| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 107 === In this thesis, first principles calculations are employed to explore stabilization behaviors of lithium polysulfides on B, N doped graphene. Lithium polysulfides are soluble intermediates produced by discharging process of Li-S batteries. If the substrates can adsorb lithium polysulfides, the cycle performance of Li-S batteries can be improved. Therefore, it would be of great interest to develop detailed atomistic scale understanding of this materials system in many fundamental aspects, especially for the interactions between lithium polysulfides and substrate.
In the first part of this thesis, we investigated the adsorption behaviors of lithium polysulfides on pure B and N doped graphene. The doping configurations of pure B and N doped graphene were first studied. The graphitic N dopants prefer to stay away from each other. In contrast, graphitic B prefer to stay together, which makes it easier to reach high dopant contents in local area. The adsorption energies of lithium polysulfides on substrates with low formation energies were calculated. The result shows that N-doped graphene can afford strong binding energy only if N dopants locate at two-fold bonded sites with vacancies. However, B-doped graphene can adsorb lithium polysulfides no matter what sites B are located. In addition, we found that one of Li would be grabbed from lithium polysulfides to N-doped substrates with vacancy. Thus, in the real discharging process, Li will first bind to substrate then interact with sulfur. So the lithium polysulfides in that kind of system will adsorb on substrate with trapped Li. Our result indicates that Li-trapping effect will lower the binding energy of lithium polysulfides, revealing that the adsorption energies calculated in previous section of N-doped graphene are overestimated. We also studied the ability of substrates to adsorb multiple lithium polysulfides. The result shows that B-doped graphene can bind more lithium polysulfides than N-doped graphene, which is one of the advantage to B-doped substrates. In summary, B is a better dopant than N due to four reasons: preference of dopant clustering, more dopant configurations capable for adsorbing lithium polysulfides, no Li-trapping effect and stronger ability to adsorb multiple lithium polysulfides.
In the second part of this thesis, we studied the anchoring mechanism of lithium polysulfides on B, N co-doped graphene. The dopant configurations are first investigated. Our result indicates that B and N can synergically reduce the formation energies of substrates, showing that high dopant contents can be easily reached. As for adsorption energy calculations, our result show that structures with B-N pairs, which thought to be able to adsorb lithium polysulfides by previous experimental studies, cannot afford strong adsorption energy due to the cancellation of p-type doping effect by N dopants. Instead, the structures without B-N pair and having more graphitic B dopants than N can anchor the lithium polysulfides. We also explored the effect of B to N-doped vacancy and the effect of N to B-doped vacancy. Adsorption energies of N-doped vacancy significantly increase with the increasing B contents. Even after Li-trapped, the substrates could still adsorb lithium polysulfides. However, the formation energies of those structures become higher in the meantime. On the other hand, with the increasing N contents, adsorption energies of B-doped vacancy decrease and the formation energies of those structures also decrease. In the view of materials design, graphene with more boron than nitrogen dopants are better cathode materials for Li-S batteries, which could not only reach high dopant concentration but also creating structures that can strongly anchor lithium polysulfides.
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