K. Amine, A. Abouimrane , J. Liu, Z. Zhang, P. Du, K.C. Lau, H-H Wang, L. Curtiss
Argonne National laboratory
Lithium-sulfur and lithium-air batteries are attractive because they have the potential of providing 2 to 5 times the energy density of the lithium-ion batteries currently on the market. However, lithium-air batteries suffer from large polarization between charge and discharge and poor cycleability due to electrolyte decomposition and the high potential needed to remove lithium from Li2O. In the case of lithium-sulfur batteries, although progress has recently been made by fabrication of a carbon-sulfur composite [1,2], substantial improvements are still needed. The main challenge is the poor cycle life resulting from the dissolution of polysulfide in the organic electrolyte and its migration to the anode. Moreover, to overcome the insulator characteristic of sulfur (5 × 10−30 S/cm at 25 °C) and Li2S (final product of Li-S cell), special carbon (e.g., carbon mesopores [1]) or high amounts of carbon [2] are needed for high current density applications. Another drawback of lithium-sulfur batteries is that the voltage output is close to 1.8 V, and the cell cannot be cycled over 3.6 V.
In this paper, we report on a new battery system based on selenium and selenium-sulfur composite. Selenium has a melting point of 217 ºC and an electronic conductivity of 10-5 S cm-1, which is 20 orders of magnitude higher than that of sulfur because the gap between the valence band and the conduction band is reduced with decreasing atomic number. In earlier preliminary work, we investigated the electrochemical properties of selenium as a host for lithium ions. We found that this new class of electrodes can compete with the lithium-sulfur system in terms of energy density, even though the theoretical capacity of the Li /Se system based on the formation of Li2Se is only 675 mAh g-1, much lower than that of the Li/S system (1675 mA g-1). However, the high density of selenium (4.82 g/cm3) versus sulfur (2.07 g/cm3) makes the volumetric capacity of these materials very close (~3253 Ah/l for selenium and ~3467 Ah/l for sulfur). Furthermore, we found that the Li/Se system delivers an output voltage at least 0.5 V higher than that of Li/S and could surpass the Li/S system in terms of volumetric energy density. Furthermore, S-Se mixtures are miscible in all proportions and many selenium-sulfur composites including Se5S, Se5S2, Se5S4, SeS, Se3S5, SeS2 are already reported. Those known Se-S materials can offer higher theoretical capacities than the selenium alone ranging from 675-1550 mAh.g-1 with improved conductivity compared to pure sulfur. The potential Se-S systems will allow for tunable electrodes, combining the high capacities of S-rich systems with the high conductivity associated with the d-electron containing Se. Unlike Li/Sulfur system, both Se and SexSy can be cycled to 4.6V without failure. We will also report on new improvement in cycle life of lithium air using two ether-based electrolytes; tetraglyme (tetra (ethylene glycol) dimethyl ether. TEGDME) and a siloxane (tri(ethylene glycol) methyltrimethyl silane, 1NM3).
References
1. J. Xiulei, T.L. Kyu, & L. F. Nazar, Nature Mater., 8, 500-506 (2009).
2. B. Zhang, X. Qin, G.R. Li & X.P. Gao, Energy Environ. Sci., 3, 1531–1537 (2010).
