鋰硫電池材料開發與電化學分析－PVDF膠體電解質對枝晶生成及碳保護層對硫電極之影響

Abstract

本研究針對目前發展中的鋰金屬電池之中極具潛力的鋰硫電池，解決目前研究遇到的困難，並且改進電池性能與壽命，主要從兩個面向切入，其一為陽極，鋰金屬電極的部分，在長時間充放電容易形成枝晶，我們利用具有機械強度的固態電解質置入對稱鋰金屬電池，分析不同硬度對於枝晶生成型態與速度的影響，我們發現楊氏模量0.05548MPa的電解質在0.1mA/cm2電流密度時，在前3000分鐘可以完全抑制枝晶的生長，並且促進電極表面愈趨平整。在實時觀察實驗中，我們發現在陽極側，電極氧化的情形十分類似腐蝕理論中的孔蝕現象，也發現隨著電解質的擴散係數變小，孔蝕現象也變得較不明顯。 鋰硫電池的陰極因為其特性，在電池使用過程中容易溶解出鋰硫化物，我們模仿Su等人使用奈米碳管製作陰極的保護層，並且使用自製碳化過的木質素做為與奈米碳管混和的替代品，結果發現使用50% 900℃碳化過的木質素製作的奈米碳管保護膜可以讓鋰硫電池的循環效率大幅提升，在60次0.1C循環充放電後保持有近1000mAh/g S的電容量。

This research is dedicating to one of the most promising lithium metal battery, lithium sulfur battery. The development of this kind of lithium metal battery is facing some challenges recently, which can split to two parts. One of them is dendrite growth on the lithium metal negative electrode, which may cause some safety issue, including short-circuited and energy capacity decay. We designed a symmetric cell to in-situ observe dendrite growth when applying a constant current. In order to study the relationship between mechanical strength and dendrite growth, we fabricated the cell with different gel polymer electrolyte with different Young’s modulus. We found that when using the gel polymer electrolyte which Young’s modulus is 0.05548MPa and the current density is 0.1mA/cm2, dendrite would not grow in the first 3000 minutes. We also found that the mechanism of oxidation of lithium metal is very similar to pitting corrosion. When using the electrolyte which diffusivity is lower, the phenomena of pitting corrosion is less apparent. The other part is the dissolution of sulfur electrode. Due to its physic properties, the lithium sulfide would gradually dissolve into the electrolyte. This may cause some energy capacity decay. We add an additional layer into the cell to be a protect layer. This layer could efficiently adsorb the lithium sulfide that dissolved into the solution, reducing the decay rate of the cell. We also mixed MWCNT with carbonized lignin, and found that 50% 900℃ carbonized lignin MWCNT film could make the cell remain 1000mAh/g S capacity after 60 cycles(0.1C).