Since the MetIL compound represents both electrolyte and the energy storing (electroactive) species, the opportunity to achieve high energy densities is possible. Currently, MetIL compounds in our library have electroactive species (metal) concentrations on the order of 1.6 M. These concentrations are low compared to what is currently achievable in aqueous vanadium redox battery chemistries, which is up to 2.5 M vanadium. However, higher energy densities are achievable by decreasing the molar volume of the MetIL through the use of smaller ligands, anions, and/or cations. Calculations of some theoretical MetIL compounds have shown metal concentrations up to 6.5 M are possible. Utilization of metals that can accommodate multi-electron processes (i.e. Mn, V, Cr, etc.) is also a path forward to high energy densities.

Increases in energy density should directly translate to lower capital costs. A MetIL based flow battery is attractive technology to achieve aggressive capital cost goals of $100 /kWh. Ligands such as ethanolamine and diethanolamine are readily available in multi-ton quantities because of current applications in gas treatment, personal care and agricultural industries. Most metal salts, particularly copper ones, are also readily available and low cost.

—Staiger et al.

Fluctuating electricity from intermittent renewable energy sources is difficult for the grid to accommodate. Improved energy storage technologies help even out the flow of such fluctuating sources, and Sandia researchers are studying new ways to develop a more flexible, cost-effective and reliable electric grid with improved energy storage.

The US and the world need significant breakthroughs in battery technology for renewable energy sources to replace today’s carbon-based energy systems. MetILs are a new, promising battery chemistry that might provide the next generation of stationary storage battery technology, replacing lead-acid and lithium-ion batteries and providing significantly higher energy storage density for these applications.

—Anthony Medina, director of Sandia’s Energetic Components Realization program

Sandia researcher and inorganic chemist Travis Anderson is leading a team developing the next generation of redox flow batteries. A redox flow battery (RFB) stores electrical energy typically in two soluble redox couples contained in external electrolyte tanks sized in accordance with application requirements. Liquid electrolytes are pumped from storage tanks to flow-through electrodes where chemical energy is converted to electrical energy (discharge) or vice versa (charge).

Redox flow batteries theoretically offer a number of advantages: high energy efficiency >75 % (> 95% found on lab scale); long calendar life, excellent cycle ability (> 10,000); flexible design; fast response time; overcharge and over discharge tolerant; low self discharge or no discharge depending on pumping of electrolyte; and easily scalable.

However, RFBs also have general issues with charge cycle efficiencies; low energy densities; raw material costs; cross contamination of anolyte and catholyte; and corrosiveness/safety issues—all leading to a high cost per kWh.

Flow batteries have been fielded in the US, Japan and Australia. A number of systems—up to 25 MW—are in the process of being demonstrated under the American Recovery and Reinvestment Act (ARRA) administered by DOE’s Energy Storage Systems Research program. Zinc bromine and vanadium redox systems are among the top contenders. But the materials involved are moderately toxic, and vanadium is subject to major price fluctuations. In addition, the aqueous solution limits the amount of material that can be dissolved and how much energy can be stored, and outside temperature can hurt performance.

Anderson and his colleagues focused on research and development efforts on non-aqueous systems because of their operational advantages and potential for reduced cost. In a 2011 paper (Staiger et al.), Anderson and his team noted that non-aqueous electrolytes address the shortcomings of current systems by potentially offering wider voltage windows, higher charge cycle efficiency, decreased temperature sensitivity, and increased cycle life.

Instead of dissolving the salt into a solvent, our salt is a solvent. We’re able to get a much higher concentration of the active metal because we’re not limited by saturation. It’s actually in the formula. So we can cost-effectively triple our energy density, which drastically reduces the necessary size of the battery, just by the nature of the material.

—Travis Anderson

The electrochemical efficiency in MetILs is greater than anything else published to date. The team has prepared nearly 200 combinations of cations, anions and ligands, and of those, five outperform the electrochemical efficiency of ferrocene, which has long been considered the gold standard.

A common problem when mixing positively and negatively charged species is that these species will start aggregating together, eventually causing the solution to turn gummy and clog the battery membrane and electrode surfaces. The team addressed that challenge by developing asymmetric cations, or positively charged ions, that resemble a soccer ball.

In this analogy, the black pentagons represent negatively charged areas and the white hexagons represent positively charged regions. Such an arrangement lowers the melting point by preventing the ionic liquid constituents from bonding and becoming a solid, while the partial charge still allows electrons to flow freely through the cell to generate a current.

The team is funded by the US Department of Energy’s Office of Electricity Delivery and Energy Reliability. Imre Gyuk, energy storage systems program manager for that office, has been a champion of Sandia’s efforts and provided the necessary funding.

The MetILs approach represents an ingenious, out-of-the-box solution to the cathode/electrolyte paradigm. Because it is based on readily available, inexpensive precursors, it may well lead to innovative, cost-effective storage systems with major impacts on the entire US grid.

—Imre Gyuk

The findings apply to new flow battery cathode materials. The next step for the Sandia team is to find similar materials for flow battery anodes.

Resources

• Harry D. Pratt III, Alyssa J. Rose, Chad L. Staiger, David Ingersoll and Travis M. Anderson (2011) Synthesis and characterization of ionic liquids containing copper, manganese, or zinc coordination cations. Dalton Trans. 40, 11396-11401 doi: 10.1039/C1DT10973A

• Chad L. Staiger, Harry D. Pratt III, Jonathan C. Leonard, David Ingersoll and Travis M. Anderson (2011) MetILs: A Family Of Metal Ionic Liquids For Redox Flow Batteries (Electrical Energy Storage Applications & Technologies (EESAT) Conference)