嵌段共聚高分子電解質應用於電化學電池之離子傳導膜

Abstract

嵌段共聚高分子電解質因為可藉由調控組成而同時具備良好傳導度、機械強度以及微結構的特性，此特性對離子傳導膜的製備具有相當程度的吸引力。磺酸化聚電解質現今用於電化學儲能裝置非常廣泛，尤其因為可藉由氫離子和鋰離子的交換反應可用於直接甲醇燃料電池(direct methanol fuel cell, DMFC)及鋰金屬電池(lithium metal battery, LMB)而變得尤其重要。在本論文中，我們主要專注利用合成手段開發高效能離子傳導膜並應用於鋰金屬電池及直接甲醇燃料電池。膜材的化學結構及組成在電池表現上扮演著極為重要的角色，因此我們針對其微結構、機械性質以及離子傳導特性進行深入研究。 根據Chalzalviel 理論模型，提高鋰離子遷移係數(TLi+)理論上可在鋰金屬電池中永久地抑制鋰金屬枝晶的成長，因此在第二、三章中，我們以陰離子聚合法及其後續化學修飾開發出新穎三嵌段聚苯乙烯-異戊二烯-磺酸化異戊二烯高分子電解質(SII)，並將其作為用於鋰金屬電池中抵擋鋰金屬枝晶之非多孔隔離膜。在特定組成中，SII膜材具備三維網路微相分離結構(磺酸化異戊二烯圍繞聚苯乙烯的碎形結構(fractal))、並具備適當機械強度及柔軟度與鋰金屬有良好的貼附。在有限的電解液吸附量(30wt%)，室溫傳導度可達到1.4510-4 S cm-1、鋰離子遷移係數(TLi+)至0.92，在極化實驗中表現出相對於Celgard多孔膜材均勻的電化學沉積，並成功地在高電流密度下抑制dendrite 的成長至725小時。在最終的電池試驗結果也充分表現出高庫倫效率(90%)。 為了理解在高電解液吸附量時，單離子導體在電解液中扮演的角色，在第四章中，我們以磺酸鋰化聚苯乙烯-乙烯/丁烯-苯乙烯(SSEBS)作為多孔隔離膜Celgard之保護層來研究電化學沉積在鋰金屬界面的穩定性。隨著提高SSEBS磺酸化程度，離子傳導度及TLi+也隨之增加。當磺酸化程度提高至40 mol % (S40)時，傳導度可在室溫下達至3×10-4 S cm-1 。在和Celgard多孔膜材搭配進行電化學極化以及量測自擴散實驗時，我們發現S40可以有效地改善原本Celgard多孔膜材的電化學沉積並抑制支晶的形成。從SEM觀察實驗後的鋰金屬也觀察到平整光滑的表面，表示鋰金屬界面產生均勻電化學沉積。在電池試驗中，相較於無搭配S40的Celgard隔離膜，可以0.5C速率穩定充放電進行300圈以上和高庫倫效率(98%)。最後我們得到結論，高離子濃度的S40膜材在鋰金屬和電解液界面可扮演整流的功能、穩定了鋰金屬界面。 最後我們利用了嵌段高分子聚電解質可進行離子交換的特性，對SSEBS進行質子化(protonation)作為質子傳導膜、並對直接甲醇燃料電池(DMFC)的表現進行研究。為了有效抵擋DMFC中的甲醇擴散，我們以”前驅物原位擴散法”選擇性地在SSEBS的親水端中引入磷酸鋯(Zirconium phosphate, ZrP)無機物來製備複合膜材(SSEBS-ZrP)。在引入3wt% ZrP於S40膜材中，由於傳導度提升及甲醇擴散大幅抑制，複合膜材可高至原先S40膜16倍的選擇性。在與Nafion 117作為對照組比較DMFC電池性質表現時，在相同操作條件最大功率可高出Nafion 50%。由DSC實驗可證實ZrP對水分子有強吸引力，即使在高溫低濕的環境下SSEBS-ZrP複合膜材仍可提升傳導度。因此SSEBS-ZrP無機複合膜可被視為未來DMFC或是PEMFC潛在質子傳導膜候選人。 高分子電解質在能源領域具備極佳的潛力，但由於隨著電荷濃度越高、高分子玻璃轉移溫度也隨之提高、間接使得膜材在實際操作上難以使用。欲改善高分子電解質的物性，我們使用高分子聚合及化學後修飾的手段來達到具備良好離子傳導膜的目的。尤其在鋰金屬電池領域中，本團隊係全球首位嘗試以合成手段製備功能性離子傳導膜兼具鋰電池隔離膜以及抑制鋰金屬枝晶的實驗室，並依據本篇論文發現高分子電解質對抑制鋰金屬枝晶的效果顯著，這對於下一世代高能量密度儲能裝置的發展，係為一關鍵性開發要素，、並可因而推知未來高分子化學於電化學領域中必能有更廣泛及嶄新之應用。

