The specific capacity declines very little during repeated charge/discharge cycles. Credit: HZB. Click to enlarge.

The lithium-sulfur battery cathode does not consist of heavy metals, but instead of sulfur—an economical and widely available material. 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 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 are working to improve cathode materials that would be able to chemically or physically confine or encapsulate polysulfides, such as with nanoparticles made of titanium dioxide (TiO₂), for example.

With respect to low density and high specific surface area, mesoporous materials have become a subject of extensive research and been exploited in many applications, including templated synthesis, catalytic support, ef cient adsorbents for controlled release, supercapacitors, cathode materials for lithium–sulfur (Li–S) batteries, light emission and gas sensors.

… Ti₄O₇ has been utilized as a Pt catalyst support for fuel cells because of its high electrical conductivity (>103 S cm⁻¹ at room temperature). Moreover, Ti₄O₇ contains polar O-Ti-O units that have a high affinity for polysulfide, which is essential for retarding the dissolution of polysulfide when applied as cathode material for Li–S batteries.

… In this work, we report for the first time the synthesis of Ti₄O₇ particles with interconnected-pore structure using PS-P2VP porous particles as template.

—Mei et al.

The HZB team headed by Prof. Yan Lu developed a special fabrication process to generate the complex, three-dimensionally interconnected pore structure. First, a template was made of a matrix of tiny polymer spheres that have porous surfaces. This template is prepared in additional steps, then submerged in a solution of titanium isopropoxide. A layer of Ti₄O7 was formed on the porous spheres; this remained after thermal treatment, which decomposed the underlying polymer. Compared with other cathode materials made of titanium oxides, the Ti₄O₇ nanosphere matrix possesses an extremely large surface area.

The porous structure of the nanoparticles is visible under the electron microscope. Credit: HZB. Click to enlarge.

X-ray spectroscopy measurements (XPS) at the CISSY experiment of BESSY II showed that sulfur compounds bind strongly to the surface in the nanomatrix.

The large surface area also accounts for the high specific capacity per gram (1219 mAh) at 0.1 C (1 C = 1675 mA g^-1). The specific capacity also declines very little during repeated charge/discharge cycles (0.094% per cycle). By comparison, the specific capacity of cathode materials made of TiO₂ nanoparticles is 683 mAh/g. To increase the conductivity of this material, it is possible to apply a supplementary coating of carbon to the nanoparticles. The highly porous structure remains 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.

—Prof. Yan Lu, corresponding author

Resources

• Shilin Mei, Charl J. Jafta, Iver Lauermann, Qidi Ran, Martin Kärgell, Matthias Ballauff, Yan Lu (2017) “Porous Ti₄O₇ Particles with Interconnected-Pores Structure as High-Efficiency Polysulfide Mediator for Lithium-Sulfur Batteries” Advanced Functional Materials doi: 10.1002/adfm.201701176