| With global warming and increasing air pollution,more and more people are calling for clean and non-polluting renewable energy sources to replace fossil fuels:such as tidal energy,wind energy,solar energy and so on.Because they eventually harvest electricity,the development of energy storage technologies is of great importance.However,current energy storage technologies are difficult to meet the growing demand for mobile or stationary power supplies in portable electronic devices,electric vehicles,and power grids.Among various clean energy sources,Li-S battery is considered to be one of the most promising electrochemical energy storage devices due to its high mass/volume energy density(2500 Wh kg-1/2800 Wh L-1).However,unlike traditional intercalating cathode materials,the elemental sulfur of the cathode in Li-S batteries has shortcomings of poor conductivity and compositional/structural changes during the charge/discharge cycles as well as the shuttle effect due to soluble polysulfides.Although efforts have been paid to accomplish great breakthroughs,there are still many problems hindering the practical use of Li-S batteries,especially the serious shuttle effect of polysulfides in Li-S batteries.To overcome these series issues,we firstly design positive carrier materials with micro/mesoporous structures,which can improve the conductivity of sulfur and alleviate its volume expansion in the cycling process,while inhibiting the shuttle effect of polysulfides;secondly,on the basis of micro/mesoporous structure,the catalysts introduced can improve the reversible conversion between soluble long-chain polysulfides and solid short-chain sulfides for better inhibiting the shuttle effect of polysulfides and improving the electrochemical performance of Li-S batteries.Studies have demonstrated that the performance of batteries are not only affected by the morphology and structure of the electrode materials,and the introduced catalyst such as Co also plays an important role in enhancement of battery performance.This thesis work especially investigate the enhancement mechanism of both pore structure and incorporated catalyst for the Li-S batteries.This thesis is divided into five chapters as follows:?1?Overviews of the working principle,challenges,research status of each component of Li-S battery,and some host materials for sulfur cathodes investigated recently.?2?The synthesis methods of electrode materials,the experimental apparatus and the characterization methods of the sample are discussed in detail.?3?Chitosan,a natural macromolecular polymer,was selected as the carbon source in this thesis work.The precursor was prepared by hydroxyaldehyde condensation reaction,and the porous carbon spheres with different morphologies were prepared by changing the carbonation temperature.When other reaction conditions remain unchanged,carbonization temperatures of 600°C,700°C,800°C and 900°C were applied;in turn,porous carbon spheres with uneven size and surface roughness morphology,honeycomb-like porous carbon spheres with uniform size and smooth surface,mixed cracked and retaining porous carbon spheres and fully fractured porous carbon spheres were produced,respectively.In addition,the N2 adsorption/desorption isotherm show that all the host materials have high specific surface area and micro/mesoporous structures.The electrochemical performance tests for these four host materials-supported sulfur cathodes in Li-S batteries show that the initial discharge specific capacities were 1001.5,1120,897 and 610 mAh g-1,respectively,and the discharge specific capacities after 300 charge/discharge cycles were 778.1,879.7,748.6,568.7 mAh g-1,respectively.Among these four materials,the honeycomb-like porous carbon sphere?SHC?material delivers the best electrochemical performance.?4?A monodisperse cobalt-doped micro/mesoporous nanomaterials?Co-N/MMC?was prepared.Compared with Fe-N/MMC,Co-N/MMC as the host materials of Li2S6,it has better morphology,higher specific surface area and higher micro/mesoporous volume,more reactive area and active sites,more stable electrode structure and higher resistance to the shuttle effect of polysulfides.Both theoretical calculations and experimental results clearly reveal that the introduction of monatomic cobalt with strong catalytic activity can not only reduce the polarization but also promote the reversible transformation between long-chain polysulfides and short-chain sulfides thus accelerating the reaction kinetics.In terms of preventing the dissolution and diffusion of polysulfides,the combination of micro/mesoporous structure and catalysts can inhibit the shuttle effect of polysulfides to a greater extent from both physical structure and chemistry aspects.Therefore,the Co-N/MMC-Li2S6 cathode exhibits higher specific capacity,better rate performance and long cycle stability.After cycling 300 cycles at a current density of 1 C,the discharge capacity of the Co-N/MMC-Li2S6 cathode is1010.3 mAh g-1,the Coulomb efficiency remains at around 101.2%,and the capacity decay rate per cycle is only 0.08%;even after 500 cycles at a current density of 2 C,the discharge capacity of the Co-N/MMC-Li2S6 cathode was reduced from the initial 942.6to 542.5 mAh g-1,the Coulomb efficiency was still as high as 101.4%,and the capacity decay rate per cycle was still 0.08%.?5?The thesis finally comes to conclusions and offers the perspectives of the future works in Li-S batteries. |
PDF Full Text Request