Among the best candidates for next-generation high-energy-storage systems, metal−sulfur batteries, such as Li−S, Na−S, and Mg−S, hold high theoretical energy densities, making them especially attractive. Of these, the Li−S battery has the highest theoretical energy density of 2567 Wh kg⁻¹, calculated on the basis of the Li anode (∼3,860 mAh/g) and the S cathode (∼1675 mAh/g), making it a promising choice for the next generation of high-energy rechargeable batteries.

The application of Li−S batteries suffers from two major issues. One involves with the use of lithium metal as anode, which may raise safety concerns for practical applications due to the growth of lithium dendrite...The other issue lies in the sulfur cathode. It is known that a Li−S battery with a cyclooctasulfur (cyclo-S₈) cathode usually discharges stepwise, with two plateaus in its voltage profile. At the first plateau (∼2.3 V vs Li⁺/Li), sulfur is reduced from S₈ to S₄²⁻, during which various electrolyte-soluble polysulfides (Li₂S_n, n = 4−8) form. The second plateau (∼1.95 V vs Li⁺/Li) corresponds to the transformation from Li₂S₄ to insoluble Li₂S₂ and finally Li₂S.

The performance of the Li−S battery is therefore limited by the insulating problem with sulfur and the dissolution and shuttling problem with polysulfides in liquid electrolyte. Many efforts have been made to improve the performance of sulfur, including the use of various conducting substrates and new electrolytes, e.g., solid-state and ionic liquid types. Though the former leads to improved electrical conductivity, it does not solve the intrinsic problem with dissolution of polysulfides due to the contained cyclo-S₈ molecules. The latter, intended to relieve the dissolution problem of polysulfides, suffers from the low ionic conductivity of electrolyte at room temperature.

—Xin et al.

In their study, the team from the Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Sciences, Beijing National Laboratory for Condensed Matter Physics, and Bosch Research & Technology Center, China, found that the confined small S₂₋₄ molecules avoid the unfavorable transition between S₈ and S₄²⁻ during discharging/ charging and typically give a single long output plateau at ∼1.9 V.

To create the material, they started with multi-walled carbon nanotubes (CNTs, average diameter = 50 nm) and then coated them with a MPC layer via a solution-based method (CNT@MPC). The resulting cumulative pore volume for the micropores <0.6 nm was calculated to be 0.46 cm³/g—corresponding to a theoretical loading of 49 wt% sulfur in the micropores, based on the density of sulfur (2.07 g/cm³).

Heating a mixture of sulfur and the CNT@MPC results in the diffusion of sulfur as chain-like molecules into the MPC layer to form the sulfur−carbon composite (S/(CNT@ MPC). The S/(CNT@MPC) contained ∼40 wt% S.

In view of the micropore size of 0.5 nm, only small S₂₋₄ molecules can be accommodated in the micropores of MPC, while the large S₅₋₈ molecules cannot be stored. In this way, the chain-like sulfur molecules in the carbon micropores could not transform to the large S₈ rings but remain as S₂₋₄ molecules.

—Xin et al.

For electrochemical testing, they assembled the S/(CNT@MPC) into Li−S batteries. For comparison, a sulfur−carbon composite containing cyclo-S₈ was also tested, synthesized by mixing sulfur powder and carbon black (S/CB) with the same sulfur content, 40 wt%.

They found that, when using a carbonate-based electrolyte of 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate (1:1 wt%), the composite exhibits an initial discharge capacity of 1670 mAh/g at 0.1 C—close to the theoretical capacity of sulfur (1675 mAh/g), and much higher than that of S/CB (∼573 mAh/g) under the same experimental conditions.

The composite also demonstrated cycling stability with 1149 mAh/g after 200 cycles, and a favorable high-rate capability of 800 mAh/g at 5 C.

The success of the novel S cathode promises a new Li−S battery with higher energy density (785 Wh/kg based on anode and cathode) than state-of-the-art Li-ion batteries (theoretically 387 Wh/kg in a LiCoO/C battery) for powering our future electronics. In view of the wide applications of sulfur such as fertilizers, medicines, fungicides, and cell nutrients, our discovery of these unusual small sulfur allotropes may trigger wide research interest in these fields as well.

—Xin et al.

Resources

• Smaller Sulfur Molecules Promise Better Lithium–Sulfur Batteries Sen Xin, Lin Gu, Na-Hong Zhao, Ya-Xia Yin, Long-Jie Zhou, Yu-Guo Guo, and Li-Jun Wan Journal of the American Chemical Society doi: 10.1021/ja308170k