透過溶劑化電解液控制鋰硫電池中硫正極的還原機制

Abstract

能源的儲存已成為再生能源供電穩定的關鍵，其中鋰離子電池為大型儲能設備的首選。然而，鋰離子電池可以提供儲存的最高能量已無法滿足交通等市場需求，因此科學家們致力發展於尋找更高電容及能量密度的電池，而鋰硫電池因其具有高電容、高能量密度，且硫因價格低廉且對環境友善，因此被譽為下一世代的電池。但是在鋰硫電池運轉時產生的穿梭效應大幅降低鋰硫電池效能，其中溶劑化電解液因其具有抑制穿梭效應的能力，因此被視為抑制穿梭效應並提升鋰硫電池性能的解決方案。 本實驗分別使用傳統鋰硫電池電解液(1 M LiTFSI 溶於DOL/DME 1:1(v/v))和不同的溶劑化電解液，包括：4.2 M LiTFSI 溶於ACN/TTE 1:1(v/v)、4.2 M LiTFSI 溶於DME/TTE 1:1(v/v)和4.2 M LiTFSI 溶於THF/TTE 1:1(v/v)進行電化學性能的測試，更使用了循環伏安法、臨場拉曼光譜及臨場X光吸收光譜等分析技術探討在不同電解液下，硫電極在充放電過程中的反應機制，並進一步的探討反應機制對電池性能的影響。 本研究結果顯示，溶劑化電解液相對於傳統鋰硫電池電解液具有更高的電容及更穩定的循環壽命。在臨場拉曼光譜中發現傳統鋰硫電池電解液會涉及複數個電化學及化學反應，因此可以觀察到各種不同的多硫化物，包括：S82-, S7-, S62-, S42-, S3●-，此現象也證明歧化反應的發生。然而若使用溶劑化電解液，只可觀察到中間產物S82-和S42-的生成，此說明了溶劑化電解液的電子轉移途徑較為單純且可以抑制歧化反應的發生。在臨場拉曼光譜實驗中更發現了除了濃度效應以外，不同溶劑對於鋰鹽的溶劑化能力不同，抑制歧化反應的能力也不同。在臨場X光吸收光譜中可以觀察到溶劑化電解液相較於傳統鋰硫電池電解液可溶解較少量的多硫化物。根據實驗結果，我們發現抑制歧化反應的發生可以讓電池具有更優異的電化學性能。Sparingly solvated electrolytes have been proposed as one of the solutions to prevent shuttle effect in lithium-sulfur (Li-S) batteries. However, the sulfur reduction mechanisms occurred in different solvent systems are still unclear. In this study, we use in situ Raman/X-ray absorption spectroscopy to investigate the effect of conventional electrolyte (DOL/DME) and solvated electrolytes (ACN-based, DME-based and THF-based electrolyte) on the Li-S battery performance and sulfur reduction mechanism. In situ Raman spectroscopy shows that polysulfides, such as S82-, S7-, S62-, S42-, S3●- are formed through multiple electrochemical and chemical reactions in conventional ether-based electrolyte system, suggesting that disproportionation processes occur in the dilute electrolyte during discharge process. In contrast, in situ Raman spectroscopy shows that sulfur is reduced and forms S82-, S42- in solvated electrolyte during discharge process which suggests that solvated electrolyte could suppress disproportionation and has an uniform electron transfer pathway. Moreover, the solvated electrolyte with good solvation ability consists of less amount of free solvent which can suppress the disproportionation process efficiently. The solvation ability of solvated electrolytes was studied in the present work in detail. In situ X-ray absorption spectroscopy of sulfur-carbon cathode suggests that less amount of polysulfide dissolution is formed in the solvated electrolyte than that of in the conventional ether-based electrolyte. Our results demonstrate that solvated electrolyte could suppress disproportionation processes resulted from the polysulfide dissolution and lead to an enhanced Li-S battery performance.