Unlike Li-ion batteries which use a process called intercalation to insert the ions between molecules in the electrode, Lithium-sulfur batteries rely on a multi-step redox reaction with sulfur that results in a number of stable intermediate sulfide ions. The storage process, in theory, reduces limitations of electrode structure, thus enabling higher capacity in similar volumes.

However, the intermediate polysulfides are soluble in the electrolyte and can diffuse to the lithium anode, resulting in undesired parasitic reactions. Li₂S₂ and Li₂S can also precipitate on the positive electrode, changing the morphology and resulting in fast capacity fading.

To tackle the polysulfide problem, researchers at Stanford University led by Dr. Yi Cui have developed a hollow carbon nanofiber-encapsulated sulfur cathode for effective trapping of polysulfides. In a paper in the ACS journal Nano Letters, they report the demonstration of experimentally high specific capacity and excellent electrochemical cycling of cells using the new cathode material.

...efficient trapping of polysulfides is highly desired for improving the cycle life of Li/S batteries...Based on our own research and literature studies in the past several years, we believe that an ideal structure for sulfur electrode should have the following characteristics: (1) a closed structure for efficient polysulfides containment; (2) limited surface area for sulfurelectrolyte contact; (3) sufficient space to accommodate sulfur volumetric expansion and small characteristic dimension of the sulfur electrode to avoid pulverization; (4) a short transport pathway for both electrons and Li ions to achieve high capacity at a high power rate; (5) a large conductive surface area for depositing insulating Li₂S₂ and Li₂S, in order to preserve the morphology of electrodes; and (6) suitable electrolyte additives to passivate the lithium surface to minimize the shuttle effect.

Some of these characteristics require structure designs that are self-conflicting, such as the minimization of sulfurelectrolyte contact and the large surface area needed for Li₂S₂ and Li₂S plating, which explain why it is very challenging to realize sulfur electrodes with high specific capacity and long cycle life.

To address these requirements, we designed a hollow carbon nanofiber-encapsulated sulfur electrode structure, comprising vertical arrays of hollow carbon nanofibers filled with melted sulfur.

—Zheng et al.

The researchers fabricated the hollow carbon nanofiber arrays using anodic aluminum oxide (AAO) templates through thermal carbonization of polystyrene. The AAO template also facilitates sulfur infusion into the hollow fibers and prevents sulfur from coating onto the exterior carbon wall. The high aspect ratio of the carbon nanofibers provides an ideal structure for trapping polysulfides, and the thin carbon wall allows rapid transport of lithium ions, the team noted in their paper.

The small dimension of the nanofibers provides a large surface area per unit mass for Li₂S deposition during cycling and reduces pulverization of electrode materials due to volumetric expansion.

The researchers observed a high specific capacity of about 730 mAh/g was observed at C/5 rate after 150 cycles of charge/discharge. Introducing of LiNO₃ additive to the electrolyte improved the Coulombic efficiency to more than 99% at C/5.

Resources

• Guangyuan Zheng, Yuan Yang, Judy J. Cha, Seung Sae Hong, and Yi Cui (2011) Hollow Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries. Nano Lett., Articles ASAP DOI: 10.1021/nl2027684