Synthesis And Electrochemical Properties Of Sulfur, Titania And Nickel Sulfide Electrodes Based On Metal-Organic Frameworks

Nowadays lithium-ion battery is indispensable and has been applied in various of portable electronic devices. Based on intercalation-deintercalation reaction, the commercial electrodes (such as graphite anode, LiCoO2, LiFePO4 and LiMn2O4 cathodes) feature stable cyclability but low theoretical capacity, which limits further development of Li-ion battery technology. Thus, the research should focus on electrode materials with high theoretical capacity based on alloying or conversion reaction mechanism. Their poor cyclability can be improved by morphological or structural design and this research is mainly about applying metal-organic frameworks (MOFs) in the synthesis of well-performed electrode materials and exploring their electrochemical properties:Used as hosts, MOFs confined sulfur molecules and improved the cycle performance of Li-S battery; and as precursors, MOFs were turned into porous TiO2 and nano NiS with outstanding battery performance by calcination.Aiming at solving the problem of fast capacity fading caused by the dissolution of polysulfides in Li-S battery, hollow carbon spheres (HCSs) were synthesized by the calcination of hollow polymer spheres and used as sulfur hosts. The shell of HCSs was about 80 nm which could provide physical confinement for the sulfur inside. Sulfur was loaded into HCSs and their micro channels were sealed by poly pyrrole (PPy) achieving cathode material with dual core-shell structure. With the good conductivity of PPy shell and the confinement of cage-like pores for sulfur, such cathode reveled reversible capacity of 844.3 mA h g-1 after 100 cycles at 0.1 C. Moreover, sulfur was encapsulated into a MOF with cage-like pores, i.e. ZIF-8 by melt-diffusing method and with the physical confinement of the hosts, S@ZIF-8 also featured good cycle performance with reversible capacity of 420 mA h g-1 after 200 cycles at 0.1 C. The continuous capacity fading of S@ZIF-8 was also investigated and the incomplete sulfur encapsulation and poor electrochemical stability of ZIF-8 limited its performance promotion.In order to study the chemical interaction between MOF hosts and sulfur, another cathode was synthesized by MOF HKUST-1 named as S@HKUST-1. Sulfur distribution in HKUST-1 crystals was studied by X-ray single crystal structural analysis. Chemical interactions between sulfur and Lewis acidic sites (LASs) had been found through XPS and TG analysis which diminished the solution problem of polysulfides and reversible capacity of 505 mA h g-1 after 170 cycles had been achieved. Combining both cage-like pores and LASs, MOF-525(M) (M= 2H, FeCl, Cu) was designed to serve as sulfur host in Li-S battery. It had higher BET surface area than ZIF-8 with proper pore structure to provide strong physical confinement for sulfur and ensure complete sulfur encapsulation. The Lewis acidic sites in MOF-525(Cu) were also more effective than those in HKUST-1, which could offer better chemical interactions with sulfur. With high sulfur loading amount, S@MOF-525(Cu) showed the best performance among all reported S/MOF cathodes with reversible capacity of 704 mA h g-1 after 200 cycles at 0.5 C. S@MOF-525(Cu) film was also prepared on Al substrate and used directly as cathode. Such idea provided effective strategy to improve energy density of Li-S battery.In order to increase Li+ diffusing efficiency in TiO2 anode and improve its rate capability, porous TiO2 was synthesized by the calcination of MOF MIL-125(Ti) under air atmosphere. It demonstrated anatase phase with truncated octahedron morphology which was constructed by small TiO2 crystals (10 nm). Inherited from the MIL-125(Ti) precursor, porous TiO2 revealed high porosity with BET surface area of 220 m2 g-1. It demonstrated good photocatalytic activity and Li+storage ability with capacity retention of 96.8% after 500 cycles at 1 C charge/discharge rate. Under reducing atmosphere, another MOF precursor, MOF-74(Ni), was turned into carbon wrapped nano nickel particles (Ni@C). The Ni particles were only 5-10 nm with carbon shell (2-4 nm) homogeneously wrapped on them showing BET surface area of 112 m2 g-1. Owing to the high reactivity of nano Ni, it can be converted to nano NiS and NiO directly in the carbon shell. Featuring small particle size with conductive carbon shell, NiS@C revealed better electrochemical performance than the commercial NiS powder with reversible capacity of 273 mA h g’1 after 100 cycles at 0.1 C.