by Allison Kerley
Yang et al. (2013) discussed their new proof-of-concept lithium/polysulfide semi-liquid battery as a potential solution to large-scale energy storage. The lithium/polysulfide (Li/PS) battery uses a simplified version flow battery system, with one pump system instead of the traditional two. The Li/PS battery was found to have a higher energy density than traditional redox flow batteries, with the 5 M polysulfide solution catholyte cell reaching an energy density of 149 W h L–1 (133 W h kg –1), about five times that of traditional vanadium redox battery. The Li/PS battery cells were also found to have a high coulomb efficiency peak around 99% before stabilizing at around 95%, even after 500 cycles. The authors conclude that the Li/PS cells maintain a steady rate of performance after 2000 cycles, and are more cost-effective than traditional flow systems as the Li/PS cells operate without the expensive ion-selective membrane and can be operated at room temperature.
In the Li/PS battery, passivated metallic lithium foils made up the anode, a liquid lithium polysulfide solution* made up the catholyte, carbon-paper acted as the current collector, and a recently developed LiNO3 electrolyte additive was used as a passivation layer. During operation, the polysulfide solution continually flows through the electrode stacks to generate or store electricity. In down time, the catholyte drains back to the reservoir tank. Due to the simple design, the battery could be scaled either up or down with relative ease.
In the electrochemical tests (which were all done in the 2032 coin cell configuration), the initial discharge capacity was found to be 172 mA h g –1, and the second discharge was found to reach a capacity of 295 mA h g –1, corresponding to a capacity of 48 mA h cm– 3 for the whole catholyte. The average discharge voltage was 2.45 V, and the average voltage was 2.30 V, which indicated that the reaction is highly reversible. In the constant voltage cycling tests, which was investigated at a 0.8 C rate, it was found that the cells without the LiNO3 electrolyte additive could not be properly charged due to a strong shuttling effect, and reached a coulomb efficiency of 15% (as compared to the coulomb efficiency of 99–95% for the cells with the additive). Cells with the additive, however, reached energy densities of 108 W h L–1 (97 W h kg–1) for 5 M catholytes and 61 W h L–1 (59 W h kg–1) for 2.5 M catholytes. The capacity fading rate was found to be as low as 8.4% (for cells with the 5 M catholyte) and 5.0% (for cells with the 2.5 M catholyte) per 100 cycles. In addition to tests run on the Li/PS battery, the authors ran tests on effects of various parameters on the cycle life and the coulomb efficiency, which are not covered in this summary.
*Lithium polysulfide (Li2S8) in 1,3-dioxolane (DOL)/1,2-dimethoxyethane (DME)
Yang, Y., Zheng, G., Cui, Y., 2013. A membrane-free lithium/polysulfide semi-liquid battery for large-scale energy storage. Energy and Environmental Science 6, 1552–1558. Abstract at http://rsc.li/1sDQCkD