Liquid Metal Batteries

Background

Although there was significant research into liquid metal batteries by the USA during the cold war the technology was shelved in favour of higher energy density Lithium-ion battery chemistries which were better suited to automotive applications. However, due to increasing interest in energy storage for grid-scale applications the technology is once again being investigated. Researchers in MIT have recently made significant progress in the field of liquid metal batteries. Most of the information contained here comes from [1].

 

Liquid metal battery

Figure: Depiction of liquid Metal batteries, (a) discharging, (b) charging.

A liquid metal battery comprises two liquid metal electrodes separated by a molten salt electrolyte that self-segregate into three layers based upon density and immiscibility. The idea is to pair a strong electron donor with a strong electron acceptor while avoiding non-metals in the choice of the latter. The strong interaction between the two liquid metals A and B provides the thermodynamic driving force (cell voltage) for the liquid metal cell. Upon discharge the thickness of the negative electrode is reduced, as metal A is oxidized (A→ Az+ + ze), and the cations are conducted across the molten salt electrolyte to the positive electrode, as electrons are released to an external circuit. Simultaneously the positive electrode layer grows in thickness, as the cations are electrochemically reduced to form a liquid A−B alloy [Az+ + ze → A(in metal B)]. This process is reversed upon charging.

Characteristics

Liquid metal batteries have the potential of being low cost, as many of the candidate electrode materials are reasonably abundant and inexpensive. The natural self-segregation of the active liquid components allows simpler, lower-cost cell fabrication compared with that of conventional batteries. Finally, perhaps the most attractive feature of these batteries is the continuous creation and annihilation of the liquid metal electrodes upon charge−discharge cycling. This feature grants liquid metal batteries the potential for unprecedented cycle life by rendering them immune to the micro-structural electrode degradation mechanisms that limit the cycle life of a conventional battery. All of these factors suggest that liquid metal batteries could be especially well-suited for the grid energy storage market. However they also have several disadvantages that led to a break in their research. The high operating temperatures and liquid form mean that they are not well suited to mobile applications, and their energy density (<200 Wh/kg) is considered less than Lithium-ion batteries.

Liquid metal batteries are a technology in the early stages of R&D and have not yet entered the demonstration phase. However, MIT spin-off Ambri have recently opened a new LMB production facility and anticipate the first demonstration units in 2014.

References

[1] Kim, H. et al., 2012. Liquid Metal Batteries: Past, Present, and Future. Chemical Reviews.