Despite the promise of high theoretical energy capacity (1673 mAh g⁻¹, 5x that of current commercial cathodes) of Li-S chemistry, practical Li-S batteries have been hindered by poor cyclability, mainly due to (1) the poor conductivity of sulfur; (2) the heavy dissolution of polysulfides; and (3) the large volumetric expansion during lithiation.

A great deal of research is targeting these issues (e.g., earlier post, earlier post, earlier post), with much effort focused on the development of novel nanostructures aiming to confining the sulfur and mitigating the shuttling effect of polysulfides.

Despite successes in stabilizing capacity over the short term, the GM team notes, “some inevitable negative effects in these processes still remain.”

• For mesoporous carbon-sulfur nanocomposites, if sulfur can diffuse into the small pores of the mesoporous carbon, the polysulfides should still be eventually diffuse out since a large amount of sulfur surface is still exposed to the electrolyte, although the weak interactions between sulfur and mesoporous carbon could alleviate the dissolution of polysulfides in the short-term.

• There has been no convincing evidence to support the assumption that the sulfur has diffused into the internal empty space of the porous carbon instead of in/on the superficial carbon pores. If the sulfur was only aggregated in/on the superficial pores of the mesoporous carbon, it would easily be dissolved out during cycling.

• For the approach of a core-shell composite (grown “bottom up” using a sulfur nanoparticle seed coated with a conductive polymer), a practical difficulty is obtaining small-sized sulfur nanoparticle seeds (≤300 nm) in order to enhance the lithium diffusion and improve the power capability, since the nanosized sulfur easily aggregates to bulky particles. Large sulfur particles may exhibit poor conductivity of both sulfur and polysulfides. In addition, the shell can block the direct contact between sulfur and conductive carbon.

Hence, developing new strategy to confine sulfur/polysulfides, control the sulfur particle size, and maintain intimate contact between carbon and sulfur simultaneously is still highly desired and critical for practical applications of sulfur-based cathode materials. In this work, we proposed the preparation of polydopamine-coated, nitrogen-doped, hollow carbon−sulfur in a double-layered core−shell architecture as an optimized design to confine the sulfur/polysulfides and keep intimate contact between sulfur and conductive carbon.

In this structure, the nitrogen-doped hollow carbon served as both carbon shell to control the size of sulfur core and conductive carbon to enhance the conductivity. The coating polymer further helped to confine the sulfur and polysulfides inside the shell. Additionally, the introduction of nitrogen facilitated better immobilizing the lithium polysulfides through the coordination interaction between lithium polysulfides and nitrogen.

… Using this unique sulfur composite, both Li/S half cells and the silicon/sulfur(Si/S) full cells showed highly improved capacity retention and Coulombic efficiency.

—Zhou et al.

They first prepared nitrogen-doped porous hollow carbon spheres, then impregnated them with sulfur under heat treatment to obtain a nitrogen-doped hollow carbon−sulfur (NHC−S) core−shell structure. The NHC−S composite was dispersed in an aqueous solution of dopamine, which would self-polymerize at alkaline pH values and spontaneously deposit on the surface of NHC−S nanoparticles to form a polydopamine-coated NHC−S composite (PDA−NHC−S) in a double-layered core−shell structure.

Scanning transmission electron microscopy (STEM) showed that the sulfur not only successfully penetrated through the porous carbon shell but also aggregated along the inner wall of the carbon shell.

To test and compare the electrochemical performance of the NHC−S and PDA−NHC−S materials, they fabricated 2032-type coin cells using lithium foil as the anode. The cathode nanocomposites were mixed with carbon black and water-soluble binder poly(vinyl alcohol) to prepare the cathode electrodes. The final sulfur ratios were around 64% and 55% in NHC−S and PDA−NHC−S electrodes, respectively.

They attributed the significantly improved cyclability in the PDA−NHC−S composite noted above to a number of factors:

• The polymer shell helps to encapsulate and confine the sulfur/polysulfides physically inside the shell.

• The unfilled pores/space in the hollow carbon could accommodate the volume expansion of sulfur/polysulfides during the lithiation.

• The thin polydopamine shell would react with the sulfide/polysulfides once they contacted with each other, leading to a three-dimensional, cross-linked polymer shell through interchain disulfide bonds interconnection and introduce some free sulfide anions among the polymer chains. This process would not only increase the mechanical strength of the polymer shells for better tethering the polysulfide species inside the polymer shells, but also facilitate the ionic transportation owing to the formation sulfide anions in/on the shells.

They did note, however, some slow capacity degradation despite the improved cycling stability.

To avoid the formation of lithium dendrites from the lithium anode during cycling—and thus the potential safety issues in the real application—they then used lithiated silicon thin film as the anode (PDA−NHC−S/Si).

They found stable capacities around 840 mAh g⁻¹ at 0.1C and 720 mAh g⁻¹ at 0.2C after 200 cycles in the full PDA−NHC−S/Si cells, with Coulombic efficiencies of more than 99.7% in both C rates. Although the capacity showed a slight decay in the initial 200 cycles from their proof-of-concept Si/S full batteries, the team said that the cycling stability and efficiency were better than other reports on Si/S full batteries.

They did also observe a slight capacity fading, especially in the higher scan rate, which they attributed to (1) the crack of the silicon film and the limited supply of lithium ions with the cycling; (2) the dissolution of the polysulfides during charge/discharge process.

Taking the advantage of this nitrogen-doped hollow−porous carbon and polymer-coated structure, both the Li/S and the Si/S batteries exhibited much improved cycling stability and Columbic efficiency. From the study of STEM images and EDX data, the sulfur not only successfully penetrated through the porous carbon shell but also aggregated into sulfur particles along the internal wall of the hollow carbon, which provided visible and substantial evidence that the sulfur prefer diffusing into the nitrogen-doped hollow carbon.

The polymer coating further helped to confine the sulfur particles inside the hollow carbon spheres, which facilitated to immobilize the polysulfides during the charge/ discharge processes and improved the cycling stability and efficiency relative to the uncoated composite. In the Si/S full batteries, a much improved cycling stability could also be observed with high efficiency. While slight capacity fading still remains in these and premier studies, developing such a strategy is highly desirable due to the fact that these results provide promising insights and novel concepts for future sulfur-based full batteries.

—Zhou et al.

Resources

• Weidong Zhou, Xingcheng Xiao, Mei Cai, and Li Yang (2014) “Polydopamine-Coated, Nitrogen-Doped, Hollow Carbon–Sulfur Double-Layered Core–Shell Structure for Improving Lithium–Sulfur Batteries,” Nano Letters 14 (9), 5250-5256 doi: 10.1021/nl502238b