Energy storage is increasingly seen as a valuable asset for electricity grids composed of high fractions of intermittent sources, such as wind power or, in developing economies, unreliable generation and transmission services. However, the potential of batteries to meet the stringent cost and durability requirements for grid applications is largely unquantified. We investigate electrochemical systems capable of economically storing energy for hours and present an analysis of the relationships among technological performance characteristics, component cost factors, and system price for established and conceptual aqueous and nonaqueous batteries. We identified potential advantages of nonaqueous flow batteries over those based on aqueous electrolytes; however, new challenging constraints burden the nonaqueous approach, including the solubility of the active material in the electrolyte. Requirements in harmony with economically effective energy storage are derived for aqueous and nonaqueous systems. The attributes of flow batteries are compared to those of aqueous and nonaqueous enclosed and hybrid (semi-flow) batteries. Flow batteries are a promising technology for reaching these challenging energy storage targets owing to their independent power and energy scaling, reliance on facile and reversible reactants, and potentially simpler manufacture as compared to established enclosed batteries such as lead-acid or lithium-ion. Broader context Cost-effective electrochemical energy storage has the potential to dramatically change how society generates and delivers electricity. A few key market opportunities include supporting high fractions of intermittent renewable energy sources and deferring upgrades of existing electricity grid infrastructure. Unfortunately, present state-of-the-art technologies are too expensive for broad deployment. Reductions in manufacturing costs and associated overheads are identied as the single largest cost-savings opportunity for today's battery-based storage options. In addition, increasing production volume and market competition will lead to lower material costs. Both aqueous and nonaqueous ow batteries are promising technology platforms capable of achieving the low costs required for widespread implementation. Non-aqueous systems enable higher cell voltages than their aqueous counterparts but also require higher active material solubility to offset higher electrolyte costs. For both battery types, a key enabling development will be the discovery of tailored molecules that are long lived, provide large cell voltages, and have costs similar to existing commodity chemicals.

Symmetric flow cell cycling of a soluble phenothiazine.

Redox-active organic materials (ROMs) have shown great promise for redox flow battery applications but generally encounter limited cycling efficiency and stability at relevant redox material concentrations in nonaqueous systems. Here we report a new heterocyclic organic anolyte molecule, 2,1,3-benzothiadiazole, that has high solubility, a low redox potential, and fast electrochemical kinetics. Coupling it with a benchmark catholyte ROM, the nonaqueous organic flow battery demonstrated significant improvement in cyclable redox material concentrations and cell efficiencies compared to the state-of-the-art nonaqueous systems. Especially, this system produced exceeding cyclability with relatively stable efficiencies and capacities at high ROM concentrations (>0.5 M), which is ascribed to the highly delocalized charge densities in the radical anions of 2,1,3-benzothiadiazole, leading to good chemical stability. This material development represents significant progress toward promising next-generation energy storage.

Simple modification of N-ethylphenothiazine (left) with electron-donating substituents (right) increases the molecular charge-storage capacity of this donor.