工學院: 材料科學與工程學研究所指導教授: 郭錦龍林鈺杰Lin, Yu-ChiehYu-ChiehLin2017-03-032018-06-282017-03-032018-06-282016http://ntur.lib.ntu.edu.tw//handle/246246/273185鈉離子電池的開發在近年來是一個備受關注的議題。由於鈉與鋰具有相似的化學性質，且在地表的蘊含量豐富而價格也相對低廉，其未來極有希望應用於大型的儲能裝置上。然而目前最大的問題是商用鋰離子電池中的石墨負極材料無法為鈉離子電池使用，因此急需相關的研究以尋找其適當的負極材料。本論文的研究目標即是以第一原理計算探討還原氧化石墨烯上的官能基的儲鈉機制以及鈉離子在其上的相關動力學行為。 在論文第一部分的研究中，我們從熱力學的層面來探討官能基對於儲鈉機制的影響。我們先針對修飾邊界─包含扶手椅型與鋸齒型邊界─的官能基之吸鈉行為進行分析。研究的結果顯示，邊界的形狀會影響鈉吸附於石墨烯奈米帶的行為，以鋸齒型石墨烯奈米帶吸鈉具有較高的吸附能。對於修飾邊界的官能基而言，含有氫基與酚的石墨烯奈米帶幾乎沒有儲鈉量，只有羰基、羰基-環醚基對、羧基與環醚基才具有增加儲鈉量的效益。此外，我們的結果也顯示鈉在含有上述四種官能基的石墨烯上最穩定的吸附位置是在邊界修飾的官能基上，而非基面上的六碳環中心，其中羰基、羰基-環醚基對與羧基這三種官能基會以物理方式吸附鈉，而環醚基則會藉由開環還原反應與鈉作用。對於儲鈉量的增益效果，由鈉化電位曲線的分析結果顯示，羰基相對其他官能基最能有效增加鈉的容量，而且從羰基與羰基-環醚基的儲鈉量比較，顯示單一環醚基的存在相對來說無法增加儲鈉量。另一方面，在邊界修飾羧基與環醚基的石墨烯奈米帶中，我們同時進行鈉和鋰系統的探討，發現到鋰系統相對鈉系統有較高的電容量。接著，我們針對存在於基面上的官能基之儲鈉行為進行探討。我們的研究結果顯示，它們會在石墨烯鈉化過程中扮演成核中心的角色，當鈉靠近這些官能基時，會與其形成氧化鈉和氫氧化鈉的小分子團簇，因而增加了石墨烯的儲鈉量。此外，從我們的計算結果也發現，基面上的官能基對於儲鈉量的增益效果是高於邊界修飾的官能基。另一方面，我們也針對分子團簇的穩定性進行研究。由吸附能與電荷轉移的分析結果顯示，這些分子團簇能持續穩定吸附在石墨烯奈米帶上而不會脫附造成不可逆的儲鈉量損失，且吸附作用力會隨著分子團簇的成長而有強化的趨勢。 在第二部分的研究中，我們從動力學層面來探討官能基對於鈉離子的擴散行為以及成核成長的影響。我們的研究結果顯示，邊界的形狀與修飾邊界的官能基種類都會影響鈉離子的擴散行為。當邊界修飾的官能基為氫基與酚的時候，鈉離子的擴散行為與沒有官能基的石墨烯沒有明顯差別；然而，當邊界修飾的官能基為羰基、羰基-環醚對與羧基的時候，石墨烯奈米帶邊界附近的鈉離子能夠輕易地擴散到邊界而最終吸附在官能基上。針對存在於基面上的官能基，我們的結果顯示，當鈉離子距離官能基一定距離以上時，其擴散行為幾乎不會受到官能基的影響。然而，當鈉離子逐漸靠近環氧基/羥基的時候會明顯的感受到往官能基方向的拖曳力，在距離官能基某一範圍內，其僅需克服極微小的能障即可往官能基的方向擴散，最終在石墨烯表面形成氧化四鈉/氫氧化二鈉的分子團簇。Sodium ion batteries (SIBs) are attractive alternatives of LIBs due to their chemical similarity and the natural abundance of Na resources. Although the energy density of SIBs is lower than LIBs, it is still a promising candidate for use in large-scale applications considering their low cost. However, Na ions cannot intercalate into graphite, the anodes of most commercial LIBs, for any appreciable extent, and most of the capacity turns out to be electrochemically irreversible. To further find appreciate materials for anodes on SIBs, great efforts have been made. In this study, we employed first-principles density functional theory calculations to investigate the sodiation process and the Na storage mechanism of reduced graphene oxide (RGO). Here we have applied various types of functional groups, which are located at edge and attached to basal plane, to investigate the Na storage and kinetic behaviors. There are two main parts in this thesis: In the first part of the thesis, we studied the effect of functional groups on the sodiation behaviors of nanoribbons (GNRs) in thermodynamic viewpoints. Our calculated results show that the adsorption is usually stronger for zigzag than armchair under the same edge groups, due to large number of states located at conduction band bottom. Furthermore, sodiation is almost unlikely to occur on pristine graphene and the GNRs terminated