官能基修飾與木質素添加奈米碳管保護層於鋰硫電池應用

Abstract

傳統鋰離子電池礙於能量密度的限制，要裝備在電動車之中所需之量非常大，因此被譽為新一代鋰離子電池的鋰硫電池，藉著它所具備的高能量密度逐漸發展起來。鋰硫電池的發展過程中，長鍊硫的穿梭效應一直限制著它的應用，因此本研究致在開發兩種不同的保護層，來限制穿梭效應對鋰硫電池的影響，並提升電池性能。其一為木質素添加於奈米碳管保護層，製備不同比例木質素添加之奈米碳管保護層。首先利用BET和SEM發現25wt%木質素添加保護層有最佳之孔徑分布與比表面積，且也針對配置各比例保護層之鋰硫電池做定電流充放電實驗，由結果得知在配置25wt%木質素添加奈米碳管保護層，具備高初始放電容量與低衰退率，說明此比例為最佳配比並能有效減緩穿梭效應的影響。 其二為胺基化奈米碳管保護層，目的在於利用官能基修飾過後的碳材表面對長鍊硫產生吸附作用，因此除了奈米碳管的物理阻隔，還外加了極性表面的吸附效果，以此來減緩穿梭效應的影響。本研究利用自行合成之胺基化奈米碳管，再利用XPS和FTIR檢驗完材料特性後，將其製備成保護層結構，配置於鋰硫電池系統中，運用CV和EIS確認具備高電化學可逆性與低電荷轉移阻抗後，以1 C rate電流密度進行充放電，發現配有胺基化奈米碳管保護層之鋰硫電池擁有相當高的初始放電容量(1470 mAh g-1)和低衰退率(0.25% per cycle)，原因是胺基與醯胺基吸附長鍊硫使穿梭效應減低，活性物質使用量增加。其後，也利用EDS及吸附實驗確認胺基化奈米碳管保護層對長鍊硫的吸附效果，並以UV-Vis量化其阻擋長鍊硫通過程度。

Lithium-ion batteries have been widely used in our daily life because the climate change, in part due to burning fossil fuels, forces human beings to alter the lifestyle. Recently electric vehicles developed rapidly and traditional lithium-ion batteries couldn’t meet the demand for the high energy density, so the lithium-sulfur (Li-S) batteries, which was considered as the next generation of lithium-ion batteries, gradually attract lots of attentions due to its high energy density. During the development of Li-S batteries, the polysulfide shuttle effect always limits its applications. Thus, in our work, we devote to exploiting two types of protection layer to alleviate the polysulfide shuttle effect and enhance the performance of the Li-S batteries. One of them is the lignin/MWCNT composite protection layer. We successfully prepare various weight ratios of lignin/MWCNT composite protection layer and find the 25 wt% lignin/MWCNT protection layer have the highest specific surface area and advantageous pore distribution on the basic of BET and SEM results. Moreover, we conduct the galvanostatic charge/discharge experiment for the Li-S batteries with various weight ratios lignin/MWCNT composite protection layer and discover that the Li-S batteries with 25 wt% lignin/MWCNT composite protection layer have the best electrochemical performance with its high capacity and low decay rate. Our results indicate that the 25 wt% lignin adding into the MWCNT protection layer is the best formula and alleviate the polysulfide shuttle effect effectively. The other type of protection layer is fabricated by amide and amine group multi-walled carbon tube (A-MWCNT), which is aimed to suppress shuttle effect by the polar surface of carbon materials to perform strong interaction with polysulfides. A-MWCNT not only have the physical hindering ability to limit polysulfide passing but also possess the chemical adsorption ability to polysulfides and so reduce the shuttle effect. In our work, we successfully synthesized A-MWCNT and confirm its characterization by using XPS and FTIR. Next, we fabricate the free-standing A-MWCNT protection layer and assemble it into the Li-S batteries. The Li-S batteries with the A-MWCNT protection layer are demonstrated to have high electrochemical reversibility and low charge transfer resistance according to the results of CV and EIS. We also find that the Li-S batteries with A-MWCNT protection layer show great electrochemical performance with high initial capacity (1470 mAh g-1) and low decay rate (0.25% per cycle) because the amide and amine interact with the polysulfide and increase the active material utilization. We further qualitatively determine the adsorption ability of A-MWCNT protection layer to polysulfide by using EDS mapping analysis and adsorption test and measure the blocking extent in quantitative by UV-Vis analysis.