Researchers from Peking University and Beijing Institue of Technology have designed a Li-sulfur battery cathode based on sulfur and a metal-organic framework (MOF) host. In a paper in the RSC journal Energy & Environmental Science, the team reports high discharge capacities and long cycle life.
Unlike an intercalation cathode which has an intrinsic host structure made of redox metal sites allowing the transport of Li+/e-, sulfur as a conversion cathode requires an additional host to store and immobilize the mobile redox centers, polysulfides. Metal-organic frameworks as a class are highly porous and well-defined crystalline materials, and a good prospect for such a host, the researchers posited.
With the appropriate electrolyte and cutoff voltage range, sulfur stored in an appropriate MOF host can take advantage of both intercalation (fast and stable) and conversion (high energy density) cathodes.
Herein, we describe a fast cathode with long cycle life based on sulfur and ZIF-8 nanocrystals. With 30 wt% sulfur loading in electrode, it achieves remarkable discharge capacities of 1,055 mAh g-1 (based on sulfur) at 0.1C and 710 mAh g-1 at 1C. The decay over 300 cycles at 0.5C is 0.08% per cycle, prominent in long-cycle Li-S batteries.
By comparing with another three distinct MOFs, MIL-53 (Al), NH2-MIL-53 (Al) and HKUST-1, as well as two sets of ZIF-8 with particle sizes in the micrometer range, it reveals that (i) small particle size of the MOF host is appreciable to achieve a high capacity and (ii) small apertures, associated with functionalities in the open framework that have affinity with the polysulfide anions, can help achieve a stable cycling. We believe that the findings are general and applicable for the rational design of new hosts for sulfur in other porous materials families to produce more effective and stable Li-S batteries.—Zhou et al.
Junwen Zhou, Rui Li, Xinxin Fan, Yifa Chen, Ruodan Han, Wei Li, Jie Zheng, Bo Wang and Xingguo Li (2014) “Rational Design of a Metal-Organic Framework Host for Sulfur Storage in Fast, Long-Cycle Li-S Batteries,” Energy Environ. Sci. doi: 10.1039/C4EE01382D