| 摘要: | 隨著科技的發展，對能源普及性和便攜性的要求越來越高，對儲能設備的要求也逐漸提高。鋰離子電池(lithium-ion battery, LIB)其高能量密度及便利性被廣泛應用於日常生活中，例如便攜式電子設備和電動汽車。然而，作為商用陽極材料的石墨其理論容量相對較低，LIB的能量密度從1990年代(80 Wh/kg)到現在(250 Wh/kg)並無太大提升。加上直接電鍍鋰的理論容量更高(>3800 mAh/g)，因此鋰金屬電池(lithium metal battery, LMB)成為下一代儲能設備的發展方向。然而，鋰沉積過程中容易形成枝晶鋰，而枝晶鋰容易穿透隔離膜造成電池短路。為了解決這些問題，已經採用了幾種有效的策略，包括鋰載體(lithium host)和人工固體電解質界面膜(artificial solid electrolyte interphase, ASEI)作為鋰存儲空間和保護層，以提高和穩定陽極性能，然而目前文獻缺乏將兩種功能合併使用之研究，特別是機制的討論。
在此，本論文分為三個部分：(1)電泳改質石墨烯修飾陽極之效能提升機制: 將循環不同圈數之石墨烯(electrochemically-exfoliated graphene, ECG)、氟化石墨烯(fluorinated electrochemically-exfoliated graphene, FECG)在手套箱內拆解，後續透過能隔絕大氣之傳輸盒傳輸至掃描式電子顯微鏡(scanning electron microscope, SEM)，以及透過聚焦離子束(focused ion beam, FIB)觀察陽極的斷面結構，進一步推論得出鋰在FECG薄膜內部沉積行為，並發現層間結構具重要影響。(2)電泳結構化石墨烯複合陽極之鋰沉積行為研究: 為增加更多的空孔讓鋰沉積，提出在塗層內引入3D結構的氟化石墨烯球(fluorinated electrochemically-exfoliated graphene ball, FECG-ball)，形成了額外儲存鋰的空間，同時增強了塗層的機械強度，從而提高了LMB的性能穩定性。之前實驗室曾提出FECG/FECG-ball複合塗層具有2:1 (wt.)的片/球比可以顯著提高LMB中的穩定性並抑制枝晶鋰的生長。為了能更有效的使鋰沉積到多孔塗層內，提出將氟化奈米碳管(fluorinated carbon nanotube, FCNT)引入薄膜內作為成核位點，誘導鋰沿著FCNT沉積，使鋰快速且均勻的沉積。此雙功能性陽極(host & ASEI)具有低的成核過電位(50.9 mV)，經過320次循環後庫倫效率為85.3%，說明FCNT的添加，確實能使鋰快速的沉積在塗層內。(3)電泳沉積多層膜(Multi-layer film)功能性陽極之特性研究: 藉由在銅箔形成ECG/FECG複合多層膜電極，藉由上述FECG的雙功能特性，使鋰離子可以快速且均勻的沉積到薄膜內，而下層的ECG具有導電特性，可還原鋰離子而促進鋰沉積，後續也透過FIB以及能量色散X射線光譜(energy-dispersive X-ray spectroscopy, EDS)確認多層膜的內部分層結構，並在後續進行的電池量測，從電化學阻抗圖譜分析(electrochemical impedance spectroscopy, EIS)可以發現相同的循環圈數(150圈) 多層膜陽極的介面傳輸阻抗(Rct)為228.5 Ω，相較於單層膜電極FECG(486.7 Ω)，其阻抗顯著的降低，說明添加導電層ECG可促進鋰離子有效率地填於複合陽極中。
;The demand for energy storage equipment is rapidly increasing with the development of technology. As one of them, lithium-ion batteries (LIB) are widely applied in portable electronic devices and electric vehicles. Cause of the low theoretical capacity of the commercial anode material (graphite), the energy density of LIB has gradually increased from the 1990s (80 Wh/kg) to the present (250 Wh/kg). Therefore, lithium metal batteries (LMB) appear to be the next-generation energy storage devices due to their high theoretical capacities (>3800 mAh/g). However, Li dendrites can form during Li deposition, which penetrates the separator and cause short circuits.
This study is divided into three parts: (1) Efficiency Improvement Mechanism of Electrophoretic Modified Graphene Anode: electrochemically-exfoliated graphene (ECG) and fluorinated electrochemically-exfoliated graphene (FECG) in different cycles are disassembled in the glove box. Subsequently, it was transmitted to a scanning electron microscope (SEM) through the air-free sample transfer shuttle, and the cross-sectional structure of the anode was observed through a focused ion beam (FIB). Further inferences are drawn to the deposition behavior of lithium inside the FECG film, and it is also found that the interlayer structure has a significant influence. (2) Study on Lithium Deposition Behavior of Electrophoretic Structured Graphene Composite Anode: In order to add more voids for lithium deposition, it is proposed to introduce a fluorinated electrochemically-exfoliated graphene ball (FECG-ball) with a 3D structure in the coating film, which forms an additional storage space for lithium and enhances the mechanical strength, thereby improving the performance stability of the LMB. It was previously proposed by the lab that the FECG/FECG-ball composite coating with a flake/ball ratio of 2:1 (wt.) could significantly improve the stability in LMB and inhibit the growth of dendrite lithium. To more efficiently deposit lithium into the porous coating, it is proposed to introduce fluorinated carbon nanotubes (FCNT) into the films as nucleation sites to induce lithium deposition along the FCNT, so that the lithium is rapidly and uniformly deposited deposition. This bifunctional anode (host & ASEI) has a low nucleation overpotential (50.9 mV) and coulombic efficiency of 85.3% after 320 cycles, indicating that the addition of FCNTs can indeed enable the rapid deposition of lithium in the coating film. (3) Study on the Characteristics of Electrophoretic Deposition of Multi-layer Film Functional Anode: With the above-mentioned dual-functionality of FECG, lithium ions can be quickly and uniformly deposited into the film, while the underlying ECG has conductive properties, which can promote lithium deposition. Subsequently, the internal layered structure of the multilayer film was confirmed by FIB and energy-dispersive X-ray spectroscopy (EDS). And in the subsequent battery measurement, from electrochemical impedance spectroscopy (EIS), it can be found that the charge transfer resistance (Rct) of the multilayer film anode for the same number of cycles (150 cycles) is 228.5 Ω, compared with the single-layer membrane electrode FECG (486.7 Ω), its resistance is significantly reduced, indicating that the addition of a conductive layer ECG can promote the efficient filling of lithium ions into the composite anode. |