Block copolymers bearing polyelectrolyte segments are attractive for ion conducting membranes because they could concurrently achieve effective ion conduction and good mechanical properties through the microphase separated nanostructure within the membrane. The transport properties could be further manipulated by varying the composition and the molecular weight of the block copolymer to tailor the microstructure, providing good opportunities to optimize the corresponding device performance. Sulfonated polyelectrolytes are particularly important because the corresponding membranes can be applied to energy storage devices including lithium battery and direct methanol fuel cell by respectively adopting SO3Li and SO3H as the ionic groups. In this thesis, we aimed to fabricate high performance ion conducting membranes based on sulfonated block polyelectrolytes, either custom developed or commercial available for the application in lithium metal batteries and DMFCs. The interplays between chemical structure of the block polyelectrolytes as well as microstructure, mechanical properties and ion transport properties are of our interest. The associate electrochemical properties and performance of the corresponding battery are also studied. In chapter 2 and chapter 3, we developed a novel triblock copolymer poly(styrene-block-isoprene-block-sulfonated isoprene) (SII) to prepare a densely packed membrane to serve as a non-porous separat for lithium metal batteries (LMB). A mechanical robust non-porous membrane SII-D was feasibly prepared from solvent casting of SII, and which possessed a bicontinuous microphase separated nanostructure with a fractal PS network embedded in a sPI matrix. With limited LE uptake, the LE saturated separator SII-LE exhibited a good ion conductivity of 1.45×10-4 S cm-1 and a high tLi+ of 0.92 at room temperature, alone with steady overpotential profile of the galvanostatic cycling. The charge-discharge tests were also performed to testify the practical battery performance. In chapter 4, we employed commercial available poly(styrene-block-(ethylene-ran-butylene)-block-styrene) (SEBS) triblock copolymer to produce SO3Li tethered SEBS (SSEBS-Li) to serve as the protective layer coupled with Celgard for LMB. By offering more lithium ions in S40, conductivity would be improved to 3×10-4 S cm-1 at ambient temperature with high lithium transference number. The S40 membrane was then introduced onto the Li metal/electrolyte interface coupled with porous Celgard membrane, an uniform distribution of lithium ions at the Li metal surface, leading to the homogeneous and stable electrodeposition in Li/Li symmetric cell. The stability of Li electrodeposition can be enhanced and the corresponding battery performance would be also pronounced to more than 300 cycles at 0.5 C rate with comparable discharge capacity and high Coulombic efficiency (98%). We then employed S40 containing SO3H to prepare composite membranes as proton conducting membranes for DMFC by introducing zirconium phosphate (ZrP) into a preformed SSEBS membrane via precursor infiltration and following conversion process in chapter 5. With incorporating 3wt % ZrP in SSEBS with 40 mol % degree of sulfonation (S40), the composite membrane exhibited a remarkable 16 folds increment in selectivity as compared to the parent S40 membrane due to enhanced proton conductivity and significantly suppressed methanol permeability. A 50% enhancement in maximum power of a DMFC single cell was achieved as compared to the reference cell using Nafion 117 under the same setup. Enhancement in proton conductivity at low relative humidity and elevated temperature was observed as well due to the strong water retention capability of ZrP, suggesting a good potential of SSEBS-ZrP composites for both DMFC and PEMFC. In summary, we demonstrate that the multi-functional architecture of SII and SSEBS-Li would offer the resulting separator significantly mitigated lithium dendrite with good ion conductivity in LMB upon limited liquid electrolyte uptake. Simultaneously promotion of methanol blocking capability and proton conductivity is achieved in SSEBS-ZrP nanocomposite membranes. All these works offer effective and feasible routes to solve the key challenges for LMB and DMFC.