Researchers at MIT have improved a proposed liquid battery system that could enable renewable energy sources to compete with conventional power plants. Professor Donald Sadoway and colleagues have already started a company, Ambri (initially Liquid Metal Battery Corporation), to produce electrical-grid-scale liquid batteries, which comprise layers of molten material which automatically separate due to their differing densities.
In a paper published in the journal Nature, they describe a lithium–antimony–lead liquid metal battery comprising a liquid lithium negative electrode, a molten salt electrolyte, and a liquid antimony–lead alloy positive electrode, which self-segregate by density into three distinct layers owing to the immiscibility of the contiguous salt and metal phases.
The new composition substitutes different metals for the molten layers used in a battery previously developed by the team; the new formula allows the battery to work at a temperature more than 200 degrees Celsius lower than the previous formulation.
The all-liquid construction confers the advantages of higher current density, longer cycle life and simpler manufacturing of large-scale storage systems (because no membranes or separators are involved) relative to those of conventional batteries. At charge–discharge current densities of 275 milliamperes per square centimeter, the cells cycled at 450 degrees Celsius with 98 per cent Coulombic efficiency and 73 per cent round-trip energy efficiency.
To provide evidence of their high power capability, the cells were discharged and charged at current densities as high as 1,000 milliamperes per square centimeter. Measured capacity loss after operation for 1,800 hours (more than 450 charge–discharge cycles at 100 per cent depth of discharge) projects retention of over 85 per cent of initial capacity after ten years of daily cycling.
—Wang et al.
In addition to the lower operating temperature, which should simplify the battery’s design and extend its working life, the new formulation will be less expensive to make, Sadoway says.
The original system, which used magnesium for one of the battery’s electrodes and antimony for the other, required an operating temperature of 700 ˚C. But with the new formulation, with one electrode made of lithium and the other a mixture of lead and antimony, the battery can operate at temperatures of 450 to 500 ˚C.
Extensive testing has shown that even after 10 years of daily charging and discharging, the system should retain about 85% of its initial efficiency: a key factor in making such a technology an attractive investment for electric utilities.
Currently, the only widely used system for utility-scale storage of electricity is pumped hydro, in which water is pumped uphill to a storage reservoir when excess power is available, and then flows back down through a turbine to generate power when it is needed. Such systems can be used to match the intermittent production of power from irregular sources, such as wind and solar power, with variations in demand. Because of inevitable losses from the friction in pumps and turbines, such systems return about 70% of the power that is put into them (“round-trip efficiency”).
Sadoway says his team’s new liquid-battery system can already deliver the same 70% efficiency, and with further refinements may be able to do better. And unlike pumped hydro systems—which are only feasible in locations with sufficient water and an available hillside—the liquid batteries could be built virtually anywhere, and at virtually any size.
The fact that we don’t need a mountain, and we don’t need lots of water, could give us a decisive advantage.
The biggest surprise for the researchers was that the antimony-lead electrode performed so well. They found that while antimony could produce a high operating voltage, and lead gave a low melting point, a mixture of the two combined both advantages, with a voltage as high as antimony alone, and a melting point between that of the two constituents—contrary to expectations that lowering the melting point would come at the expense of also reducing the voltage.
The researchers had hoped that the characteristics of the two metals would be nonlinear, Sadoway said—i.e., the operating voltage would not end up halfway between that of the two individual metals.
They proved to be [nonlinear], but beyond our imagination. There was no decline in the voltage. That was a stunner for us.
Our results demonstrate that alloying a high-melting-point, high-voltage metal (antimony) with a low-melting-point, low-cost metal (lead) advantageously decreases the operating temperature while maintaining a high cell voltage. Apart from the fact that this finding puts us on a desirable cost trajectory, this approach may well be more broadly applicable to other battery chemistries.
—Wang et al.
The research was supported by the US Department of Energy’s Advanced Research Projects Agency-Energy and by French energy company Total.