鋰基電池材料特性之理論探討

Abstract

本論文將關注三種在能量儲存上具有前景的電化學電池，分別是鋰硫電池、鹼金屬硫電池和鋰有機硫電池。討論分為三部分。在論文的第一部分，我們使用密度泛函理論（Density Functional Theory, DFT）搭配SMD溶劑模型（Solvation Model Based on Density）並考慮電解液介電常數及donor number（DN）的效應，探討鋰硫電池當中電解液對多硫化物的電化學反應路徑及相關生成物的影響。我們發現當電解液具有低介電常數或高介電常數但低DN時，鋰硫電池中可能的電化學反應路徑為2Li+S8 → Li2S8 → Li2S6 → Li2S4。另外，當電解液具有高介電常數及高DN時，可能的電化學反應路徑為S8 → S82- → S4•- → S42-或S8 → S82- → S62- → S3•-，其中S82-亦有可能不經由S62-而生成S3•-，即S82- → S3•-。 論文的第二部分討論鹼金屬硫電池的材料特性，我們使用DFT搭配SMD溶劑模型來探討不同鹼金屬的硫化物在二甲基亞碸溶劑中的穩定性。研究結果顯示，相較於溶合更強，與更多溶劑分子鍵結而形成較大團簇的鋰離子，銣離子與短鏈多硫化物陰離子（硬鹼）的靜電力較強，而使得短鏈多硫化物陰離子可以被穩定。此外，鹼金屬硫化物M2S的溶解度會影響電容量。 論文的第三部分，我們使用DFT方法搭配SMD溶劑模型來探討鋰有機硫電池中不同取代基多硫化物對電池電容量的影響。我們發現添加二烯丙基二硫化物或二烯丙基三硫化物於對稱取代有機硫化物（例如：二苯基二硫化物）作為反應物時，系統中會形成非對稱取代有機多硫化物；並且因烯丙基自由基容易生成，可促成非對稱取代有機硫化物中的碳-硫鍵斷鍵。此碳-硫鍵斷鍵會產生烯丙基自由基和有機二硫/三硫自由基（例如：苯基二硫自由基或苯基三硫自由基）。有機二硫/三硫自由基進一步還原，並生成S2-，使得電池陰極於放電過程中可以得到較多電子，從而提升電池的電容量。因此二烯丙基二硫化物或二烯丙基三硫化物是提升電池電容量的重要因素。

We focus on the theoretical investigations that are related to three kinds of promising battery devices with application potentials in the electric energy storage. They are lithium-sulfur batteries, alkali metal-sulfur batteries and rechargeable batteries. This thesis includes three parts. In the first part, we apply density functional theory (DFT) combining with the solvation model based on density (SMD) model to consider the effect of the electrolyte dielectric constant and donor number (DN) on the electrochemical reaction pathways and the corresponding polysulfide products. We find that in solvents with either a low dielectric constant or a high dielectric constant but a low donor number, the reaction pathway follows the order of 2Li+S8 → Li2S8 → Li2S6 → Li2S4. On the other hand, the electrochemical reactions in the solvent with high dielectric constant and high DN can be described by the following reaction pathways: S8 → S82- → S4•- → S42- , S8 → S82- → S62- → S3•-, and S82- → S3•-. In the second part, we turn to the material properties of alkali metal-sulfur batteries. DFT and SMD Model are employed to investigate the stabilities of alkali metal polysulfides in dimethyl sulfoxide. We find that, in contrast to the lithium ion that shows stronger solvation and forms a larger cluster by binding with more solvent molecules, the rubidium ion exhibits stronger electrostatic interaction with strongly negatively charged short-chain polysulfides. Besides, the solubility of alkali metal sulfides M2S would affect the capacity of batteries. In the last part, we apply DFT and SMD methods to study potential capacity enhancement of organosulfides as catholytes with different organic substituents in rechargeable lithium batteries. We find that when the diallyl di/trisulfides are added to the symmetrically substituted organosulfides such as diphenyl disulfides, asymmetric substituted organosulfides will be formed in the system. This promotes the bond breaking of carbon-sulfur bonds of asymmetric substituted organosulfides due to the generation of allyl radical formation. The organo di/trisulfide radicals thus formed, such as phenyl di/trisulfide radicals, will then be reduced to produce sulfide ions (S2-), leading to more electron gain in the cathode during discharging. Therefore, diallyl di/trisulfides play the role of an activator to increase in the battery capacity.