聚苯乙烯-b-(聚異戊二烯-g-聚乙二醇)嵌段共聚高分子於鋰金屬電池固態高分子電解質中的應用

Abstract

傳統以聚乙二醇(poly(ethylene oxide), PEO)為材料的固態高分子電解質(SPEs)因其半結晶的特性使其在常溫的鋰離子傳導度僅有約10-7~10-8 S cm-1，為提升其在常溫下的離子傳導度，本研究開發了一種A-block-(B-graft-C) type雙嵌段共聚高分子作為固態高分子電解質材料，其中A嵌段為具有較高玻璃轉換溫度的聚苯乙烯(polystyrene, PS)嵌段，在固態高分子電解質中可形成剛硬的高分子骨架以提升整體的機械強度；B嵌段為性質柔軟的聚異戊二烯(polyisoprene, PI)嵌段，其具備側鏈垂懸雙鍵可進行後續的官能基修飾以及接枝反應；C嵌段為小分子量的聚乙二醇，利用低分子量高分子熔點較低的特性，解決聚乙二醇於常溫下結晶的問題以提升鋰離子傳導度。本研究期望藉由嵌段共聚高分子具備奈米尺度微相分離結構的特性，使此固態高分子電解質同時兼具優異的機械強度與鋰離子傳導能力。 研究中以陰離子聚合技術合成不同分子量的PS-b-PI高分子，再以官能基轉換與PEO側鏈接枝反應製備不同PEO接枝率的PS-b-(PI-g-PEO)雙嵌段共聚高分子，將各高分子與鋰鹽混和後以溶劑揮發法製備固態高分子電解質薄膜，並系統化討論主鏈分子量、PEO接枝率及鋰鹽濃度對相分離結構、機械強度及鋰離子傳導度的影響。於小角度X光散射試驗中，各SPEs皆出現層狀的奈米尺度的相分離結構，且各樣品相分離結構中的domain spacing皆有隨鋰鹽濃度上升而增加的趨勢。差示掃描量熱法與X光繞射結果均證實，低分子量 PEO側鏈接枝的分子設計於常溫下不具結晶性，此特性有助於提升SPEs在室溫下的離子傳導表現。動態機械分析試驗中P2-G15R05樣品展現出最佳的機械強度，其楊氏模數達134.5 MPa，結果顯示，提升PS嵌段的分子量及其含量，能有效增強固態高分子電解質的機械強度。變溫鋰離子傳導度的測試中，P2-G35R05樣品於常溫下表現出最高的鋰離子傳導度，達4.14×10⁻⁶ S cm⁻¹，歸因於其較大的傳導通道與高PEO含量。在鋰離子傳遞活化能的分析中，較大尺寸的微結構可以提供較低的活化能，顯示鋰離子主要通過PI-g-PEO區域中傳遞。鋰鹽濃度上升對傳導度的貢獻並不明顯，而活化能反而有增加的趨勢，表示SPEs中鋰鹽的添加限制了PEO鏈段的運動，鋰離子在高鋰鹽濃度下較難藉由PEO鏈段的運動傳遞。

Traditional solid polymer electrolytes (SPEs) based on poly(ethylene oxide) (PEO) exhibit limited lithium ion conductivities of only about 10-7~10-8 S cm-1 at room temperature due to the semi-crystalline properties of PEO. To improve the ionic conductivity at room temperature, this study developed an A-block-(B-graft-C) type diblock copolymer as a solid polymer electrolyte material. The A block is polystyrene (PS), which has a high glass transition temperature and serves as a rigid polymer framework to improve the overall mechanical strength of the SPE. The B block is polyisoprene (PI), which contains pendant double bonds for subsequent functional group modification and grafting reaction. The C segment is short-chain PEO, designed to lower the crystallinity of PEO at room temperature, thereby enhancing lithium ion coductivities. This study hopes to make the SPEs have both excellent mechanical strength and lithium ion conductivities by using the characteristics of block copolymers with micro-phase separation structure. In this study, PS-b-PI polymers with different molecular weights were synthesized by anionic polymerization technology, and then PS-b-(PI-g-PEO) diblock copolymers with different PEO grafting ratio were prepared by functional group conversion and PEO side-chain grafting reaction. Free standing SPE membranes are obtained by solvent casting from the blends of lithium salts and the block copolymer in designated composition. In this work, we systematically study the interplays among the backbone molecular weight, PEO grafting ratio, and lithium salt concentration on phase-separated structure, mechanical strength, and lithium-ion conductivity of the resulting SPEs. Small-angle X-ray scattering (SAXS) measurements revealed nanostructured lamellar (LAM) phase separation in all SPEs. The results of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) confirmed that the low-molecular-weight PEO side chains were amorphous at room temperature, which contributed to improve lithium ion transport. In the dynamic mechanical analysis (DMA) test, the P3-G15R05 sample showed the highest mechanical strength, with a Young's modulus of 134.5 MPa. The results showed that increasing the molecular weight and content of the PS block can effectively enhance the mechanical strength of the SPEs. In the test of lithium ion conductivity, the P2-G35R05 SPE showed the highest lithium ion conductivity at room temperature, reaching 4.14×10⁻⁶ S cm⁻¹, which is attributed to its larger conduction pathways and high PEO content. In the analysis of activation energy of lithium ion transfer, larger micro-phase separation sturcture domains are associated with lower activation energies, indicating that lithium ions primarily migrate through the PI-g-PEO regions. The increase in lithium salt concentration does not significantly enhance ionic conductivity; instead, it leads to a rising trend in activation energy. This suggests that the addition of lithium salt in the SPEs restricts the mobility of PEO chains, making it more difficult for lithium ions to migrate via the segmental motion of PEO at high salt concentrations.