Semi‐Solid Lithium Rechargeable Flow Battery

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 effi ciency 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 active solutions. Aqueous-chemistry fl ow batteries are of much current interest for stationary applications due to their scalability, relative safety, and potentially low cost. However, they currently use low energy density chemistries limited by electrolysis to ≈ 1.5 V cell voltage and have low ion concentrations (typically 1‐2 M ), yielding ≈ 40 Wh L − 1 energy density for the fl uids alone. [ 10 ] Furthermore, the large fl uid volumes that must be pumped produce parasitic mechanical losses that detract signifi cantly from round-trip effi ciency. The fl ow-cell’s design advantages are therefore offset by the use of low-energy-density active materials. In a new system we call a semi-solid fl ow cell (SSFC, Figure 1 ), the inherent advantages of a fl ow architecture are retained while dramatically increasing energy density by using suspensions of energy-dense active materials in a liquid electrolyte. [ 11 ] This approach to fl owable electrodes can produce more than 10 times the charge storage density of typical fl ow-battery solutions, due to the much greater energy density inherent to solidstorage compounds. For example, in molarity units the concentration of reversibly-stored lithium in lithium-ion cathodes such as LiCoO 2 , LiFePO 4 , LiNi 0.5 Mn 1.5 O 4 , and 0.3 Li 2 MnO 3 ‐0.7 Li M O 2 ( M = Mn, Co, Ni), and anodes such as Li 4 Ti 5 O 12 , graphite, and Si (assuming ≈ 1000 mAh g − 1 reversible capacity), is 51.2, 22.8, 24.1, 39.2, 22.6, 21.4, and 87 M , respectively. [ 2 ] Assuming a solids content of 50% (up to 70 vol% solids have been achieved in fl suspensions of other materials), the volumetric capacity of the semi-solid suspensions is 5‐20 times greater (e.g., 10 to 40 M ) than that of aqueous redox solutions ( ≈ 2 M). The semi-solid approach may be applied to aqueous chemistries, in which case the volumetric energy density is also 5‐20 times greater since cell voltages remain limited by electrolyte hydrolysis to ≈ 1.5 V. [ 12 , 13 ] When applied to nonaqueous Li-ion chemistries, however, energy density is further increased by another factor of 1.5‐3, in direct proportion to cell voltage. (Energy density is the product of volumetric charge capacity, e.g., in molarity or Ah l − 1 units, and the cell voltage. Specifi c values for the systems studied are given later.) In this work, we demonstrate working prototype SSFCs using fl owable suspensions having up to ≈ 12 M concentration. We show that in addition to energy density advantages, the SSFCs can operate at low fl ow rates with very low mechanical energy dissipation. The design fl exibility inherent in the SSFC approach may enable new use-models for electrical storage, such as rapid refueling