Sodium Sulfur Flow Battery for Low Cost Electrical Storage
DOI: 10.1002/aenm.201701991 is proportional to the size of the tank. The decoupled system energy and power makes the battery easy to scale up. However, the energy density of the aqueous solution is limited by the solubility of the redox species (e.g. 1 2 m for vanadium redox flow battery, VRFB) and the electrochemical window of water (1.5 V) which leads to relatively low energy density and contributes to their high system cost.[2b,c] Promising new approaches to increase the energy density of the reactants include semi solid flow batteries (SSFBs) using suspensions of solid storage compounds as flow electrodes,[3] and several new classes of solution reactants including lithium polysulfide solutions,[4] all organic redox couples,[5] and ionic liquid based complexes.[6] Among these new approaches, SSFBs have the potential to overcome both limitations of aqueous solution energy density.[3d,7] Semi solid suspension is composed of nanoscale carbon forming continuous conductive network and densely packed larger solid storage materials particles suspended in electrolytes. SSFB approach can significantly increase the system energy density by storing energy in solid phase instead of liquid phase that increases nominal molar concentration of redox active species and expanding electrochemical window when nonaqueous solvent is used in the electrolyte. Another aforementioned promising battery technology for grid storage, high temperature Na S battery, has many appealing features such as low cost raw materials, high theoretical specific energy, high energy efficiency, and good cycle life.[8] Traditionally, these batteries operate at high temperatures (300 350 degC) In addition to incurring significant cost in thermal management, many other safety and reliability issues arise from the high operating temperature, e.g. the corrosion of current collectors and battery casing, the degradation of solid electrolyte membrane (typically made of b' Al2O3, abbreviated as BASE) and potential for explosion when the membrane is damaged. To resolve these issues, a number of groups have studied room temperature Na S batteries using polymeric membranes and organic electrolytes. While exciting progresses have been made, these batteries often suffer from problems such as chemical self discharging due to the shuttling mechanism, formation of metallic sodium dendrites, and the poor utilization of the sulfur cathodes.[9] An alternative strategy of mitigating the issues of high temperature and room temperature Na S batteries is to operate the battery at intermediate temperature. While this concept was first introduced in the 1980,[10] it received significant attention only quite recently due A new sodium sulfur (Na S) flow battery utilizing molten sodium metal and flowable sulfur based suspension as electrodes is demonstrated and analyzed for the first time. Unlike the conventional flow battery and the high temperature Na S battery, the proposed flow battery system decouples the energy and power thermal management by operating at different temperatures for the storage tank (near room temperature) and the power stack (100 150 degC) The new Na S flow battery offers several advantages such as easy preparation and integration of the electrode, low energy efficiency loss due to temperature maintenance, great tolerance of the volume change of the metal anode, and efficient utilization of sulfur. The Na S flow battery has an estimated system cost in the range of $50 100 kWh 1 which is very competitive for grid scale energy storage applications.