Publicação
Novel separators and electrodes for the Li-S system
| Resumo: | As one of the prospective energy storage systems, rechargeable lithium-sulfur (Li-S) batteries offer possibilities of low-cost and high energy density since sulfur offers a high theoretical capacity of 1675 mAh g-1 and a high specific energy of 2600 Wh kg-1. However, commercialization of the current liquid type Li-S batteries is still hindered by insufficient cycle life with rapid capacity fades and low practical energy densities. Much work has been reported to address the aforementioned obstacles, but to gear the Li-S battery research towards practical applications, new approaches should be adopted. In this dissertation, binder-free ultra-thin carbon films were deposited on a standard separator, exploiting a variant of the Langmuir-Blodgett technique (LB), termed as Langmuir-Blodgett Scooping (LBS). The developed methodology was obtained through adjustment of the parameters that compromised the stability and homogeneity of the film upon the formation at the air-water interface and further transfer to the substrate. Subsequently, the functionalized separators comprised of 1, 5 and 10 layers of Ketjenblack (KB), Super C (SC) and multiwalled carbon nanotubes (MWCNT), were systematically studied for their ability to improve cycling behavior, polysulfide retention and reduction potential in Li-S cells. As result, cells based on carbon coated separators yield superior capacity and retention rates, compared with pristine separator. Nevertheless, carbon-coatings of MWCNT, followed by SC, evidenced higher active material utilizations, while cells employing separators coated with KB showed no improvements. Further galvanostatic studies were conducted on cells employing carbon-coated separators of MWCNT, but in the absence of the additive lithium nitrate (LiNO3), in order to investigate the trapping of soluble polysulfides. From the results, it was demonstrated that even with the best-performing carbon coating, the polysulfide shuttling was not inhibited. However, a retention of capacity was observed on following cycles, evidencing that lithium polysulfides could be trapped on the carbon film and remained electrochemically accessible. To prove this statement, additional SEM investigations were performed on cycled anodes retrieved from cells employing carbon-coated separators of MWCNT. The SEM examinations showed pronounced surface deterioration. However, unaffected areas were also detected, demonstrating that certain zones of the carbon film efficiently blocked the polysulfides. Additionally, solvent-free lithium sulfide (Li2S) cathodes were produced through a heat-press technique. The cathodes formulations comprised pristine and core-shelled Li2S, KB, and as binders polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO). Regarding the manufacturing method, and the cathode compositions adopted, it was found that the core-shell structure greatly enhanced the mechanical integrity of the cathodes, serving as an interparticle binding agent. Moreover, enhancements on the rate capabilities and cycling performances of cathodes employing core-shelled Li2S particles was observed. On the other hand, cathodes containing pristine Li2S particles demonstrated poor structural integrity, which resulted in major losses of material. Furthermore, lower discharge capacities were reported from cells employing cathodes with pristine Li2S particles. |
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| Autores principais: | Luís, Paulo Filipe Serobaba Soares |
| Assunto: | Baterias de Li-S Revestimentos de carbono ultrafino Cátodos de Li2S Teses de mestrado - 2017 |
| Ano: | 2017 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | As one of the prospective energy storage systems, rechargeable lithium-sulfur (Li-S) batteries offer possibilities of low-cost and high energy density since sulfur offers a high theoretical capacity of 1675 mAh g-1 and a high specific energy of 2600 Wh kg-1. However, commercialization of the current liquid type Li-S batteries is still hindered by insufficient cycle life with rapid capacity fades and low practical energy densities. Much work has been reported to address the aforementioned obstacles, but to gear the Li-S battery research towards practical applications, new approaches should be adopted. In this dissertation, binder-free ultra-thin carbon films were deposited on a standard separator, exploiting a variant of the Langmuir-Blodgett technique (LB), termed as Langmuir-Blodgett Scooping (LBS). The developed methodology was obtained through adjustment of the parameters that compromised the stability and homogeneity of the film upon the formation at the air-water interface and further transfer to the substrate. Subsequently, the functionalized separators comprised of 1, 5 and 10 layers of Ketjenblack (KB), Super C (SC) and multiwalled carbon nanotubes (MWCNT), were systematically studied for their ability to improve cycling behavior, polysulfide retention and reduction potential in Li-S cells. As result, cells based on carbon coated separators yield superior capacity and retention rates, compared with pristine separator. Nevertheless, carbon-coatings of MWCNT, followed by SC, evidenced higher active material utilizations, while cells employing separators coated with KB showed no improvements. Further galvanostatic studies were conducted on cells employing carbon-coated separators of MWCNT, but in the absence of the additive lithium nitrate (LiNO3), in order to investigate the trapping of soluble polysulfides. From the results, it was demonstrated that even with the best-performing carbon coating, the polysulfide shuttling was not inhibited. However, a retention of capacity was observed on following cycles, evidencing that lithium polysulfides could be trapped on the carbon film and remained electrochemically accessible. To prove this statement, additional SEM investigations were performed on cycled anodes retrieved from cells employing carbon-coated separators of MWCNT. The SEM examinations showed pronounced surface deterioration. However, unaffected areas were also detected, demonstrating that certain zones of the carbon film efficiently blocked the polysulfides. Additionally, solvent-free lithium sulfide (Li2S) cathodes were produced through a heat-press technique. The cathodes formulations comprised pristine and core-shelled Li2S, KB, and as binders polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO). Regarding the manufacturing method, and the cathode compositions adopted, it was found that the core-shell structure greatly enhanced the mechanical integrity of the cathodes, serving as an interparticle binding agent. Moreover, enhancements on the rate capabilities and cycling performances of cathodes employing core-shelled Li2S particles was observed. On the other hand, cathodes containing pristine Li2S particles demonstrated poor structural integrity, which resulted in major losses of material. Furthermore, lower discharge capacities were reported from cells employing cathodes with pristine Li2S particles. |
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