Metal-Doped ZIF-8 Derived Hollow Carbon for High-Performance Lithium-Sulfur Batteries
Authors: Murashko Kirill , Subedi Nabin , Lähde Anna , Jokiniemi Jorma 
Published: February 2024

Lithium-ion (Li-ion) batteries have been ubiquitously utilized in the modernized world in portable electronic, electrical vehicles, and grid-scale energy storage. However, demand satisfaction for traditional Li-ion batteries is becoming difficult due to their limited capacity and energy density. Moreover, there are challenges related to the availability of non-renewable critical materials for Li-ion batteries such as Co, Ni, and graphite, and environmental pollution during battery creation and recycling. Further neglect of these problems will lead to dramatic consequences at which batteries will become a part of the environmental problems but not a solution to these problems as they should be. Therefore, it is highly imperative to explore novel battery systems that show high energy density, low production cost and low environmental pollution during production, use and recycling. One of the most promising candidates is the lithium-sulphur (LiS) battery, which offers various advantages such as high energy density, high theoretical specific capacity, and affordable cost, because of the high availability of elemental sulphur and carbon used in this type of battery. The theoretical gravimetric energy density of LiS batteries is 2567 Wh/kg which is 4.5 times higher than the energy density of conventional Li-ion batteries. Despite the richness of sulphur in the Earth’s crust and its prominent merits in energy density and cost, several problems have restricted the development of LiS batteries for commercialization. The most notorious and intractable problem is the high solubility of the sulphur species (Li2Sx, 3≤x≤8) in common Li-ion battery electrolytes that leads to the so-called lithium polysulfide (LiPS) shuttle effect and loss of battery capacity, Li metal anode corrosion, and low coulombic efficiency during interaction of the LiPS with Li metal anode. The most prospective solution to address the above drawbacks is sulphur incorporation with a porous, high-conductive carbon host that will tolerate sulphur volume expansion, improve the conductivity of the electrode, prevent the shuttle effect, and allow reversible lithium-ion migration during charging and discharging.
However, to achieve the excellent operation performances of the porous carbon host its properties should be carefully tuned and the LiPS absorption ability of the carbon host should be improved for example by metal oxide doping. Many different strategies for the creation of the required material were proposed in recent years. Unfortunately, many proposed synthetic methods require a hard template to provide fine control of the pore distribution in the materials, which is a significant drawback as in this case harsh conditions are required for template removal. In addition, the formation of doped carbons with good electrical conductivity often demands high temperatures, simultaneously causing the undesirable reduction of metal cations to their metallic state followed by the aggregation to form nanoparticles, together with the collapse of the carbon matrices. Therefore, it still urgently needs to find a new strategy to produce a porous carbon host for the sulfur with fine porosity and good lithium polysulfide absorption ability, which purpose to accelerate the successful commercialization of this battery technology on the market.
In our current work, we are investigating the possibility to synthesize finely dispersed metal oxide catalysts on hollow N-doped porous carbon capsules produced from zeolitic imidazolate framework (ZIF-8). The coating of the ZIF-8 with metal–tannic acid coordination polymer allows to uniformly dispersed metal oxide nanoparticles, such as Fe2O3, MnO, CoO etc., on hollow carbon capsules after the pyrolysis process. Moreover, the carbon produced by ZIF-8 pyrolysis is expected to be both doped with nitrogen and highly porous, which should have a positive effect on the maximum sulfur loading possibility in the carbon host and lithium polysulfide absorption ability. In addition, the used approach for the synthesis of the hollow N-doped porous carbon allows the distribution of multicomponent metal oxide nanoparticles on the surface of the hollow carbon capsules, which may have a synergetic effect and improve the lithium polysulfide absorption ability of the produced materials. The structural, compositional, and morphological properties of the created material are investigated using SEM, EDS, XPS, Raman spectroscopy, FTIR and TGA techniques. The effect of the different metal oxide nanoparticles and their combinations on the lithium polysulfide absorption ability of the synthesized materials are investigated by analysis of the electrochemical performances of the Li-S batteries, where synthesized material was used as the sulfur host during the creation of the Li-S battery cathode.