U. Waterloo / BASF team reports new strategy for stabilizing high-performance Li-S cathodes; “transfer mediator”
A team from the University of Waterloo in Canada and BASF has devised a successful new strategy to stabilize cathode in Li-S batteries, thereby significantly improving performance and cycle life. In a study exploring the mechanism published in the journal Nature Communications, the researchers, led by Prof. Linda Nazar at the University of Waterloo, showed a capacity of 1,300 mAh g−1 at C/20 with only a modest drop in capacity at a 20x higher current density (a C rate) to 950 mAh g-1.
At C/5, the initial discharge capacity was 1,120 mAh g-1, with 1,030 mAh g-1 sustained after more than 200 cycles—representing excellent capacity retention of 0.04% per cycle. At higher current densities (1C), the composite cathode still delivered reversible capacity of 800 mAh g-1 after 200 cycles.
The approach relies on a chemical process in which a host reacts with the initial lithium polysulfides formed on cycling to form surface-bound intermediates. These in turn function as a redox shuttle to catenate and bind the higher polysulfides, and convert them on reduction to insoluble lithium sulfide. In the paper, the team used nanosheets of manganese dioxide (MnO2) as the host prototype to stabilize the cathode. The researchers also showed that this mechanism extends to graphene oxide and suggested it could be employed more widely.
Lithium–sulfur batteries are one of the most promising candidates to satisfy emerging market demands, as they possess a theoretical capacity and energy density of 1,675 mAh g-1and 2,500 kW kg-1, respectively, superior to current lithium-ion batteries. In addition, they present an inherently low competitive cost due to the high natural abundance of sulfur. These advantages suggest that the lithium–sulfur battery should be capable of energy storage several times greater than conventional lithium-ion batteries, with reduced cost. However, practical applications are currently hindered by several obstacles. These predominantly relate to the insulating nature of sulfur and lithium sulfides, which require addition of conductive additives (hence lowering the active sulfur mass fraction), pronounced capacity fading on cycling and an internal redox shuttle that lowers Coulombic efficiency.
Over decades, much effort has been expended to try to solve these problems by trapping the polysulfides within the cathode structure. … Herein, we present a quite different chemical approach to polysulfide retention in the sulfur cathode, which relies on mediating polysulfide redox through insoluble thiosulfate species in a two-step process.—Liang et al.
The first step is the creation of the thiosulfate groups in situ by oxidation of the initially formed soluble lithium polysulfide (LiPS) species on the surface of ultra-thin manganese oxide nanosheets. As the reduction proceeds, the surface thiosulfate groups anchor newly formed soluble higher polysulfides by forming polythionates and converting them to insoluble lower polysulfides.
Nazar and her colleagues thus describe the polythionate complex formed on the surface as a “transfer mediator”. This process curtails active mass loss during the discharge/charge process and suppresses the polysulfide shuttle to deliver high performance with high sulfur loading and cycle stability.
Unlike previous strategies to trap polysulfides by physical barriers or simple surface interactions, this chemistry is quite efficient. The discovery and understanding of a transfer mediator, which binds polysulfides and promotes stable redox activity, addresses one of the important challenges that face this chemistry. Along with future anticipated improvements in electrolytes and the lithium-negative electrode, this brings the Li–S battery a step closer to practical realization.—Liang et al.
This is a major step forward and brings the Li-S battery one step closer to reality.—Linda Nazar
Postdoctoral research associate Xiao Liang, the lead author, and graduate students Connor Hart and Quan Pang are currently investigating other oxides to find the best sulfur retaining material.
BASF International Scientific Network for Electrochemistry and Batteries funded the research. The paper’s co-authors include Arnd Garsuch and Thomas Weiss of BASF.
X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss, and L.F. Nazar (2015) “A Highly Efficient Polysulphide Mediator for Lithium Sulfur Batteries,” Nature Communications in press. doi: 10.1038/ncomms6682