What do you get when you cross a rechargeable battery with a fuel cell? The answer, according to researchers at the Massachusetts Institute of Technology, is a semi-solid flow cell (SSFC), which combines the high energy density of rechargeable batteries with the advantages of fuel cells.
Writing in a recent edition of the journal Advanced Energy Materials, researchers say they store energy in suspensions of solid storage compounds that charge up and discharge through networks of nanoscale conductors. Potential advantages of the SSFC approach include energy densities more than ten times those of aqueous flow batteries, and simplified low-cost manufacturing resembling that of large-scale storage systems.
The researchers point out that most battery designs aren't much different from Volta’s galvanic cell of 1800, and as such have make inherently poor use of active materials. Even the highest energy density lithium ion cells currently available have less than 50 vol% active material.
The problem, explain the MIT researchers, arises because high-energy-storage compounds in batteries are diluted by inactive components needed to get power out: current-collector foils, tabs, separator film, liquid electrolyte, electrode binders and conductive additives, and external packaging. Energy density also gets cut by about a factor of two between the cell and system level.
One way to improve this situation is to decouple power components from energy-storage components. Some battery technologies do, in fact, take this approach. However, most of them are aqueous-chemistry batteries which are hampered by low energy density chemistries limited by electrolysis to a cell voltage of ≈1.5 V. Furthermore, they generally entail pumping large fluid volumes that produce parasitic mechanical losses detracting significantly from round-trip efficiency.
The researchers say their SSFC dramatically boosts energy density by using suspensions of energy-dense active materials in a liquid electrolyte. This approach can produce more than 10 times the charge storage density of typical flow-battery solutions because there is much greater energy density inherent to the solid-storage compounds they use.
Researchers say their SSFC design should provide lower materials and manufacturing cost than conventional lithium ion battery technology when scaled up. At near-term costs of $10 to $15/kg for active materials and $14/kg for nonaqueous electrolyte, the semi-solid suspensions they use have an energy-specific cost of $40 to $80/kWh depending on the specific chemistry, which leaves substantial room to get system-level cost targets of $250/kWh and $100/kWh for transportation and grid level storage, respectively. Researchers say a simplified and streamlined manufacturing process would use prepared semi-solids to fill assembled systems, bypassing many of the unit operations (e.g., electrode coating, calendering, slitting and tabbing, cell assembly, module assembly) and associated capital equipment costs of current battery manufacturing.
Researchers put together a working prototype SSFC that not only demonstrates a high energy density but also can operate at low flow rates with low mechanical energy dissipation. They say the design approach may enable new use-models for electrical storage, such as rapid refueling of vehicles by fuel or fuel tank exchange, tuning of suspensions as needed for power, energy, and operating temperature, and extension of service life by renewing suspension chemistry.
To transfer charge from the active material particles to the current collectors of the cell, researchers say they used particle aggregation behavior to produce novel, electrochemically active composites. By testing numerous nanoscale carbons, researchers found that what they call percolating networks form in typical nonaqueous lithium-conducting electrolytes at particle concentrations less than 1 vol%. Into this structure they added micrometer-scale particles of electrode-active cathodes and anodes. We found that besides ‘wiring’ the active material for charge transfer the nanoscale conductor network stabilizes the larger particles from settling out of suspension.
In one of their tests, researchers used a LiCoO2 suspension that continuously circulated at 20.3 mL/min through a single channel half-flow-cell. Single-channel flow cells were machined from 101 copper alloy (negative pole) and 6061 aluminum alloy (positive pole). The working surface of the aluminum was sputtered with gold to reduce interfacial impedance. Intermittent flow experiments took place using manually pumped, calibrated, gas-tight syringes.
Here is a link to the full paper: http://onlinelibrary.wiley.com/doi/10.1002/aenm.201100152/full