similarly to a Li-ion battery, Ventosa et al. reported proof-of-concept for a non-aqueous, semi-solid flow battery based on Na-ion chemistry using P2-type Nax Ni0.22 Co0.11 Mn0.66 O2 and NaTi2 (PO4 )3 as pos- itive and negative electrodes, respectively, although the energy density of the cell was around 9 Wh/L [90]. Similarly to the traditional RFB, the E/P ratio can be tuned in the design of a semi-solid flow battery to reduce the cost. In addition, low-cost active materi- als in powder form and low-cost carbon-conductive materials can be used. The battery-manufacturing approach can be similar to RFBs, which can be very different from the manufacturing approach for tra- ditional Li-ion batteries [91]. A typical lithium-ion battery-manufacturing process starts with metal foil, and then layers liquid ‘ink or paint’ on it to form its electrodes; these steps are followed by drying and calendering. These processes need to be conducted in dry rooms or protected atmospheres. However, for a semi-solid flow battery, the manufacturing pro- cess can be simplified with the focus on prepara- tion of semi-solid flowable inks. The cost of semi- solid flow systems was predicted to be less than 50% of that of Li-ion battery and could be less than $100/kWh [91]. The semi-solid flow battery still needs to be care- fully engineered and optimized. Scale-up of the sys- tem could still be difficult. Current RFBs depend on multiple reaction stacks to increase the energy and power capacity. In a semi-solid system, the elec- trolyte (i.e. a suspension) needs to be conductive enough to transfer the electrons. Conductive elec- trolyte systems cannot be used in multiple stacks be- cause of the shunt current through the electrolyte connecting multiple series-connected cells in a stack driven by the voltage difference between the cells. In addition, the chemistry and the physical properties of the suspension need to be carefully controlled. A high solid loading is normally desired to achieve high energy, but the high suspension viscosity could con- tribute to pumping-related energy losses. As a result, it is necessary to operate in either stoichiometric or intermittent flow mode. It has been suggested that either of these operating modes is capable of reduc- ing pumping energy loss to <1% [92]. In traditional Li-ion batteries, performance de- pends on the formation of stable solid electrolyte in- terphase layers on the solid electrode materials [93]. Such stable interfaces and interphases may not exist in semi-flow battery configurations. The long-term stability of the electrode materials and the efficiency of such systems need to be carefully evaluated. PROSPECTIVE Overall, discussions so far suggest that aqueous re- dox batteries with highly soluble, low-cost materi- als have potential for ultra-low-cost solutions. The aqueous system is also a safer option. In 2014, the DOE released the Energy Storage Safety Strategic Plan [94]. Validation of energy-storage safety and reliability has attracted significant attention. Several safety concerns should be addressed, such as re- lease of the stored energy during an incident, cas- cading failure of battery cells, fires, etc. Therefore, an aqueous flow battery is a prime candidate because of its safety, reliability and low cost. Redox materi- als, such as highly soluble iodine, polysulfide and all- organic materials, have demonstrated that low-cost, aqueous redox batteries also can achieve high energy or high power. Still, in these studies, stable and op- timized redox couples have not been demonstrated for practical battery applications. In addition, there is great potential in rational de- sign of organic redox couples based on lessons from solid-state polymer electrode materials through sys- tematically tuning the redox potential, conductiv- ity, stability and solubility for both aqueous and non-aqueous systems. Hybrid battery designs in- volving either metal anodes or particulate suspen- sions should be investigated further. Such hybrid concepts begin to blur the boundary between tradi- tional batteries (e.g. Li-ion battery) and RFBs, but the metal anode must be protected to prevent the metal from reacting with the electrolyte or forming dendrites [95]. There also are some non-traditional chemistries and technologies that appear to have potential to compete with Li-ion batteries in terms of energy density [96]. A very careful examination revealed that the electrolyte actively participated in electro- chemical reactions in these systems. In this case, en- ergy density is not entirely determined by the ca- pacity of the solid-state electrode materials. Rather, it can be dictated by the concentrations of the re- dox species in the electrolytes. Therefore, such non- traditional chemistries actually behave more like the redox batteries we have discussed in this article. 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