反應分子動力學探討氟化鋰人工固態電解質之機械性質

Abstract

近年來，鋰金屬電池之相關研究由於其擁有高理想電容量而備受各界關注。然而，介於鋰金屬負極與電解液之間的固態電解質介面(solid electrolyte interphase, SEI)將因其組成不均勻與表面不平穩而導致鋰枝晶生長並困擾著鋰金屬電池的實際應用。 為了解決這些問題，越來越多研究致力於製成一均勻、穩定且擁有高機械強度之人工固態電解質介面(artificial solid electrolyte interphase, ASEI)以取代自然生成之SEI。此次研究我們將選用擁有高機械強度的氟化鋰(lithium fluoride, LiF)作為主要塗層材料，並參照他人文獻中使用無毒氣體試劑與鋰金屬反應之製成技術，利用反應分子動力學(reactive molecular dynamics, RMD)之模擬方式放入氣相1,1,1,2-四氟乙烷(1,1,1,2-Tetrafluoroethane, Freon R134a)與鋰金屬基板反應形成富含LiF的ASEI薄膜並探討其生成結果、機械性質改善結果與溫度對其生成之影響。 本次研究採用反應力場(reactive force field, ReaxFF)描述分子間的化學成、斷鍵現象，並使用LAMMPS程式作為模擬工具。首先，我們透過反應分子動力學模擬生成了一富含LiF之ASEI並驗證了系統生成LiF之反應途徑。接著，我們在反應過程中發現副產物卡賓類化合物 (carbene) 與1,1-二氟乙烯氣體 (1,1-Difluoroethylene, C2H2F2) 的生成與堆積將可能會使LiF較難形成良好晶體並降低表面膜的穩定性，成為此製成方法的潛在風險。而透過拉伸試驗模擬我們可知，本次研究所生成之ASEI擁有比使用標準電解液所生成的SEI更好的極限應力、韌性值與楊氏模數，也證實了本次生成之ASEI雖並不足以完全抑制鋰枝晶生長，但仍能夠有效地改善SEI薄弱之機械性質。最後，我們探討了溫度對於生成結果之影響並發現調節系統反應溫度不但能控制欲生成之ASEI厚度，還能夠透過提升溫度以增加ASEI內的LiF數量，進而提升機械性質。本團隊也將在未來持續優化本模擬模型，以生成更加理想且更堅固之LiF-rich ASEI並針對反應途徑進行更加全面的探討。我們相信本研究方法不限於單一ASEI塗層材料的探討，也期望在不久的將來能夠將其實際運用在商業用途上。

Research on lithium metal battery has been attracting high attention nowadays because of its high theoretical capacity. However, an unstable and accumulated solid electrolyte interphase (SEI) film, which is a layer between anode and electrolyte, will result in dendrite growth and ineluctably plaguing the practical applications of Li metal batteries. To solve these issues, efforts have been made to replace natural SEI with artificial SEI (ASEI), which is more stable, uniform and robust. Herein, we choose LiF which has high mechanical strength as the main coating material and present reactive molecular dynamics (RMD) simulations of the LiF-rich ASEI formation by using Freon R134a gas reagent and Li metal. We investigated the formation and the mechanical properties of the LiF-rich ASEI, and we explored the effect of temperature on ASEI formation by changing the simulated temperatures. Our RMD simulations were based on the reactive force field (ReaxFF) to simulate bond formation and bond breaking in chemical reactions. First of all, we successfully formed a LiF-rich ASEI by using reactive molecular dynamics simulation. Further, Our study shows that the formation and accumulation of the by-products (carbene and 1,1-difluoroethylene) may affect the structure of LiF and reduce the stability of the ASEI and become a potential risk of this method. From the tensile test results, we know that the LiF-rich ASEI we formed has better mechanical properties (ultimate stress, toughness and Young's modulus) than natural SEI, which proves that although the ASEI we formed is not robust enough to resist lithium dendrites growth comprehensively, but it can indeed improve the weak mechanical properties of SEI. Lastly, we found that increasing the reaction temperature of the system not only effectively increases the reaction rate of the system and LiF production, but also improves the thickness of ASEI to improve the mechanical properties of ASEI. In the future, we will keep on optimizing the simulation model to form a more ideal and robust LiF-rich ASEI and conduct a more comprehensive discussion on the reaction pathway. We believe this research method is not limited to the study of a single ASEI coating material, and we expect it to be practically used in commercial applications in the near future.