A KAIST research team has developed ultra-stable, high-rate lithium-sulfur batteries (LSBs) by using hierarchically porous titanium nitride (h-TiN) as a sulfur host. In a paper in the journal Advanced Materials, the researchers report that h-TiN/S shows a reversible capacity of 557 mAh g⁻¹ even after 1000 cycles at 5 C rate with only 0.016% of capacity decay per cycle.

Lithium–sulfur batteries (LSBs) have garnered much attention as alternatives for the existing lithium ion batteries owing to their high theoretical energy density of 2600 Wh kg⁻¹. In particular, sulfur, whose theoretical capacity is 1672 mAh g⁻¹, is naturally abundant and environmentally benign. How-ever, the practical use of LSB is still largely hindered by poor conductivity of sulfur, the dissolution of lithium polysulfides (LiPSs), and the sluggish redox kinetics.

Hitherto, in contrast with other batteries, encapsulating sulfur into porous materials has been considered as a powerful strategy.Tailoring the porous architecture is a key for the further advance of LSBs because it plays crucial roles in determining LSB performance.

… Herein, we report the synthesis of co-continuous hierarchical macro- and mesoporous titanium nitride (h-TiN) as a multifunctional sulfur host. Metal nitrides are versatile materials in various research fields owing to their high electrical conductivity and excellent mechanical strength. Even though many researchers have attempted to control the nanostructure of metal nitrides, this work is the first to introduce a well-developed hierarchical macro- and mesoporous architecture on metal nitrides.

—Lim et al.

To tackle those issues, Professor Jinwoo Lee from the Department of Chemical and Biomolecular Engineering and his team synthesized a well-developed hierarchical macro/mesoporous titanium nitride as a host material for sulfur.

The titanium nitride has a high chemical affinity for sulfur and high electrical conductivity. As a result, it prevents the dissolution of active materials and facilitates the charge transfer. Moreover, the synergistic effect of macropore and mesopore structures allows the stable accommodation of large amounts of sulfur and facilitates the electrolyte penetration.

a,b) Schematic illustrations for the synthetic route of co-continuous h-TiN (a) and its application as host material for sulfur (b). The nitric acid and controlled evaporation (NICE) process induces macrophase separation via spinodal decomposition (SD). Meanwhile, the EISA method is widely used to synthesize the mesoporous structure via phase separation between a hydrophobic polymer block and a hydrophilic polymer/inorganic precursor block. c–f) SEM images of h-TiO₂ (c,d) and h-TiN (e,f). The hierarchical multiscale porous structure is still retained without any collapse after the conversion to h-TiN. The good retention of the porous structure is attributed to the thick pore wall of the h-TiO₂ derived from the block copolymer self-assembly. Lim et al.

Previously reported polar inorganic materials have a high affinity for sulfur, but it was challenging to control the porous architecture suitable to the sulfur host. This work breaks such limitations by developing a synthetic route to easily control the porous architecture of inorganic materials, which led to obtaining superior cycle stability and high rate capabilities.

The unique porous architecture of TiN is easily induced by multiscale phase separation. The evaporation rate control of the volatile solvent simultaneously induces the macrophase separation via SD, and mesophase separation via the self-assembly of the block copolymer. The coexistence of macro- and mesopore enables the effective control of the shuttle effect, stable accommodation of large amount of sulfur, and facile transport of electron/Li⁺ ions. Furthermore, the non-carbonaceous TiN surface exhibits the catalytic activity and a high affinity with LiPS. The synergistic effect of the multiscale porous structure and the surface properties of TiN enables the outstanding performance … Thus, the facile introduction of the multiscale porous architecture to non-carbonaceous host material will create a promising avenue for high performance LSBs.

—Lim et al.

Resources

• Won-Gwang Lim, Changshin Jo, Ara Cho, Jongkook Hwang, Seongseop Kim, Jeong Woo Han, and Jinwoo Lee (2019) “Approaching Ultrastable High-Rate Li–S Batteries through Hierarchically Porous Titanium Nitride Synthesized by Multiscale Phase Separation” Advanced Materials doi: 10.1002/adma.201806547