At present, lithium batteries are one of the best options for storing electrical power in a small space. Lithium ions in these batteries migrate from the anode to the cathode during the discharge cycle; in current lithium batteries, the anode and cathode generally consist of heavy-metal compounds that are expensive and toxic.

One interesting alternative is the lithium-sulfur battery. In this case, the cathode is made from sulfur – an economical and widely-available material –rather than heavy metals, but this creates a problem. As lithium ions migrate to the cathode during the discharge cycle, a reaction takes place there that forms lithium sulfide (Li2S) via various intermediate lithium polysulfides. During cycling, dissolution of these lithium polysulfides causes the battery's capacity to decline over the course of multiple charging cycles via the so-called ‘shuttle effect’. For this reason, researchers the world over are working to develop improved cathode materials that could chemically or physically confine or encapsulate the polysulfides, such as using nanoparticles made of titanium dioxide (TiO2), for example.

A team from Helmholtz-Zentrum Berlin für Materialien und Energie (HZB) in Germany, headed by Yan Lu, has now fabricated a cathode material that is even more effective. Once again, nanoparticles are used to confine the sulfur, but rather than TiO2 nanoparticles they comprise Ti4O7 molecules arranged on a porous spherical surface. These porous nanoparticles bind polysulfides with substantially greater strength than TiO2 nanoparticles. The team report their advance in a paper in Advanced Functional Materials.

"We have developed a special fabrication process to generate this complex, three-dimensionally interconnected pore structure", explains Lu. This process involves first fabricating a template comprising a matrix of tiny polymer spheres with porous surfaces; this template is then submerged in a solution of titanium isopropoxide.

This causes a layer of Ti4O7 to form on the porous spheres; this layer remains after thermal treatment, which decomposes the underlying polymer. Compared with other cathode materials that incorporate TiO2, the Ti4O7 nanosphere matrix possesses an extremely large surface area: just 12g of this material would cover a football field.

X-ray spectroscopy measurements (XPS) confirmed that sulfur compounds bound strongly to the surface of this nanosphere matrix, which accounts for its high specific capacity (1219 mAh/g) at 0.1°C. The specific capacity also declines very little during repeated charge/discharge cycles (0.094% per cycle). By comparison, the specific capacity of cathode materials with TiO2 nanoparticles is 683mAh/g. To increase the conductivity of the nanosphere matrix, a supplementary coating of carbon can be applied to the nanoparticles, with the highly porous structure remaining intact after this process.

"We have been working to improve the repeatability of this synthesis for over a year. Now we know how to do it. Next, we will work on fabricating the material as a thin-film," says Lu. Furthermore, future commercial development of the cathode should be aided by the fact that all the processes involved in fabricating the material, from the colloid chemistry to the thin-film technology, are scalable.

This story is adapted from material from HZB, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

The porous structure of the Ti4O7 nanoparticles is visible under an electron microscope. Image: HZB/adfm.201701176.
The porous structure of the Ti4O7 nanoparticles is visible under an electron microscope. Image: HZB/adfm.201701176.