A team of researchers in South Korea and Italy has demonstrated a highly reliable lithium–sulfur battery showing cycle performance comparable to that of commercially available lithium-ion batteries while offering more than double the energy density. The team, led by a group from Hanyang University, used a highly reversible dual-type sulfur cathode (solid sulfur electrode and polysulfide catholyte) and a lithiated Si/SiOx nanosphere anode.
In a paper in the ACS journal Nano Letters , they reported that the lithium–sulfur cell showed superior battery performance in terms of high specific capacity, excellent charge–discharge efficiency, and remarkable cycle life, delivering a specific capacity of ~750 mAh g–1 over 500 cycles (85% of the initial capacity). These promising behaviors may arise from a synergistic effect of the enhanced electrochemical performance of the newly designed anode and the optimized layout of the cathode, they suggested.
Lithium-sulfur batteries offer a great deal of promise as a next-generation battery technology, with the natural abundance and low cost of sulfur, coupled with the high theoretical energy density of sulfur-based cathodes: 1675 mAh g-1 and 2500 Wh kg-1. They also carry a number of well-known challenges limiting their commercialization, including low active material utilization, and low stability of the sulfur electrodes due to the formation of soluble lithium polysulfides during cell operation.
There has been consistent progress recently toward optimizing the sulfur electrodes, for example, by the use of conductive carbonaceous matrices and metal-organic framework (MOF) for sulfur impregnation, as well as by the choice of suitable electrolytes. Recently, several research groups have reported that the addition of lithium polysulfide to the electrolyte could improve the performance of the Li/S batteries in terms of cycle performance and energy density.
Another major concern regarding the lithium-sulfur battery system is its use of a lithium-metal anode, which is well-known to have some critical problems including chemical reactivity in commonly used organic electrolytes and dendritic growth of lithium during cycling, leading to poor cycle performance and safety problems. In addition, when coupled with a sulfur cathode, the lithium metal anode reacts with lithium polysulfide to form an insoluble Li2S phase on lithium-metal surface, leading to the loss of lithium metal and eventually causing poor cycle performance of the system.
To minimize the problems associated with lithium-metal anode an excess amount of lithium metal usually is needed to construct the full cell to secure its long cycle life, which might lead to degradation of both the energy density and the safety of the full cell. Recently, alloy-type anode materials have been suggested as alternatives to replace the lithium-metal anode. However, even though some examples have shown promise for practical use with sulfur cathodes, the cycle performance and the energy density of lithium-sulfur battery system adopting alloy-type anode materials needs to be advanced further before they can penetrate the rechargeable battery market.
(—Lee et al.)
To address these issues, the researchers designed a LiS cell using a dual-type hybrid sulfur cathode and a lithiated Si/SiOx nanosphere anode with an optimized liquid electrolyte.
The cathode consists of an activated carbon-sulfur composite on a gas diffusion layer (GDL) electrode in contact with a catholyte solution to which Li2S8 has been added. This cathode system delivers a maximum capacity of ~1300 mAh g-1 with respect to the overall mass of sulfur (about 1.2 mg) from both the solid sulfur (about 0.2 mg on the electrode) and the dissolved lithium polysulfide (1.024 mg in 80 µL of the polysulfide-containing electrolyte).
At a rate of C/3, the cathode shows a capacity of ~1000 mAh g-1; Coulombic efficiencies of more than 99.3% except for the first cycle; and a maintenance of the capacity above 99% of the initial capacity even after 100 cycles.
The lithiated Si/SiOx nanosphere anode used shows highly stable cycling behavior over 100 cycles with a capacity of as high as 800 mAh g-1 and cycling efficiency approaching 100%.
The full lithium-ion sulfur cell presented in the study delivers a capacity of ~750 mAh g-1 with an average working voltage of about 1.8 V, corresponding to the energy density of 497 Wh kg-1 based on the weight of active materials on the cathode and anode.