Sulphur is a promising cathode material with a theoretical specific capacity of 1,673 mAh g^-1, which is ~5 times that of existing materials based on transition metal oxides and phosphates. However, many challenges remain in developing a practical lithium–sulphur battery for commercialization. It is known that sulphur particles suffer from the problems of (a) poor electronic conductivity, (b) dissolution of intermediate polysulphides and (c) large volumetric expansion (~80%) upon lithiation, which results in rapid capacity decay and low Coulombic efficiency.

Over the years, extensive efforts have been devoted to addressing the first two problems, by encapsulating sulphur particles with conducting materials, including porous carbon, graphene oxide and conductive polymers, in an attempt to improve their electronic conductivity and limit polysulphide dissolution. However, insufficient emphasis has been placed on dealing with the third challenge—the large volumetric expansion of sulphur during lithiation coupled with polysulphide dissolution. This poses a critical problem because volume expansion of the sulphur core will cause the protective coating layer to crack and fracture, rendering the conventional core–shell morphology ineffective in trapping polysulphides.

...Herein, we demonstrate for the first time, the design of a sulphur–TiO₂ yolk–shell nanoarchitecture for stable and prolonged cycling over 1,000 charge/discharge cycles in lithium–sulphur batteries.

—Seh et al.

Schematic of the lithiation process in various sulfur-based nanostructure morphologies. (a) Bare sulfur particles undergo large volumetric expansion and polysulfide dissolution upon lithiation, resulting in rapid capacity decay and low Coulombic efficiency. (b) Although the core–shell morphology provides a protective coating, cracking of the shell will occur upon volume expansion of sulfur during lithiation, leading to polysulfide dissolution as well. (c) The yolk–shell morphology provides internal void space to accommodate the volume expansion of sulfur during lithiation, resulting in a structurally intact shell for effective trapping of polysulfides. Seh et al. Click to enlarge.

To prepare the material, the team reacted sodium thiosulfate with hydrochloric acid to create monodisperse sulfur nanoparticles (NPs); these NPs were then coated with TiO₂, resulting in the formation of sulfur–TiO₂ core–shell nanoparticles. This was followed by partial dissolution of sulfur in toluene to create an empty space between the sulfur core and the TiO₂ shell, resulting in the yolk–shell morphology.

For electrochemical testing, they fabricated 2032-type coin cells with the material; using lithium foil as the counter electrode, the cells were cycled from 1.7–2.6 V versus Li⁺/Li.

They estimated the volume of empty space in the yolk–shell nanostructures to be 37%; this space can accommodate ~60% volume expansion of the sulfur present within the shell, allowing for 1,250 mAh g^-1—i.e., 75% of the maximum theoretical capacity of sulfur, to be utilized (assuming volume expansion is linearly dependent on the degree of lithiation). Experimentally, they were able to achieve a maximum discharge capacity of 1,215 mAh g^-1.

It basically worked the first time we tried it. The sulfur cathode stored up to five times more energy per sulfur weight than today’s commercial materials.

After 1,000 charge/discharge cycles, our yolk-shell sulfur cathode had retained about 70 percent of its energy-storage capacity. This is the highest performing sulfur cathode in the world, as far as we know. Even without optimizing the design, this cathode cycle life is already on par with commercial performance. This is a very important achievement for the future of rechargeable batteries.

—Yi Cui

Funding for the project came from the DOE Office of Basic Energy Sciences through SLAC’s Laboratory Directed Research and Development Program, which directs a percentage of the lab’s funding to high-risk, high-payoff research that, if successful, can lead to future program opportunities.

Over the past seven years, Cui’s group has demonstrated a succession of increasingly capable anodes that use silicon rather than carbon because it can store up to 10 times more charge per weight. Their most recent anode also has a yolk-shell design that retains its energy-storage capacity over 1,000 charge/discharge cycles.

The group’s next step is to combine the yolk-shell sulfur cathode with a yolk-shell silicon anode to see if together they produce a high-energy, long-lasting battery.

Resources

• Zhi Wei Seh, Weiyang Li, Judy J. Cha, Guangyuan Zheng, Yuan Yang, Matthew T. McDowell, Po-Chun Hsu & Yi Cui (2013) Sulphur–TiO₂ yolk–shell nanoarchitecture with internal void space for long-cycle lithium–sulfur batteries. Nature Communications 4, Article number: 1331 doi: 10.1038/ncomms2327