Applications of MOF-based derivatives in Li-S and Na-S batteries

Lithium-sulfur (Li-S) and sodium-sulfur (Na-S) batteries hold great promise for sustainable and cost-effective energy storage due to their overwhelming advantages of high theoretical capacity, low cost, and resource abundance. Up to now, enormous efforts are invested in developing reliable cathode, separator, electrolyte, and safe anode. Nevertheless, it remains a great challenge to achieve high capacity and cycling stability. On one hand, the low conductivity of S and the sluggish kinetics of between Li/Na and S hinder the practical capacity and S utilization. On the other hand, the polysulfide intermediates show complicated behaviours in ether and carbonate ester electrolytes. These problems cause fast capacity fading and rapid loss of active materials in both Li-S and Na-S batteries. In this doctoral thesis, we develop metal–organic frameworks (MOF) -based derivatives to not only enhance the cycling performance in Li-S and Na-S batteries but study the electrochemical behaviour in ether and carbonate ester electrolytes. It is worth noting that MOF-based derivatives obtain porous space for storing S and improved conductivity via carbon skeleton. Significantly, various metal doping in carbon framework shows potential catalysis toward polysulfide conversion. These benefits render MOF-based derivatives various promising applications in Li-S and Na-S batteries. In the first work, we employ a facile and general vacuum-filtration approach to coat hierarchical MOF derived Co-C polyhedrons onto traditional separator for Li-S batteries. Lithium polysulfides (LiPSs) can dissolve in ether electrolyte, in which solvated LiPSs can freely shuttle to Li anode causing self-discharging. The resultant condensed separator with Co-C coating can effectively confine the shuttle effect via decreasing the pore size of commercial separator from 200-300 nm to 2.2-10 nm. In spite of physical confinement, the embedded Co nano-nodes in Co-C polyhedrons are capable of capturing LiPSs via strong chemical adsorption of solvated LiPSs. By these advantages combined with the rapid Li-ion transport through the intragranular pores between the Co-C polyhedrons, a sulfur-rich cathode (72% sulfur) has achieved outstanding performance when using the Co-C@separator, delivering decent cycling stability (675 mAh g-1 after 300 cycles at 0.1 A g-1) and high rate performance (401 mAh g-1 after 600 cycles at 1.0 A g-1). This approach serves as a facile strategy for remedying the shuttle phenomenon in Li-S batteries.