Researchers from the US Department of Energy’s (DOE) SLAC National Accelerator Laboratory and Stanford University have designed a new lithium/polysulfide (Li/PS) semi-liquid (flow) battery for large-scale energy storage, with lithium polysulfide (Li2S8) in ether solvent as a catholyte and metallic lithium as an anode.
Unlike previous work on Li/S batteries with discharge products such as solid state Li2S2 and Li2S, the catholyte is designed to cycle only in the range between sulfur and Li2S4. Consequently, the team points out in a paper describing there work published in the RSC journal Energy & Environmental Science, all detrimental effects due to the formation and volume expansion of solid Li2S2/Li2S are avoided.
This novel strategy results in excellent cycle life and compatibility with flow battery design. The proof-of-concept Li/PS battery could reach a high energy density of 170 Wh kg−1 and 190 Wh L−1 for large scale storage at the solubility limit, while keeping the advantages of hybrid flow batteries. We demonstrated that, with a 5 M Li2S8 catholyte, energy densities of 97 Wh kg−1 and 108 Wh L−1 can be achieved. As the lithium surface is well passivated by LiNO3 additive in ether solvent, internal shuttle effect is largely eliminated and thus excellent performance over 2000 cycles is achieved with a constant capacity of 200 mA h g−1. This new system can operate without the expensive ion-selective membrane, and it is attractive for large-scale energy storage.—Yang et al.
The work is some of the earliest supported by the DOE’s new Joint Center for Energy Storage Research battery hub (JCESR). (Earlier post.)
For solar and wind power to be used in a significant way, we need a battery made of economical materials that are easy to scale and still efficient. We believe our new battery may be the best yet designed to regulate the natural fluctuations of these alternative energies.—Stanford Prof. Yi Cui
Currently the electrical grid cannot tolerate large and sudden power fluctuations caused by wide swings in sunlight and wind. As solar and wind’s combined contributions to an electrical grid approach 20%, energy storage systems must be available to smooth out the peaks and valleys of this intermittent power—storing excess energy and discharging when input drops.
Among the most promising batteries for intermittent grid storage today are “flow” batteries, as it is relatively simple to scale their tanks, pumps and pipes to the sizes needed to handle large capacities of energy. The new flow battery developed by Cui’s group has a simplified, less expensive design that presents a potentially viable solution for large-scale production.
Today’s flow batteries pump two different liquids through an interaction chamber where dissolved molecules undergo chemical reactions that store or give up energy. The chamber contains a membrane that only allows ions not involved in reactions to pass between the liquids while keeping the active ions physically separated.
This battery design has two major drawbacks: the high cost of liquids containing rare materials such as vanadium—especially in the huge quantities needed for grid storage—and the membrane, which is also very expensive and requires frequent maintenance.
The new Stanford/SLAC battery design uses only one stream of molecules and does not need a membrane at all. Its molecules mostly consist of the relatively inexpensive elements lithium and sulfur, which interact with a piece of lithium metal coated with a barrier that permits electrons to pass without degrading the metal. When discharging, lithium polysulfides absorb lithium ions; when charging, they lose them back into the liquid. The entire molecular stream is dissolved in an organic solvent, which doesn’t have the corrosion issues of water-based flow batteries.
Initial lab tests showed that the new battery retained excellent energy-storage performance through more than 2,000 charges and discharges, equivalent to more than 5.5 years of daily cycles, Cui said.
In the future, Cui’s group plans to make a laboratory-scale system to optimize its energy storage process and identify potential engineering issues, and to start discussions with potential hosts for a full-scale field-demonstration unit.
Yuan Yang, Guangyuan Zheng and Yi Cui (2013) A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage. Energy Environ. Sci., 6, 1552-1558 doi: 10.1039/C3EE00072A