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Thrust 4b: Develop New Concepts for Large-Scale Energy Storage and Conversion Rather than storing energy in solid electrode materials, energy can instead be stored in redox species dissolved or suspended in a liquid phase, such as redox flow batteries.25 In a redox flow battery, the cathode and anode materials consist of aqueous or non-aqueous electrolyte solutions (catholytes and anolytes) in which the energy is stored. The anolyte and catholyte are pumped through porous electrodes at each side of a cell stack, where they are separated by an ion-exchange membrane or porous separator to prevent crossover of the active species, while the electrochemical redox reactions occur on the electrode surfaces. (See also the “Flow Systems” sidebar in Panel 1 Report.) The unique architecture and working mechanism allow the energy and power to be controlled independently. The power is defined by the size and design of the electrochemical cell (the stack) whereas the energy depends on the concentration of redox species and the size of the external tanks in which they are stored. In this research thrust, concepts related to the storage of energy within liquid electrolytes will be discussed. Inspiration from Flow Systems for Novel Chemistries: The unique architecture of redox flow batteries may provide viable solutions to issues inherent in other next generation battery chemistries. As an example, the Li-sulfur battery system, based on the reaction between S8 and Li (Figure 2.4.2),26 represents a promising energy storage technology due to high energy density and low cost. However, this battery chemistry falls short of expected performance due to dissolution of lithium polysulfide species, which are intrinsically insulating materials, and the depletion of electrolyte after long-term cycling. Since polysulfides are highly soluble in both water and ether-based solvents, partially liquid batteries based on polysulfides have been explored with the goal of achieving extremely low cost and long cycle life.27-29 This unique battery design holds promise for addressing the issues of the conventional Li-S battery and can be scaled up for large applications. The key challenges that remain for liquid batteries based on the Li-S chemistry is to understand the solution chemistry and chemical speciation, and to control the dissolution, nucleation, and precipitation cycles. Similarly, aqueous polysulfide-based batteries have also been proposed to circumvent the issue of organic solvents. In a typical aqueous Li-S battery design, for example, electrochemical potential control is employed to take advantage of the reaction between Li2S4 and Li2S, both of which are soluble in water.30 The higher ionic conductivity of aqueous electrolyte is also appealing to provide high power density. However, the compatibility between the aqueous catholyte and the organic electrolyte in the anode, as well as Li metal protection, still remains to be addressed. Li metal anode Li2S2 Li2S Li2Sn S8 Li2S8 Li2S6 Li2S4 Charge E/V 2.5 2.0 1.5 1.0s8 Li2S2 Li2S Discharge Li2S4 Li2S2 Li2S Polysulfide shuttle Li2Sn-2 Figure 2.4.2. Working principle of conventional lithium-sulfur battery, which involves the formation of soluble lithium polysulfides and their diffusion in the liquid electrolyte. From Ref. 26. Copyright Elsevier, 2014. NEXT GENERATION ELECTRICAL ENERGY STORAGE Porous carbon/S cathode Discharge PRIORITY RESEARCH DIRECTION – 4 59PDF Image | Next Generation Electrical Energy Storage
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