Technology Today

2012 Issue 1

Liquid Metal Battery
Innovation in Applying Materials Technology to Large-Scale Energy Storage

As in many emerging technology domains, large-scale energy storage technologies are ever changing and evolving, with market needs driving the demand for completely new and novel technologies. The liquid metal battery (LMB) is a game-changing, innovative energy storage technology that meets the needs of fixed installations that require utility-scale energy storage to enhance grid stability, security and reliability, while contending with the increasing impact resulting from renewable energy insertion.

LMB technology owes its origin to research performed at the Massachusetts Institute of Technology (MIT) by Prof. Donald Sadoway (John F. Elliott Professor of Materials Chemistry at MIT) and Dr. David Bradwell, who completed his doctoral research on liquid metal batteries in the Sadoway laboratory. Its inception was a result of calculated efforts to utilize economies of scale and to develop a large-scale, yet inexpensive battery made out of earth-abundant materials, as opposed to ignoring such factors and simply hoping to chase down the cost curve regardless of the specific chemistry.

Figure 1Using the aluminum smelting industry and the Hall-Héroult process for the production of aluminum as inspiration, the liquid metal battery concept was born. A modern aluminum smelter is a perfect example of a giant current sink requiring a very large current density to produce aluminum, yet the aluminum is produced at a very low net cost. If this well established process could be reversed, then a large battery cell providing a very high current density output could be created out of relatively inexpensive materials. Turning an aluminum smelter into a battery posed a unique challenge. Electrolysis of aluminum oxide (Al2O3) causes carbon monoxide (COgas) and carbon dioxide (CO2,gas) to be produced at the anode, and these outgas and leave the system, thus creating an irreversible process. Researchers at MIT solved this problem by choosing liquid metals for both electrodes, which are regenerated upon charging of the battery. Furthermore, earth-abundant materials constitute the electrodes, as well as the molten salt electrolyte.

This research led to a first-generation battery with electrodes composed of a magnesium (Mgliq) anode, an antimony (Sbliq) cathode and an electrolyte composed of molten salts (MgCl2-KCl-NaCl) (Figure 1). The cell operates around 700°C in order for the components to stay in a molten liquid state. At operating temperature, the anode, cathode and electrolyte naturally self-segregate based on their densities (similar to oil and vinegar), forming the battery. This feature is unique because it requires no barrier membranes or materials to separate the components, thereby eliminating internal stresses in the electrodes and allowing for faster ion transfer as compared to traditional solid-state batteries.

Figure 2The LMB electrochemical reaction functions by using an alloying mechanism (Figure 2). Upon charging the battery, the liquid metal cathode (Mg-Sbliq) releases two electrons (2e-), forming a positive ion (Mg2+) that travels through the molten electrolyte, where it recombines with electrons, forming more of the liquid anode material (Mgliq). Upon discharging, the process is reversed — the liquid anode (Mgliq) releases two electrons (2e-), forming the ion (Mg2+) that travels through the electrolyte, where it recombines with two electrons to form more of the liquid cathode (Mg-Sbliq).

MIT was one of the first-round recipients of a contract award by the newly created Advanced Research Projects Agency – Energy (ARPA-E) in 2009 to advance LMB technology from basic research and to scale-up design to the fabrication of 200Ah battery prototypes. MIT is scheduled to finish development of a five-cell 720 Wh (200 Ah) Alpha unit by the end of 2012. In the process of developing the technology, MIT researchers have discovered new electrode couples that have twice the cell voltage, operate at significantly lower temperature and use materials with lower overall cost. During early stages of development, Raytheon began collaborating with the core MIT researchers and is now providing systems engineering expertise to MIT as part of their ARPA-E contract. This aligns with Raytheon’s collaboration strategy, and it can be cited as a good example of how new partnerships enable our engineers and supply chain to provide “affordable and innovative solutions” to our customers.

Figure 3As the LMB technology readiness level advanced beyond university research, a new spinout company was formed, Liquid Metal Battery Corporation (LMBC), to commercialize LMB and bring it forward to the market. The Raytheon and LMBC teams actively collaborate and engage military customers to explore demonstration opportunities in support of very challenging but important DoD energy goals.

After the development of the initial 200 Wh prototypes, LMBC is setting their sights on larger cells, about the size of a coffee table. These cells would be stacked into battery modules (Figure 3). The goal is to generate a battery system capable of delivering hours of utility-scale power, yet stay within the size constraint of a shipping container.

Figures 1 and 2 source: Bradwell, D. J., Kim, H., & Sadoway, D. R. (2011). Magnesium−Antimony Liquid Metal Battery for Stationary Energy Storage. Journal of the American Chemical Society. Figure 3 source: MIT GroupSadoway.

Alf Carroll, Ryan Faries
Contributors: Gami Maislin, Peter Morico

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