Li-ion batteries offer considerable advantages such as high gravimetric and volumetric energy densities, and good calendar life and shelf life compared to aqueous systems such as Pb-acid, Ni-Cd, and Ni-MH. Various Li-ion chemistries with different Li-intercalating cathodes have attained significant maturation for commercial applications, including portable electronics and hybrid electric vehicles. Energy densities of ≈3 to 4 times compared to those of aqueous systems are now possible, yet these systems are inadequate to meet the needs of electric vehicles.
Flow batteries are attractive for stationary applications. The state-of-the-art flow batteries for stationary applications (e.g., zinc-bromine batteries or vanadium flow cells) are grossly inadequate to meet the daunting requirements of large-scale (>1 MW) energy systems, which include long lifetimes, safety, and low cost.
This work focuses on the use of sulfur (polysulfides) as the anode in a flow cell, combined with various highly soluble organometallic complexes with high reduction potentials. Lithium polysulfides are soluble from Li2S8 to Li2S3. The specific capacity for the soluble range of polysulfides is about 900 to 1,000 mAh/g at a voltage of -1.8V. When combined with various organometallic complexes with high reduction potentials of 3.5 to 4.5V, the new flow cells will deliver high specific energy. In addition, they will be inexpensive, safe, and extremely durable.
There are several unique attributes of this flow cell, which is similar to the non-aqueous, high-voltage lithium flow battery. The reaction involves the shuttling of lithium ions from the anolyte to catholyte, much like with traditional Li-ion cells. The energy densities are expected to be high due to high cell voltages and the high degree of solubility of these active materials in typical organic solvents. There is inherent safety in this system, unlike in Li-ion cells. The flow cell exhibits good longevity based on the absence of any interfacial changes at the electrode, and is as highly scalable as any flow cell. All these features will, in principle, contribute to a long cycle life, calendar life, and safety, with low self-discharge rates. Another significant novelty is the use of a low-cost, high-energy sulfur cathode and the low-cost, high-voltage manganese complexes in a flow cell architecture.
Because of their low cost, durability, scalability, and safety, these cells will be excellent candidates for large energy storage systems required in stationary applications. Furthermore, because of their high specific energy, especially in large systems, they will be ideal candidates for energy storage on planetary landers and habitats, as well as deep space missions.