Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Wang, Hongqiang, Design and modification of cathode materials for high-performance lithium-sulfur and lithium-selenium batteries, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2016. http://ro.uow.edu.au/theses/4839
Lithium-sulfur (Li-S) batteries have been considered as most promising candidates for the next generation lithium-ion batteries, as sulfur features high theoretical capacity (1675 mAh g-1), high specific energy density (2600 Wh kg-1), cost effectiveness, and nontoxicity. However, the broad application of Li-S batteries is limited by several persistent problems, such as the low electrical conductivity of sulfur and its discharge product, the high solubility and diffusivity of polysulfide intermediates in organic electrolytes, and the related side reaction “shuttle effect”, as well as large volume change caused by the intercalation of Li into the sulfur. Therefore, improving the conductivity of the sulfur cathode, enhancing the tolerance for volume change, maintaining the soluble polysulfides within the cathode region, and limiting polysulfide diffusion are the key points for the development of Li-S batteries. In this doctoral work, a series of nanostructured sulfur-based materials were prepared for Li-S batteries, including TiO2-coated hierarchically ordered porous sulfur composite, porous carbon-sulfur composite derived from cotton, and ordered mesoporous-carbon-fiber–sulfur cathode material, as well as a novel design for an integrated flexible sulfur cathode for Li-S batteries. On the other hand, selenium, as a congener of sulfur, possesses similar chemical behaviour and could react with lithium to form selenides. The theoretical volumetric capacity of selenium (3253 mAh cm-3) is comparable to that of sulfur (3467 mAh cm-3). A novel type of one-dimensional organic selenium-containing fiber was synthesized for lithium-selenium (Li-Se) and sodium-selenium (Na-Se) batteries.
A three-dimensional (3D) hierarchically ordered mesoporous carbon-sulfur composite coated with a thin TiO2 layer has been synthesized by a low-cost process and investigated as a cathode for Li-S batteries. The TiO2-coated carbon-sulfur composite works as a suitable cathode material for lithium-sulfur batteries. The hierarchical architecture provides a 3D conductive network for electron transfer, open channels for ion diffusion, and strong confinement of soluble polysulfides. Meanwhile, the TiO2 coating layer could further effectively prevent the dissolution of polysulfides and also improve the strength of the entire electrode, thereby enhancing the electrochemical performance. As a result, after TiO2 coating, the electrode demonstrates excellent cycling performance, with a discharge capacity of 608 mAh g-1 at the 0.2 C current rate and 500 mAh g-1 at the 1 C current rate after 120 cycles, respectively.
A new type of low-cost activated micro-macroporous carbon suitable for mass production that is derived from cotton was successfully prepared by using potassium hydrate in a chemical activation method. The activated carbon exhibits a hierarchically porous microstructure and high specific surface area (989 m2 g-1). The micro-macroporous structure allows a large amount of sulfur (68%) to be infiltrated into the micropores of the host. This unique micro-macroporous structure is found to be directly related to the battery performance. The macroporous structure of this carbon provides channels for sulfur to be loaded into the micropores and enhances fast transport of electrons/ions and electrolyte, while the microporous structure can trap elemental S and lithium polysulfides during cycling. When evaluated as a cathode for lithium-sulfur batteries, the hierarchically porous carbon-sulfur composite electrode exhibits excellent cycling stability and good performance. The resulting composite electrode possesses a reversible capacity of 760 mAh g-1 after 200 cycles at current density of 0.2 C.
One-dimensional ordered mesoporous carbon fiber has been prepared via the electrospinning technique, using resol as the carbon source and triblock copolymer Pluronic F127 as the template. Sulfur is then encapsulated in these ordered mesoporous carbon fibers by a simple thermal treatment. The interwoven fibrous nanostructure has favourable mechanical stability and can provide an effective conductive network for sulfur and polysulfides during cycling. The ordered mesopores can also restrain the diffusion of long-chain polysulfides. All these features lead to excellent electrochemical performance. The ordered mesoporous-carbon-fiber -sulfur (OMCF-S) composite with 63% S exhibits high reversible capacity, good capacity retention, and enhanced rate capacity when used as cathode in rechargeable lithium-sulfur batteries. The OMCF-S electrode maintains a stable discharge capacity of 690 mAh g-1 at 0.3 C, even after 300 cycles. In particular, the OMCF can be mass produced in a simple way at low cost, which makes our sulfur-based electrode highly promising for practical application in lithium-sulfur batteries.
A strategy for configuration of an integrated sulfur cathode is presented, which is composed of an integrated carbon/sulfur/carbon sandwich structure on polypropylene separator that is produced via the simple doctor blade technique. The integrated electrode exhibits excellent flexibility and high mechanical strength. The top and bottom carbon layers of the sandwich-structured electrode not only work as double current collectors, which effectively improve the conductivity of the electrode, but also serve as good barriers to suppress the diffusion of the polysulfides and buffer the volume expansion of the active materials, leading to suppression of the shuttle effect and low self-discharge behaviour. The integrated sulfur cathode delivered a high reversible capacity of 730 mAh g-1 over 500 cycles, with capacity decay as small as 0.058% per cycle and a low self-discharge constant of 0.0293 per week.
Organic selenide fibers composed of carbonized polyacrylonitrile/selenium (CPAN/Se) have been synthesized by heating polyacrylonitrile-selenium (PAN-Se) fibers via the electrospinning technique at 600 oC. The Se molecules are confined by N-containing carbon ring structures in the form of energy-storing selenium side chains in the carbonized PAN matrix. This unique stable chemical structure with a conductive carbon skeleton connected to the selenium side chains and excellent mechanical stability can allow the CPAN/Se composite cathodes to be charged and discharged in a low-cost carbonate-based electrolyte with excellent long cycling stability and quite good rate performance. The superior electrochemical performance of the CPAN/Se electrodes has been demonstrated in both lithium-ion and sodium-ion batteries, where it has delivered a high capacity of nearly 600 mAh g-1 for 500 cycles in lithium-selenium (Li-Se) batteries and 410 mAh g-1 for 300 cycles in sodium-selenium (Na-Se) batteries at 0.3 C.