高能量矽硫電池之合成與分析

Abstract

由於汽車工業的持續發展，對高能量密度二次電池的需求逐步增加，鋰硫電池開始走進人們的視野，鋰硫電池的理論比能量高達2600 Wh/kg，而單質硫的理論比容量也高達1675 mAh/g；同時，硫的儲存量豐富、廉價、並且環境友好，雖然可充電鋰硫電池相比於傳統鋰離子電池有諸多優勢，但目前其可實現的實際比容量仍遠低於理論比容量，循環壽命也較短等弊端限制了其大規模應用。此外，鋰金屬作為電極的使用具有非常嚴重的安全性問題，而這也是導致鋰硫電池無法商業化的最大阻礙。 在這項研究中，我們採用矽基材料取代金屬鋰作為負極，形成了所謂的矽硫（Si-S）電池新概念。首先，我們研究了不同類型電解質中矽基材料的穩定性，通過系統性的討論，我們發現矽表面上穩定的固體電解質界面（SEI）形成尤其重要，特別是在醚基電解質中。另外，藉由使用循環伏安法（CV）測量，我們發現醚基電解質的主要問題是來自二甲氧基乙烷（DME）溶劑。其中，我們也意外地發現發現石墨（KS6）作為導電添加劑會導致剝離效應，這意味著石墨（KS6）不適用於醚基電解質。然而，在將少量氟代碳酸亞乙酯（FEC）添加到電解質中之後，矽基材料的性能有顯著的改善。結合循環伏安法測量（CV）數據和文獻回顧，我們了解到電性的提升是來自氟代碳酸亞乙酯（FEC）在第一次放電步驟中的還原反應，其會在矽表面上產生氟化鋰(LiF)分子，並且作為“晶種”以誘導更多更穩定固體電解質界面(SEI)形成。 最後，我們組裝了兩種矽硫電池，一種是結合了矽-碳化矽-鎳-碳複合材料負極和硫/聚丙烯腈複合材料正極 (SSNC-S/PAN)；另一則是結合氧化矽-碳複合材料負極與硫/碳複合材料正極 (SiOx@C-S/C)組成矽硫全電池，有趣的是第一種搭配在初始循環時成功地達到幾乎與理論值一樣的可逆電容量（1600mAh / g基於硫）。此外，透過添加氟代碳酸亞乙酯（FEC）於電解質當中，也能有效的改進循環穩定性和變速率性能，我們將此提升歸功於SEI穩定的形成。另一方面，我們設計了一個有趣的實驗，首先，我們將矽基材料(SSNC)先在酯類電解質中做一圈的充放電測試以形成穩定的固體電解質界面(SEI)，然後再將電池拆解並改變至醚類電解質中做充放電測試，以觀察舊有的穩定固體電解質界面(SEI)是否可以始終穩定。不意外地，充放電數據結果表明此電池在改為醚類電解質後開始有明顯的電容量衰減。這也是一個證據可以說明如果電解質不兼容，就算已形成的穩定固體電解質界面(SEI)仍有可能遭到破壞。因此，為了將矽硫電池實現為商用產品，我們建議添加適當量的氟代碳酸亞乙酯(FEC)添加劑於酯類電解質中，此配方對於我們的SSNC-S/PAN電池也是最佳的選擇。

Because of automotive industry sustainable development, the demand for higher energy density rechargeable batteries make the lithium-sulfur (Li-S) batteries become one of the most attractive candidates. The Li-S systems have a theoretical specific energy of 2600 Wh/kg while the theoretical capacity of sulfur is 1675 mAh/g. Sulfur is abundant, low cost and environment friendly. Although the rechargeable Li-S batteries possess more advantages over the conventional lithium-ion batteries, the practical use faces with a variety of problems such as low specific capacity and short cycle life. In addition, the usage of lithium metal as electrode has safety issues which is the most difficulty leads to inability of commercialize. In this study, we replace the metallic lithium with silicon-based materials to make a new concept of silicon-sulfur (Si-S) batteries. At first, we studied the stability of silicon-based materials in the different type of electrolyte, through the systematic discussion we found that the stable solid electrolyte interface (SEI) formation on silicon surface is extremely important especially in ether-based electrolyte. In addition, by using the cyclic voltammetry (CV) measurement, we figure out the major problem of ether-based electrolyte is the dimethoxyethane (DME) solvent. Also, we accidently discovered that the graphite (KS6) as conductive additive will cause the exfoliation effect which means KS6 was not suitable in ether-based electrolyte. However, after we add the small amount of fluoroethylene carbonate (FEC) into electrolyte, the performance of silicon-based material has significant improvement. Combined with the CV data and paper review, we realized the enhancement is coming from the reduction reaction of FEC in the first discharge step which will produce the LiF molecules on the surface of silicon and act as a “seed” to induced the much more stable SEI formation. Finally, we assembled two kinds of Si-S batteries, one is combined the Si-SiC-Ni-C anode with Sulfur/Polyacrylonitrile cathode (SSNC-S/PAN) and the other one is combined SiOx@C anode with Sulfur/Carbon cathode (SiOx@C-S/C), more interestingly, the former can successfully demonstrate nearly theoretical capacity (1600 mAh/g based on S) at initial cycle. In addition, by introducing the FEC as electrolyte, the cycling stability and rate performance has effective improvement which contributed to much stable SEI formation. Furthermore, we design one interesting experiment which is testing the Si-based cell in carbonate-based electrolyte first to form a stable SEI, and then change to ether-based electrolyte to see if the stable SEI can be always stable. Not surprisingly, the results indicated that the design cell starts the obvious capacity fading after changing to ether-based electrolyte. This is also an evidence to confirm that the stable SEI will be destroyed if the electrolyte is incompatible. Therefore, to fulfill the Si-S batteries into commercial product, we suggest that using all-carbonate electrolyte with appropriate amount of FEC additive which is also the best choice for our SSNC-S/PAN combination.