高性能鋰硫電池之製備與分析

Abstract

在二十一世紀社會中，電能的貯存是生活必需之一。鋰離子電池已經成功地證明可應用在大規模能量貯存。因此，尋求下一代可提供高能密度且便宜之新系統電池顯得更為重要。鋰硫電池極有潛力作為高能蓄電池的電化學系統之一。在這個研究計畫中探討鋰硫電池中硫電極之型態變化、電極材料之修飾及電解液對電池效能之影響。 首先，透過臨場穿透式X光顯微技術觀察鋰硫電池在整個電化學充/放電過程中，其中間產物-多硫化鋰之溶解與再沉積現象並以定量之模式進行研究。由實驗結果中發現多硫化鋰在碳-硫複合物上的溶解與再沉積造就了各式各樣的硫形態，這同時也使得活物密度的變化不定，且絕大多數的硫顆粒變成Li2S4後均溶解消失。從溶解和再沉積的動力學結果顯示溶解速率常數與顆粒大小、電位及電化學操作模式息息相關。在鋰遷出過程中，硫或多硫化鋰的再沉積受到在碳表面成核之限制，導致硫顆粒在再成長過程中的凝聚與聚集。因此從這些觀察中討論設計一高循環穩定性的鋰硫電池。 再者，利用液相混合法將Nafion高分子均勻批覆於S-pearl奈米複合物表面，將此複合體當做鋰硫電池之正極材料。經過50圈不同充/放電速率測試，最後在0.2C rate下仍保有930 mAh g-1之可逆電容量，且在1C rate較高充/放電速率下還保有80%之電容量，此複合材料改善了鋰硫電池之循環性能及庫倫效率。利用臨場穿透式X光顯微技術觀察電極中純硫顆粒和經Nafion高分子批覆之硫顆粒在鋰嵌入過程中形態變化。由結果發現經Nafion高分子批覆之硫顆粒形成一核-殼之結構，不但允許鋰離子之傳送且抑制了多硫化鋰在電解液中之傳遞。從臨場穿透式X光顯微技術結果顯示Nafion高分子層能有效地減少多硫化鋰在正負極間的穿梭且提升了電極之穩定性和可逆性。 最後，本研究亦注重鋰硫電池中電解液之鋰鹽濃度影響，因為液態電解液在鋰離子電池中扮演極重要之角色。在吾人研究中利用2.5M較高濃度鋰鹽當做電解液，由實驗結果證實經過50圈充/放電循環測試，仍保有1000 mAh g-1之電容量，與第一圈相比約具有80%之可逆電容量可供使用。另外，透過多硫化鋰在不同鋰鹽濃度電解液中之擴散實驗，驗證高濃度鋰鹽電解液有抑制多硫化鋰擴散之能力，減緩多硫化鋰在正負極間之穿梭。同時在臨場穿透式X光顯微技術下也觀察到在高濃度鋰鹽電解液中硫/多硫化鋰有較慢之溶解現象發生。所以使用高濃度鋰鹽電解液在鋰硫電池中，不但抑制多硫化鋰之溶解且改善多硫化鋰之穿梭現象發生。因此得到具有高循環壽命且保有100%庫倫效率之鋰硫電池。

Electrical energy storage is one of the most critical needs of 21st century society. Li-ion batteries have proven successful in large-scale energy storage. Therefore, new systems are being sought for the next-generation batteries to provide much higher energy density, and reduce cost factors. The lithium-sulfur (Li-S) battery is a promising electrochemical system as a high-energy secondary battery. This study aims at significantly raising the process efficiency from the viewpoints of morphology changing, decoration of the electrode material and influence of electrolyte in Li-S battery. First, the dynamics of lithium polysulfide dissolution and re-deposition during electrochemical charge/discharge of the Li-S cell has for the first time been revealed in a quantitative manner based on in-operando transmission X-ray microscopy (TXM) analysis. Extensive dimensional variations of S particles have been observed on an S-carbon composite electrode to mainly result from polysulfide dissolution/re-deposition, which obscures the effect of lithiation-induced density change. Essential all S particles dissolve when turning into Li2S4. The kinetics of both dissolution and re-deposition are shown to follow classical dissolution theory with the rate constants exhibiting strong dependence on particle size, potential and electrochemical operation mode. S/polysulfide re-deposition upon de-lithiation is found to be nucleation-limited on carbon surface, leading to considerable particle growth and aggregation of the S-containing active mass. The implications of these observations on the designing of Li-S cell with enhanced cycling stability are discussed. Moreover, a novel composite consisting of sulfur-pearl (S-P) nanocomposite particles coating with a Nafion polymer film was prepared via a liquid mixing method as a cathode material for Li-S batteries. The reversible capacity of 930 mAh g-1 at 0.2C rate after being continuously cycled for 50 cycles at various rates, compared with higher rate of 1C, which also contents 80 % capacity retention. It also improves the cycle performance and Coulombic efficiency of Li-S cell. The dynamics of S and Nafion-coated S during electrochemical lithiation of Li-S cell have for the first time been revealed by in-operando TXM analysis. The Nafion polymer coated exhibits a containing core-shell interior structure that not only allows penetration of lithium ions transmission but also effectively prevents polysulfide anions transporting in the electrolyte. It is demonstrated from an in-operando measurements that the Nafion layer is quite effective in reducing shuttle effect and enhancing the stability and the reversibility of the electrode. Finally, the lithium salt concentration in liquid electrolyte of Li-S battery also researched, because liquid electrolyte plays an important role in commercial lithium-ion batteries to allow conduction of lithium-ion between cathode and anode. Using 2.5 M lithium salt concentration electrolyte in our research, it maintains a reversible capacity of 1000 mAh g-1 with capacity retention of 80 % after 50 cycles. From diffusion experiment, it shows the lithium polysulfide diffusion rate in high salt concentration electrolyte slower than in low one. The S/polysulfide dissolution also restrain in high salt concentration electrolyte. The advantage of high salt concentration electrolyte is further demonstrated that lithium polysulfide dissolution is inhibited, thus overcoming the polysulfide shuttle phenomenon. Consequently, a Coulombic efficiency nearing 100% and long cycling stability are achieved.