- 學位論文
利用氟化物進行鋰離子電池矽碳負極表面改質
Surface Modification of Li-ion Battery Silicon-Carbon Anode with Fluorinated Polymers
摘要
穩定性佳的鋰離子電池（LIBs）如今已成為最重要的儲能設備之一。然而，近年來便攜式電子設備與電動汽車的飛速發展，開發新一代具更高能量密度與更長循環壽命之鋰離子電池是眼前最重要的課題。當前，除石墨因循環壽命較佳穩坐於商業化之負極材料代表。矽基材料的負極其因來源豐富、理論比容量高（3600mAh/g，Li15Si4）與低工作電位平台而引起人們的興趣。不幸的是，其最大的問題在於重複的充放電過程中會產生嚴重的體積膨脹(420%)進而造成活性物質與集電器的分離，矽基材料顆粒本身因破碎與反覆生成過厚且不穩的固體-電解質介面膜(SEI)，最終導致電容量快速衰退與較差的循環穩定性。 於本研究中，藉由打造一層具良好機械性質、離子導電率、鋰離子遷移率(tLi+)、熱穩定性及化學穩定性之高分子人造固體電解質介面膜（A-SEI）於矽-碳負極表面以減少活性物質與電解質之間的直接接觸，從而減少電解液因界面不可逆之反應被持續消耗的問題。同時我們發現該層鍍膜有助於增加矽顆粒結構的穩定性。我們主要選用含氟高分子材料進行矽-碳負極(MS-51)的表面改質。 Poly (vinylidene fluoride) (PVDF)因常被用於鋰離子電池中之黏著劑，此可間接證明PVDF具化學惰性，故其為此系統中之首選。將PVDF直接披覆於矽-碳極片表面，藉由負壓環境使高分子滲入極片中顆粒與顆粒間的空隙，使其不僅能減少電解液的接觸亦可以同時作為緩衝層，此方法有著可在不破壞電極片結構優勢下進行改質。接著我們發現於高分子膜中添加少量路易士酸式無機氧化物(Al2O3，AlF3)有助於吸收充放電過程中不斷產生的自由基以減少不可逆反應發生，而更加有效地提升循環壽命及穩定性。再者，為了增加實際應用之多元性及機動性，同時希望可更完全包覆活物粉體之表面以更有效的隔絕與電解液的直接接觸，除了針對電極片直接批覆改質，我們另外嘗試對顆粒進行鍍膜並且得到更好的循環表現結果。 經過實驗驗證高分子表面改質的可行性，我們進一步開創不同的製程方法，以新的高分子材料披覆於矽-碳負極極片表面。首先，使用PVDF的共聚合物Poly (vinylidene fluoride-co-hexafluoropropylene)(PVDF-HFP)，其因HFP鏈段的加入改變原屬塑性的PVDF性質，使其具彈性且柔軟的特質以提升包覆矽-碳負極顆粒表面的完整性。再將PVDF-HFP與交聯劑經由光固化製程形成網狀結構，使其在保有彈性的性質下同時具抵抗溶劑分解的能力。除機械性質的改變，我們也發現高分子膜因結晶度的下降使其有較高的離子導電度，且量測鋰離子遷移率亦有所提升，此不僅能延長循環壽命，同時提升其於高速率充放電下的表現。此新型高分子在循環測試中表現優於純PVDF膜。 經過實驗得知，此款矽-碳負極(MS-51)衰退的機制其主因為不穩定的顆粒結構導致，最外層披覆的石墨層在充放電過程中因應力改變而有脫落的可能性，此將造成電解液因二次接觸新鮮露出的矽顆粒而持續消耗形成過厚的SEI。因此，針對如何完整包覆顆粒且不影響電子傳導路徑之問題，我們嘗試利用氟烷基矽烷與矽-碳負極表面原生的羥基或氧基團，使其藉由自組裝反應於其表面形成低聚合物膜。在所有改質中，此新方法展現最佳的循環壽命及穩定性。總體而言，我們藉由不同方法於矽-碳負極極片或顆粒上建造出不同特性的高分子膜作為人造固體電解質介面膜（A-SEI）。並找出適合本研究主要材料MS51之高分子特性。成功地有效提升其整體電性表現。
並列摘要
Lithium-ion batteries (LIBs) with good stability have become one of the most popular energy storage devices. However, in recent years, the rapid development of portable electronic devices and electric vehicles, the development of a new generation of lithium-ion batteries with higher energy density and longer cycle life is the most important issue at hand. The silicon-based anode materials have attracted people’s interest because of its abundant sources, high theoretical specific capacity (3600mAh/g, Li15Si4), and low working potential platform. Unfortunately, the major problem with Si-based anode is the extreme volume expansion (420%) during the repeated lithiation /de-lithiation process, causing delamination between the active materials and the current collector, pulverization of Si materials, and repeated generation of thick unstable solid-electrolyte interface (SEI) film, resulting in rapid capacity fading and poor cycling stability. In this study, by engineering layer of polymer artificial solid electrolyte interface film(A-SEI) with good mechanical strength, ionic conductivity, Li+ transference number(tLi+), thermal stability and chemical stability on the Si anode surface, which can prevent the direct contact between the active material and the electrolyte to reduce the irreversible reaction and help to increase the stability of the Si particles structure. We mainly choose the fluorine-containing polymer to modify the Si/C anode (MS-51) surface. Poly (vinylidene fluoride) (PVDF) is the first choice due to good electrochemical stability. And we use electrode (impregnation) coating, which makes the polymer penetrate into the gap of the electrode to form an A-SEI layer without destroying the structure of the electrode itself under a negative pressure environment, it can not only reduce the contact of the electrolyte but also act as a buffer layer at the same time. Then we found that Lewis acid inorganic oxides (Al2O3, AlF3) added into the polymer film will more effectively improve the performance due to the absorption of the free radicals generated during the charge/discharge process can reduce the irreversible reactions. In addition to direct modification of the electrode, we additionally tried to coat on the particles, which is more completely covered, and get better cycle performance results. The feasibility of polymer surface modification is confirmed, we further develop different methods to modify the silicon electrode surface with new polymer materials. First, the PVDF copolymer (Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) is used due to the addition of HFP segments, making it flexible and soft to improve the coating integrity, which is formed into a network structure with cross-linker through a photo-curing process. And we also found that polymer films have higher ion conductivity due to the decrease in crystallinity, and the measured tLi+ has also been increased. This not only extends the cycle life but also improves the rate capability. It is known the mechanism of the decline of the electrode (MS-51) is mainly due to the unstable particle structure. The pitch coating on the outermost layer may fall off due to stress changes during charge and discharge. This will cause the second exposure of fresh silicon particles to form an excessively thick SEI. Thus, for the problem of how to completely coat the particles without affecting the electron conduction path, we tried to use the fluoroalkyl silane and the native hydroxyl or oxygen groups on the surface of the silicon-carbon anode with self-assembled reaction to form an oligomer layer. This new method exhibits the best cycle life and stability. In general, we use different methods to build polymer films with different characteristics on the Si/C electrode or particles as artificial solid electrolyte interface films (A-SEI). And find out the suitable polymer for the main material of this study MS51. Successfully and effectively improve its overall electrical performance.