with OH and H groups. Only GNRs terminated with ketone, K-E pair, carboxylic acid and cyclic ether can effectively enhance the Na storage capacity. During the sodiation process, these edge-oxidized groups are always the most favorable sites for Na adsorption rather than the hollow sites on the basal plane. And most functional groups can physically adsorb Na except of cyclic ether, which was found to occur via a reductive ring-opening process instead of the general physical adsorption. The results of sodiation show that the Na/O atomic ratio for ketone-terminated GNRs is found to be around 1.0~2.0 depending on concentration of ketone and types of edge, while that for K-E pair is found to be around 0.25~0.5. This indicates that the ether group in K-E pair has no appreciable effect on Na storage enhancement. However, the Na/O atomic ratio for cyclic ether is found to be around 0.25~0.33, thereby, the capacity enhancement of cyclic ethers is only available when they are aggregated at the graphene edges. As for the in-plane functional groups, they were found to serve as the nucleation centers for Na clustering during the sodiation process, which can thus enhance the Na storage capacity of GNRs. And the Na/O atomic ratio for epoxy and hydroxyl is found to be 4 and 2 respectively. Compared the Na/O atomic ratio with edge functional groups, the in-plane functional groups appear to be more effective in enhancing the Na storage capacity than the edge-oxidized groups in terms of the calculated Na/O atomic ratios. Furthermore, the adsorption for NanO/Nan(OH) becomes stronger with cluster growth. In the second part of the thesis, we studied the effect of functional groups on the kinetic behaviors in reduced graphene oxide. Our calculated results show that the migration barrier of Na on the basal plane is similar to that on pristine graphene when Na is located away from the edge functional groups. As the edge sites are terminated with the ketone, K-E pair, or the carboxylic groups, Na atom on an edge hollow site is found to diffuse easily towards the edge terminations and then adsorb onto the edge functional groups with a very small energy barrier. As Na is in the vicinity of an epoxy/hydroxyl group, it can diffuse readily towards the functional group and then form a Na-O/Na-(OH) pair on the basal plane without any sizeable energy barrier. Furthermore, the Na-O/Na-(OH) pair can keep growing into a bigger compound cluster like the Na4O/Na2(OH) by overcoming a small energy barrier.4705074 bytesapplication/pdf論文公開時間: 2019/8/25論文使用權限: 同意有償授權(權利金給回饋學校)鈉離子電池還原氧化石墨烯第一原理計算Sodium-ion BatteryReduced graphene oxidesFirst Principles calculationsthesis10.6342/NTU201603469http://ntur.lib.ntu.edu.tw/bitstream/246246/273185/1/ntu-105-R03527061-1.pdf