製備層狀MnO/C奈米複合材料與鋰離子電池負極材料之應用

Fabrication of Layered MnO/C Nanosheets as Anode Materials for Li-ion Battery

林瑞恩(Jui-En Lin)

指導教授 ： 董瑞安

國立清華大學/原子科學院/生醫工程與環境科學系/碩士(2014年)

摘要

MnO具有環境友善、高理論容量(756 mAh/g)及較低的電位延遲等特點，所以被視為極具發展性的鋰離子電池負極材料。但MnO在初次放電過程有會有極大的不可逆反應產生，包含固體電解質膜的形成或是顆粒的聚集等現象，因此大幅降低鋰離子電池之效率。而使用碳材包覆於MnO表面不僅可減少固體電解質膜得形成且避免顆粒聚集，同時也可提升MnO電化學性能，在適當調控下，表面披覆的碳可以分散各MnO顆粒，並具有減緩體積變化以及作為電子與離子傳輸通道的功能，因此MnO/C複合材料為近年來研究的主軸之一。
本研究是利用油酸錳作為碳與錳元素之前驅物，配合低成本之硫酸鈉作為模板，以鍛燒方式形成MnO/C二維平面複合材料。同時探討鍛燒氣氛、鍛燒溫度與時間對其形貌之影響，最後進行材料於鋰離子電池負極應用效能之評估。鍛燒氣氛方面，發現選用氮氣時可以得到高純度之MnO晶相。鍛燒溫度方面，隨溫度之提高可提升MnO之結晶性，但碳含量卻下降，由500 C的12.9 wt%，降至 800 C的0.9 wt%。碳含量下降會使複合物之形貌由二維層狀結構轉變成三維堆疊結構，造成反應面積的下降，在考量MnO的結晶性與碳含量之情況下，得到最佳之鍛燒溫度為600 C。就鍛燒時間而言，較長的鍛燒時間會使複合物中碳含量緩慢下降，其下降趨勢和溫度相比較不明顯，而經鍛燒3小時可以得到最佳化的參數。將各材料組成半電池並經由充放電儀測試，其結果發現複合物中碳含量對於電化學效果呈現火山型的結果，碳含量高於10 wt%時會造成整體電容值的下降、過多的電解質分解和較厚的固體電解質膜。當碳含量小於10 wt%時，MnO顆粒間缺乏有效的阻隔，經多次循環後會因顆粒聚集而使電容值嚴重衰退。同時考量MnO顆粒與碳含量因素下，本實驗中最佳化之碳含量為11.9 wt%，在100 mA/g掃速下，電容值可達到500 mAh/g，並維持庫倫效率達97%以上；當掃速提高至1000 mA/g時，其放電電容值約130 mAh/g，庫倫效率維持在96%以上。本研究外層包覆的碳材受限於氧化錳長晶的條件，所以其導電性還有加強的空間，將來可以使用氮參雜或其他方式來提升奈米材料整體性能。

關鍵字

鋰離子電池 ； 碳包覆 ； 氧化錳 ； 負極材料

並列摘要

MnO is an attractive anode material for Lithium ion battery(LIB) because of the low conversion potential, low voltage hysteresis (< 0.8 V), low cost, high capacity(756 mAh/g), environmental friendliness, and the high abundance of Mn. However, MnO will have a large amount of irreversible reaction , including formation of solid electrolyte interphase and aggregation of particles during the initial discharge process, leading to the significant reduction of the performance of LIBs. The use of carbon coating on MnO nanoparticles can not only reduces the formation of solid electrolyte interface and avoids particle aggregation, but can also improve the electrochemical performance of MnO. Under appropriate control, surface coating carbon can disperse MnO particles well, limit the volume change of particles and act as a channel for electrons and ions transportation. In summary, MnO /C composite material gets a lot of attention in recent years.
In this study, we use manganese oleate as a precursor for both carbon and manganese element, mixing with low-cost sodium sulfate template. After calcination and washing process, the final product is two-dimensional MnO/ C composite. The influence of calcination atmosphere, calcination temperature and holding time on the morphology of composites was examined, and then lithium ion battery anode material. Calcined under nitrogen atmosphere can get high purity MnO. Higher temperature can get better crystallinity of MnO, but the carbon content has dropped. The carbon content is decrease from 12.9 to 0.9 wt% when temperature increased from 500 C to 800 C. The morphology of the composites changed from the layered structures into three-dimensional stacked structures, resulting in the decrease in reaction area, as the carbon amount decreased. In the presence of MnO crystalline and the carbon content, the optimum calcination temperature is 600 C. The long calcination time decreased carbon content slowly. Comparing to temperature effect, the carbon loss tendency in time effect is not obvious, but calcination for 3h can get the most suitable carbon thinckness. Each material will test in the form of half-cell by charging and discharging test and cyclic voltammetry. The carbon content affecting the electrochemical performance show a volcano type result. When the carbon content is higher than 10 wt%, it will show the capacity decrease, excessive electrolyte decomposition and thicker SEI film. In contrast, less carbon content causing the less barriers between MnO particles and fast capacitance decrease after cycling. Considering the carbon content and MnO particles simultaneously, the optimized carbon content in this study is 11.9 wt%. The discharge capacitance can be up to about 500 mAh/g at 100 mA/g scan rate, and the coulomb efficiency is maintained more than 97%; when the scan rate increased to 1000 mA/g, the discharge capacitance value is about 130 mAh/g, coulomb efficiency is maintained above 96%. The manganese oxide crystal growth will limit the improvement of conductivity for carbon content under calcination process.

並列關鍵字

MnO ； anode ； lithium ion battery ； composite ； carbon

參考文獻

78. 李孟倫，快速充電鋰離子二次電池負極複合碳材之研究，國立清華大學材料科學工程學系碩士論文，2009.

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