Left: Crystal structure of Ni-MOF containing two different types of pores represented by dark yellow sphere and blue sphere: mesopore (yellow sphere indicates pore volume; gray, C; red:,O; green, Ni; blue, N); micropore (blue sphere indicates the pore volume). Right: Cycling performance of Ni-MOF/S composite at 0.1 and 0.2 C rates at a voltage range of 1.5−3.0 V; inset of panel is a schematic diagram illustrating the interaction between polysulfides (e.g., Li2S8/ Li2S6/Li2S4, and so forth) and paddle-wheel unit in Ni-MOF. C, O, N, S, Li and Ni atoms are represented by gray, red, blue, yellow, pink, and green spheres, respectively. Credit: ACS, Zheng et al. Click to enlarge.

Lithium–sulfur (Li–S) batteries promise high specific capacity (1,675 mAh g^–1 based on sulfur). However, they suffer from rapid capacity degradation, mainly caused by polysulfide dissolution, hampering many practical applications. Developing a solution for that problem is thus a key focus.

In recent years, many efforts have been pursued to overcome the hurdles in Li−S battery technology. Various approaches have been proposed, spanning from immobilization of sulfur in different kinds of hosting materials, sulfur cathode surface modification, electrolyte modification, and anode protection by employing LiNO3 as the electrolyte additive. While the development of new electrolyte/additive and lithium anode protection remain challenges for a long history in lithium batteries, more progress was achieved in the sulfur cathode modulation.

… Recent work from this group further correlates carbon properties with the real current density and revisits their functions at different current densities. Polymers, porous aromatic framework (PAF), intercalation compounds, and silica have also been extensively investigated for Li−S battery system. Another type of high surface area hosts, metal organic framework (MOF), however, has received much less attention probably due to its poorly conducting nature compared to the carbon scaffold.

… In the present work, we report a novel nickel-based MOF (Ni-MOF) for sulfur impregnation. … Here, we demonstrate that polysulfides can be effectively harnessed by this novel Ni-MOF, displaying remarkably improved cycling performances.

—Zheng et al.

Metal organic frameworks (MOFs) are crystal-like compounds made of metal clusters connected to organic molecules, or linkers. Together, the clusters and linkers assemble into porous 3-D structures. MOFs can contain a number of different elements. PNNL researchers chose the transition metal nickel as the central element for this particular MOF because of its strong ability to interact with sulfur.

The framework’s positively charged nickel center tightly binds the polysulfide molecules to the cathodes. The result is a coordinate covalent bond that, when combined with the framework’s porous structure, causes the polysulfides to stay put.

Having shown the effectiveness of their MOF cathode, PNNL researchers now plan to further improve the cathode’s mixture of materials to improve its energy capacity. The team also needs to develop a larger prototype and test it for longer periods of time to evaluate the cathode’s performance for real-world, large-scale applications.

MOFs are probably best known for capturing gases such as carbon dioxide. This study opens up lithium-sulfur batteries as a new and promising field for the nanomaterial.

—Jie Xiao

This research was funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy. Researchers analyzed chemical interactions on the MOF cathode with instruments at EMSL, DOE’s Environmental Molecular Sciences Laboratory at PNNL.

Resources

• Jianming Zheng, Jian Tian, Dangxin Wu, Meng Gu, Wu Xu, Chongmin Wang, Fei Gao, Mark H. Engelhard, Ji-Guang Zhang, Jun Liu & Jie Xiao (2014) “Lewis Acid-Base Interactions Between Polysulfides and Metal Organic Framework in Lithium Sulfur Batteries,” Nano Letters doi: 10.1021/nl404721h