PNNL team develops composite sulfur/Ni-MOF composite cathode for Li-S batteries showing excellent capacity retention
Researchers at Pacific Northwest National Laboratory (PNNL) have used a novel Ni-based metal organic framework (Ni-MOF) significantly to improve the performance of Li-sulfur batteries by immobilizing polysulfides within the cathode structure through physical and chemical interactions at molecular level.
In a study reported in the ACS journal Nano Letters, the use of a sulfure/Ni-MOF composite cathode resulted in capacity retention of up to 89% after 100 cycles at 0.1 C. The research team attributed the excellent performance to the synergistic effects of the interwoven mesopores (2.8 nm) and micropores (1.4 nm) of Ni-MOF, which provide an ideal matrix to confine polysulfides, as well as the strong interactions between Lewis acidic Ni(II) center and the polysulfide base, which significantly slow down the migration of soluble polysulfides out of the pores.
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.
|Li-S anode work|
|Lead PNNL Jie Xiao and some of her PNNL colleagues earlier reported designing a lithium–sulfur battery using electrically connected graphite and lithium metal as a hybrid anode to block polysulfides.|
|Lithiated graphite placed in front of the lithium metal functions as an artificial, self-regulated solid electrolyte interface (SEI) layer that actively controls the electrochemical reactions and minimizes the deleterious side reactions, leading to significant performance improvements.|
|Lithium–sulfur cells incorporating such hybrid anodes deliver capacities of >800 mAh g−1 for 400 cycles (4x the cycle life compared to a conventional anode) at a high rate of 1,737 mA g−1, with only 11% capacity fade and a Coulombic efficiency of more than 99%. (Earlier post.)|
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.
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