Global energy and climate-change concerns have accelerated the electrifi cation of vehicles, aided by advances in battery technology. It is now recognized that low-cost, scalable energy storage will also be key to continued growth of renewable energy technologies (wind and solar) and improved effi ciency of the electric grid. While electrochemical energy storage remains attractive for its high energy density, simplicity, and reliability, existing battery technologies remain limited in their ability to meet many future storage needs. Here we propose and demonstrate a new storage concept, the semi-solid fl ow cell (SSFC), which combines the high energy density of rechargeable batteries with the fl exible and scalable architecture of fuel cells and fl ow batteries. In contrast to previous fl ow batteries, energy is stored in suspensions of solid storage compounds to and from which charge transfer is accomplished via dilute yet percolating networks of nanoscale conductors. These novel electrochemical composites constitute fl owable semi-solid 'fuels' that are here charged and discharged in prototype fl ow cells. Potential advantages of the SSFC approach include projected system-level energy densities that are more than ten times those of aqueous fl ow batteries, and the simplifi ed low-cost manufacturing of large-scale storage systems compared to conventional lithiumion batteries.Demand for batteries of higher energy and power has driven several decades of research in electrochemical storage materials, resulting recently in signifi cant improvements in the stored energy of cathodes and anodes. [ 1 , 2 ] However, most batteries have designs that have not departed substantially from Volta's galvanic cell of 1800, and which accept an inherently poor utilization of the active materials. [ 3 ] Even the highest energy density lithium ion cells currently available, e.g., 2.8-2.9 Ah 18650 cells having > 600 Wh L − 1 , have less than 50 vol% active material. The reduced energy density, along with higher cost, result because the high-energy-storage compounds are diluted by inactive and costly components necessary to extract power (e.g., currentcollector foils, tabs, separator fi lm, liquid electrolyte, electrode binders and conductive additives, and external packaging). Further dilution of energy density, by about a factor of two, occurs between the cell and system level. [ 4 ] Electrode designs that minimize inactive material, bio-and self-assembly, and 3D architectures are new approaches that promise improved design efficiency but have yet to be fully realized. [ 5 -9 ] Decoupling power components from energy-storage components so that stored energy can be scaled independently of power is a strategy for improving system-level energy density. Redox fl ow batteries have such a design, in which active materials are stored within external reservoirs and pumped into an ion-exchange/electron-extraction power stack. [ 10 ] As the system increases in capacity, its energy density may asymptotically approach that of the redox acti...
