Abstract
Owing to the rapid depletion of fossil fuels, various clean energies have been extensively explored as alternatives. First released by Sony in 1991, lithium-ion batteries (LIBs), which convert chemical energy into electric energy, are presently the most widely used energy storage device. However, due to the scarcity of lithium, the production cost of LIBs is inevitably rising. Sodium-ion batteries (SIBs) are regarded as alternatives for LIBs because of the low cost of Na resources. Although the working mechanisms of SIBs are expected to be analogous to those of LIBs, unfortunately, the commercially used electrode materials (such as graphite) for LIBs are not well suited for Na+ ion intercalation and deintercalation. Therefore, there are still challenges lying ahead regarding appropriate electrode materials for Na+ storage. In this thesis, different kinds of metal-organic frameworks (MOFs) derived electrode materials were explored, with appealing electrochemical performances.In Chapter 2, two Co-MOFs were designed as templates to fabricate CoS2-based anodes for sodium-ion batteries. Specifically, starting from micron-sized Co(IPC)·H2O (IPC: 4-(imidazole-1-yl) phthalic acid) and polydopamine as the MOF precursor and the carbon source, the CoS2/C/C composite constituting CoS2 nanoparticles decorated with N-doped carbon layers were obtained. N-doped carbon layers derived from the MOF precursor and polydopamine, provided a robust network for the CoS2 nanoparticles, enhancing the structural integrity and electronic conductivity of the resulting CoS2/C/C composite, which exhibited electrochemical performance superior to most existing CoS2 composites, and one of the best amongst all MOF derived CoS2 anode for SIBs. Then, another material of MoS2 was deposited on the Co-MOF derived CoS2/C composite to fabricate a hierarchical nanostructure of CoS2/N-doped carbon@MoS2 composite for Na+ storage. It offers several advantages for sodium storage: (i) accelerated sodium ion diffusion kinetics due to its heterogeneous interface; (ii) shortened ion diffusion path and exposed active sites for sodium storage due to its hierarchical nanosheet architecture; and (iii) homogeneous nitrogen doping of the MOF-derived carbon, which is beneficial for the electronic conductivity. Due to these merits, this composite exhibited an excellent electrochemical performance with a specific capacity of 596 mAh g-1 after 100 cycles at 0.1 A g-1, and 395 mAh g-1 at 5.0 A g-1.
Chapter 3 introduces MOF-derived materials as cathodes for sodium-sulfur batteries. Currently, emerging room-temperature sodium-sulfur (RT Na-S) batteries still suffer from insulating properties of elemental sulfur, a large volume expansion (≈170%) of S cathode, and more importantly, the formation of soluble sodium polysulfides (Na2Sn, n = 4–8) in the liquid electrolyte, which has seriously impeded their practical application. To address the above-mentioned issues, three different approaches were applied. Firstly, a new strategy to generate short-chain sulfur in larger pores (>0.5 nm) by the synergistic catalytic effect of CoS2 and proper pore size was developed. The use of short-chain sulfur (S2-4) could prohibit the generation of soluble polysulfides during the sodiation process directly. Based on density functional theory (DFT) calculations, we predicted that CoS2 can serve as a catalyst to weaken the S-S bond in the S8 ring structure, facilitating the formation of short-chain sulfur molecules. By experimentally tuning the pore size of CoS2-based host and evaluating its performances as cathodes in Na-S batteries, we conclude that such catalytic effect depends on the proximity of sulfur to CoS2.
Secondly, to enhance the electrochemical kinetics between S and Na+, an N,O-doped carbon matrix with Cu single atoms was derived from a bimetallic Cu-Zn MOF. Solid-state nuclear magnetic resonance, synchrotron X-ray absorption spectroscopy and single-crystal X-ray diffraction analysis showed that single atoms of Cu are coordinated with two N and two O atoms within the produced carbon-based composite material. Similar to CoS2, those copper sites can weaken S-S bonds in the S8 ring structure, and thus are able to catalyze the formation of short-chain sulfur molecules in even larger-size pores. In addition, Cu atoms facilitate conversion between short-chain sulfur and Na2S. As a result, when the produced sulfur-loaded carbon framework containing atomic Cu catalyst was used as a cathode for sodium-sulfur batteries, it exhibited a superior capacity of 776 mAh g-1 with a high sulfur utilization (1158 mAh gs-1 normalized with respect to the sulfur content) after 100 cycles at 0.1 A g-1, and excellent rate performance of 483 mAh g-1 (720 mAh gs-1) at 5 A g-1.
Thirdly, similar to short-chain sulfur, the use of covalently bonded sulfur can avoid the generation of long-chain polysulfides completely. Therefore, starting from a novel 3D Zn-based MOF with 2,5-thiophenedicarboxylic acid and 1,4-bis(pyrid-4-yl) benzene as ligands, a S,N-doped porous carbon host with 3D tubular holes for the sulfur storage was fabricated. Different from the commonly used melt-diffusion method to confine sulfur physically, we utilized a vapor-infiltration method to achieve sulfur/carbon composite with covalent bonds, which could join electrochemical reactions without low voltage activation. A polydopamine-derived N-doped carbon layer was further coated on the composite to confine the high-temperature-induced gas-phase sulfur inside the host. S and N dopants increased the polarity of the carbon host to restrict the diffusion of sulfur, and its 3D porous structure provided a large storage area for sulfur. As a result, the obtained composite showed outstanding electrochemical performance with a high reversible capacity of 467 mAh g-1(1262 mAh g-1(sulfur)) after 100 cycles at 0.1 A g−1.
Chapter 4 introduces MOF-derived materials as cathodes for sodium-selenium (Na-Se) batteries. Similar to Na-S batteries, Na-Se batteries have their drawbacks of large volumetric expansion and shuttle effect related to generation of soluble sodium polyselenides. To address these issues, an N and O codoped flower-like porous carbon host was derived from a Ni-based MOF for Se storage. The use of 4,4'-oxybisbenzoic acid and 4,4'-dimethyl-2,2'-bipyridyl ligands ensured the in-situ doping of the carbon host with oxygen and nitrogen upon carbonization of MOF, which increased its polarity and restricted the diffusion of selenium. Highly porous microflower-like carbon morphology provided sufficient space for Se storage. To avoid undesirable aggregation of Se particles which happens when using the conventional melt-diffusion method, we employed a vapor-infiltration method for Se loading, which ensured the homogeneous dispersion of selenium and high utilization of pores in the carbon host. DFT calculations confirmed that the increased polarity of the carbon host due to N,O codoping promotes the adsorption of polyselenides. When tested as a cathode for Na-Se battery, the optimized carbon/selenium composite showed an excellent energy storage performance with a large reversible capacity of 512 mAh gSe-1 (normalized with Se mass) at 0.1C after 100 cycles, accompanied by excellent cycling stability of 431 mAh gSe-1 after 1000 cycles at 1C.
| Date of Award | 25 Aug 2021 |
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| Original language | English |
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| Supervisor | Andrey ROGACH (Supervisor) |