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Applications Of Polymer And Polymer-Derived Carbon In Lithium-Sulfur And Lithium-Selenium Batteries

Posted on:2016-06-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:Z Q YiFull Text:PDF
GTID:1221330467498376Subject:Materials science
Abstract/Summary:PDF Full Text Request
High energy density rechargeable batteries have received more and more attention due to the increasing demands from the adjustment of energy structure:from fossil fuels to the available alternative energy sources, such as wind and solar energy. Sulfur can provide high theoretical capacity (1675mAh g-1) and energy density (2600Wh kg-1) when coupled with lithium metal. Meanwhile, sulfur has advantages of natural abundance, low cost and low toxicity. So, lithium-sulfur battery has been recognized as one of the most promising candidate for next-generation energy strorage devices. While the realization of lithium-sulfur battery is still plagued with intrinsic drawbacks of sulfur such as the low conductivity and solubility of intermediate polysulfide in electrolyte. Selenium, as a congener of sulfur, has similar chemical properties with sulfur but higher conductivity and electrochemical reactivity. Therefore lithium-selenium battery deserves reasonable attention as a rechargeable battery candidate with high energy-density. This article is intended to obtain high-performance sulfur/selenium composite as cathode materials for lithium-sulfur or lithium-selenium batteries. The mainly research results are shown as below:1. To enhance the conductivity of sulfur and to suppress the solubility of polysulfides in electrolyte, ID conductive carbon network with hierarchical porous structure was synthesized to accommodate sulfur. With linear polypyrrole as a precursor, the hierarchical porous carbon was obtained by carbonizing the mixture of carbonaceous species and KOH at high temperature. In this unique structure, the macropores and micropores can not only host the sulfur species, but also provide three-phase reactive sites for lithium ion, electron and active material; the porous conductive network an provide a continuous electron pathway to ensure good electrical contact and facilitate electron/lithium ion transport by shortening diffusion pathways; nitrogen doping further enhancethe conductivity of the carbon matrix. So, the porous carbon/sulfur composite achieves outstanding cycliability and rate capability:the initial discharge capacity at0.2C reached1419mAh g-1and remained 650mAh g-1after100cycles with a capacity retention of60%; a discharge capacity of345mAh g-1was displayed at a higher rate of5C.2. A linear polyacrylonitrile was prepared using a facile electro-spinning method, and a ID nanofiber/sulfur composite (FPAN/S) based on PAN fibers was fabricated to accommodate sulfur. It’s demonstrated by emission scanning electron microscopy (SEM) that the ID structure maintaines stable during synthesization. Compared with particle-shaped sample, the nano fibers can shorten diffusion pathways of lithium ions and electrons which can facilitate the electrochemical reaction. Electrochemical tests show that the FPAN/S composite can obtain higher cycliability and much better rate ability with carbonate-based electrolyte than with ether-based electrolyte:it mains a reversible capacity of478mAh g-1calculating based on composite and1049mAh g-1based on sulfur alone after400cycles at a current density of500mA g-1, corresponding to a capacity retention of88%. Even ar a current density as high as5C, the FPAN/S composite still can delieve a capacity of380mAh g-1(calculated based on the whole composite). Meanwhile, the chemical properties under different temperatures were also tested with carbonate-based electrolyte and the stability of cells turned out to be extremely good. By comparing the as-synthesized nanofiber-sulfur composite with cathode material using powder-shaped material, it can be found that the unique secondery morphology of the material can help to achieve better electrochemical performance.3. A N-dopes carbon possessing hierarchical porous structure with regular macropores and interconnected micropores (CP) was used to accommodate selenium. The as-synthesized CP/Se composite showed excellent cycling performance and rate capability. With the carbonate-based elctrolyte, at1C current, the CP/Se composite can maintain506mAh g-1after150cycles, when the current density increased to20C, it still can delivere a capacity over300mAh g-1. The outstanding performance of CP/Se can be ascribed to the ultrahigh surface area of carbon host and the attachment between microporous carbon and Se and LiSex. Furthermore, nitrogen doping can not only enhance the conductivity of CP/Se composite, but also can greatly facilitates the interaction between carbon and Li2Se, which has bben evaluated by first-principle calculation.4. Carbon materials with specific and fancy morphology always suffered from complicate synthesization and high cost. Here we developed a very simle methodto prepare a polymer/selenium composite (PAN/Se). The PAN/Se composite shows excellent cycling performance in carbonate-based electrolyte, exhibiting an initial discharge capacity of525mAh g-1and a reversible capacity of477mAh g-1after1200cycles at1C with a capacity retention as high as91%. And the coulombic efficiency is almost maintained at100%.
Keywords/Search Tags:lithium-sulfur batteries, lithium-selenium batteries, porous carbon, thecomposite materials, electrochemical performance
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