The new semi-solid flow cells, which can use established lithium intercalation compounds, could deliver energy densities of 300–500 Wh L -1 (specific energy of 130–250 Wh kg -1 ) at system-level costs, depending upon the chemistries, of $250 kWh -1 and $100 kWh -1 for transportation and grid level storage, respectively, the researchers conclude.

Researchers at MIT, led by Dr. Yet-Ming Chiang (co-founder of A123 Systems), report on their development of a new energy storage concept—a semi-solid flow cell (SSFC) combining the high energy density of rechargeable batteries with the flexible and scalable architecture of fuel cells and flow batteries—in a paper published in the journal Advanced Energy Materials. In August 2010, A123 spun out 24M to commercialize this type of new technology. ( Earlier post .)

Dr. Yet-Ming Chiang will give a talk on “Scaling Lithium Ion (or other) Chemistries Using a Flow Battery Architecture” at the upcoming 4th Symposium on Energy Storage: Beyond Li-ion , to be held at Pacific Northwest National Laboratory, 7-–9 June 2011.

In contrast to previous flow batteries, the SSFC stores energy in suspensions of solid storage compounds to and from which charge transfer is accomplished via dilute yet percolating networks of nanoscale conductors. The new semi-solid lithium flow cell offers energy densities that are an order of magnitude greater than previous aqueous flow batteries. In addition, the SSFCs offer simplified low-cost manufacturing of large-scale storage systems compared to conventional lithium-ion batteries.

...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. 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., current collector foils, tabs, separator film, 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. 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.

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 flow batteries have such a design, in which active materials are stored within external reservoirs and pumped into an ion-exchange/electron-extraction power stack. As the system increases in capacity, its energy density may asymptotically approach that of the redox active solutions. Aqueous-chemistry flow 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 fluids alone. Furthermore, the large fluid volumes that must be pumped produce parasitic mechanical losses that detract significantly from round-trip efficiency. The flow-cell’s design advantages are therefore offset by the use of low-energy-density active materials. —Duduta et al.

The new SSFC system, the researchers say, retains the inherent advantages of a flow architecture while dramatically increasing energy density by using the suspensions of energy-dense active materials in a liquid electrolyte. Assuming a solids content of 50%, 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 (≈2m). The semi-solid approach may be applied to aqueous chemistries, the team notes, 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. However, when applied to nonaqueous Li-ion chemistries, energy density is further increased by another factor of 1.5–3, in direct proportion to cell voltage.

The authors note that their proposed concept cannot work without effective charge transfer from the active material particles to the current collectors of the cell. To accomplish this, they take advantage of two limiting cases of particle aggregation behavior to produce novel, electrochemically active composites: 1) diffusion-limited cluster aggregation (DLCA) of conductive nanoparticles at low volume fraction to form percolating conductor networks; and 2) volumetrically dense packing of micrometer-scale storage particles to maximize storage energy density.

After confirming that their semi-solid suspensions were electrochemically active in nonflowing cells, the team subsequently tested them under flowing conditions. Half-cells in which cathode suspensions were cycled against a fixed Li-metal negative electrode were tested under two pumping modes: 1) a continuously circulating mode; and 2) an intermittent mode. They then moved to full-cell testing in intermittent mode.

They found that a semi-solid system with 40 vol% solids in each suspension has the following theoretical energy densities for the two semi-solids combined:

LiCoO 2 –Li 4 Ti 5 O 12 (2.35 V average discharge voltage) has 397 Wh L - 1 (168 Wh kg -1 );

LiNi 0.5 Mn 1.5 O 4 –Li 4 Ti 5 O 12 (3.2 V average discharge voltage) has 353 Wh L -1 (150 Wh kg -1 ); and

LiCoO 2 –graphite (3.8 V average discharge voltage) has 615 Wh L-1 (309 Wh kg-1).

With reasonable allowances for the stack, storage vessels, and balance-of-plant, we estimate that optimized SSFC systems using established lithium intercalation compounds could have energy densities of 300–500 Wh L-1 (specific energy 130–250 Wh kg-1 ), which would satisfy metrics considered necessary for widespread adoption of all-electric vehicles. Further improvements would be possible by ‘dropping in’ higher-energy-density or lower-cost storage compounds in the SSFC platform as they are developed.

For large-scale applications, the SSFC design should also provide lower materials and manufacturing cost than conventional lithium-ion battery technology. At near-term costs of $10–15 kg-1 for active materials and $14 kg-1 for nonaqueous electrolyte, the semi-solid suspensions alone have an energy specific cost of $40–80 kWh-1 depending on the specific chemistry, which leaves substantial room to achieve system-level cost targets of $250 kWh-1 and $100 kWh-1 for transportation and grid level storage, respectively. —Duduta et al.

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