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Posted: Apr 14, 2016
Phosphorene as a promising anchoring material for lithium-sulfur batteries
(Nanowerk Spotlight) Lithium-sulfur (Li-S) batteries, which employ sulfur as cathode and metallic lithium as anode materials, have been extensively studied as promising alternatives to the widely used lithium-ion batteries because – theoretically – they can render 3-6 times higher energy density (2600 Wh kg-1).
In practice, though, it has proven challenging to approach that theoretical value. Specifically, the rapid capacity fading, low Coulombic efficiency, and irreversible loss of active materials have impeded large-scale commercial use of Li-S batteries.
Researchers now have shown that trapping lithium polysulfide (Li2Sx) species on (nanoscale) host materials is an effective way to overcome these challenges.
Phosphorene is a single atomic layer of black phosphorus that has recently garnered the attention of researchers due to its unique puckered structure, outstanding anisotropic physical and mechanism properties, and wide potential applications for the design of nanodevices.
"In our recent work, by means of density functional theory computations, we have explored the potential of phosphorene as a host material to anchor Li-S species," Chen tells Nanowerk. "According to our computations, all Li2Sx species can moderately bind with phosphorene, exhibit ultrahigh diffusivity along the zigzag direction of phosphorene, and enhance the electrical conductivity of phosphorene."
The team's results indicate that the performance of Li-S species anchored on phosphorene is comparable to well-established host materials such as 2D metal sulfide materials.
"Our results showed that Li2Sx species with long chains can be moderately adsorbed on the phosphorene surface with moderate binding strength, and their structures are well maintained, thus avoiding their dissolution into the electrolyte," explains Chen. "Due to the puckered structure, the diffusion of LiSx species with a longer chain length along the zigzag direction is highly favorable with a small energy barrier of 0.20 eV."
"Furthermore," he adds, "a certain amount of charge is transferred from Li2Sx species to phosphorene, which accounts for the band gap reduction after the deposition of Li2Sx species on phosphorene. Therefore, our theoretical results suggest that phosphorene is a highly promising anchoring material for high performance Li–S batteries."
Chen's team will continue their efforts to develop low-dimensional nanostructures for stabilizing Li-S species, especially S8 species.
"In our opinion" he concludes, "a major challenge facing future research in this area is to reveal the detailed mechanisms of Li-S batteries on host materials from the theoretical viewpoint, which are in progress in our laboratory."