| 摘要: | 本研究主要以化學還原法為基礎,利用超臨界二氧化碳製備二氧化錫 與碳材之複合材,並應用於鈉離子電池負極材料。藉由超臨界之高擴散性, 表面張力趨近零等優點,以提高二氧化錫於石墨烯上分散性,並觀察其電化 學特性。
實驗結果指出,超臨界製備之二氧化錫顆粒尺寸(SnO2-SC, 2.5nm)小於 傳統大氣製程(SnO2-air, 5 nm),在 0.02 A/g 充放電速率下,得到 95 mAh/g 之可逆電容。藉由添加相同含量(20 wt.%)之石墨烯與碳管藉此提升其導電 性並作為緩衝基底,並比較不同碳材之特性。20 w%之碳管添加僅能提升至 149 mAh/g,而 20 wt.%石墨烯則可提升至 275 mAh/g 的高可逆電容量,並 同時提升高速充放電能力,這是由於石墨烯擁有較高導電性與表面活性位 置。並當做緩衝區緩衝二氧化錫之高體積膨脹,使循環壽命得以提升,且在 超臨界的均勻分散下,在 100 圈之充放電後仍具有 66 %的電容維持率。
進一步探討石墨烯添加量、製程參數、電解液對於電化學特性之影響, 本研究以 10 wt.%、20wt%、35wt%三種不同石墨烯含量分別進行材料分析 與電化學測試,實驗結果得知 35 wt.%雖然穩定性較佳,但其可逆電容量不 高,而 20 wt.%之添加量能得到最佳之可逆電容量且壽命衰退量較小。超臨 界之流體密度會隨著臨界溫度與臨界壓力改變而有所變動,實驗結果表明, 適中的流體密度能擁有最好的電化學特性,於 145bar, 80°C 下可以得到 240 mAh/g 的可逆電容量,並於 100 圈充放電後仍有 77 %的維持率。
從實驗結果得知,超臨界合成之二氧化錫/石墨烯於納離子電池負極中 得到良好的可逆電容量,為了比較與傳統硬碳之間差異性,本研究使用商用 硬碳與二氧化錫/石墨烯做比較,結果顯示,超臨界二氧化錫/石墨烯於 0.02 A/g 充放電中有優於硬碳 230 mAh/g 的可逆電容量,並且於高速下能也能有
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較優異的電性以及維持率。
為了探討不同電解液在 SnO2/Graphene 負極材料中對於鈉離子電池中
的影響,以及安全性問題,因此分別使用有機溶劑 PC-EC、PC-FEC、EC- PC-FEC 和離子液體 PMP-FSI 三種電解液做為測試,結果指出,在 25°C 下 有機溶劑仍有較好的可逆電容量,然而因電解液不斷形成 SEI 膜而使電容 量衰退明顯。相較於有機溶劑,雖然離子液體在低溫時仍因黏滯性過高而電 容量不高,但在 100 圈充放電後仍然有 100 %的維持率。當操作溫度提升至 60°C 後,有機溶劑因電解液的分解,造成電容量明顯衰退,相反的,離子 液體則因黏滯性降低而明顯提高整體導電性,並在 0.02 A/g 下得到 346 mAh/g 的可逆電容量。
最後進一步探討電容量與理論電容量差異,藉由 Ex-situ XRD、Ex-situ EXAFS 觀察鈉化與去鈉化反應過程,以及 HRTEM 觀察鈉化後之電極。Ex- situ EXAFS 中觀察發現,SnO2 確實有明顯價數偏移,從原本高價數偏移至 低價數,證明確實有轉化反應發生。而從 Ex-situ XRD 中也看到了在鈉化過 程中 Sn 峰值以及鈉錫合金峰值產生,而在去鈉化過程中,Sn 峰值減弱以鈉 錫合金峰值消失,證明合金化反應可逆性。而HRTEM 中發現並非所有 Sn 顆粒皆反應完全,而使同時擁有 Na9Sn4 與最終相 Na15Sn4 存在以及一些中 間相(NaSnO2),因此可能為電容量無法達到理論電容量原因之一。;This study is based on chemical reduction method, using of supercritical carbon dioxide technology to production of tin dioxide and Carbon nanocomposites, and applied as an anode material for the sodium-ion battery. With supercritical high diffusivity, surface tension approaches zero, etc., to improve the dispersibility of tin dioxide on Graphene and observed the electrochemical properties.
The experimental results show that the supercritical prepared of tin dioxide (SnO2-SC) compare the traditional atmospheric synthesis (SnO2-air) has small nanoparticles shows a reversible capacity of 95 mAh/g (at 0.02 A/g). Adding Garphene and Carbon nanotubes as to enhance its electrical conductivity and as the buffer. The carbon nanotubes (SnO2-CNT20wt%-SC) only can up to 149 mA/g. The grapheme (SnO2-G20wt%-SC) shows a clearly higher capacity of 275 mAh/g as same condition, and also enhance the high-rate capacity, it is due to Graphene has high electrical conductivity and surface active position. And with buffer of Graphene can effectively suppress the volume expansion, and uniformly dispersed in supercritical exhibits a capacity retention ratio of approximately 66 % after 100 cycles.
In order to study the added amount of the carbon and the critical temperature (Tp) and critical pressure (Tc) impact electrochemical performance, this study was to use 10 wt.%, 20 wt.%, 35 wt.% different carbon content were analyzed and electrochemical tests.The experimental results that 35 wt.% has better stability, but its reversible capacity were not very high, the amount of 20 wt.% have good reversible capacity and a smaller amount of life decline. Change the Tp and Tc were be relatively close to the supercritical density and it will affect the state of the supercritical fluid. The results shows that high supercritical density have more high solubility, but the diffusion coefficient decreases, the electrochemical results
also show the 145 bar, 80°C has batter capacity and stable cycle performance. iii
Electrolyte were to affect the capacity and stability, in our results show the ionic-liquid has batter cycle stability with a capacity retention of 100% after 100 cycles, but the relatively capacity were small. Although, PC-FEC has the good capacity, but the cycle performance were unstable.
Finally, in order to study the reason of capacity why different to theoretical capacity, we use the Ex-siut EXAFS and Ex-situ XRD to observe the sodiation/desodiation reaction, electrode to charge on 0.01V and analysis by HRTEM. The results shows the conversion reaction was observed. But not form the final Na15Sn4 phase. Therefore, the capacity can’t not be achieved to the theoretical capacity. |
