可逆光交聯聚乙二醇接枝共聚物之開發及其在鋰金屬電池之類固態電解質應用

Abstract

鋰金屬電池（LMBs）因其極高的理論比容量，被視為最具潛力的新一代高能量密度電池。然而，鋰金屬負極在循環過程中不均勻的鋰離子沉積與剝離，常導致鋰枝晶的形成，進而引發內部短路與易燃等安全問題。為了解決鋰金屬電池的安全性疑慮，類固態高分子電解質（QSPEs）被認為是極具潛力的解決方案。此類電解質將有機溶劑固定於高分子基質中，既能防止電解液洩漏與爆炸，又能維持良好的電池性能並抑制鋰枝晶生長。本研究設計並合成了一種接枝共聚物，其結構包含低分子量的聚乙二醇（PEG）側鏈與具有光響應性的香豆素基團。在365nm紫外光照射下，香豆素基團會發生光二聚化反應，形成機械性質強韌的交聯網絡，而PEG鏈段則有助於鋰離子傳輸。我們將光固化後的共聚物夾於兩層多孔纖維素複合隔離膜（Nano Base 2®）之間，製備出複合膜，作為QSPE的基質。藉由調整共聚物的組成，可調控其交聯密度，而此交聯密度對於QSPE的電化學性能至關重要。經過優化的QSPE，其電解液吸收率可達258 wt%，在20°C時的離子導電度為1.49 × 10-3 S cm-1 ，鋰離子遷移數為0.58。在0.2 mA cm⁻²的電流密度下進行定電流充放電（充放電各2小時），對應的鋰對稱電池展現了超過400小時的穩定循環，此性能顯著優於僅使用浸潤電解液之Nano Base 2®隔離膜的液態電池，證明了此QSPE抑制鋰枝晶生長的能力。值得注意的是，即使在-20°C的低溫環境下，其離子導電度仍可達9.30x10-4 S cm-1，顯示其具備在低溫環境下運作的良好潛力。此外，在254nm紫外光照射下，香豆素部分會發生逆二聚化反應，使QSPEs 解體。此可逆特性已透過宏觀形貌變化、溶液的螢光響應以及微觀結構的改變得到驗證，賦予此QSPE在回收與廢棄電池處理方面的潛力。結合纖維素纖維的生物可再生特性，本研究開發的QSPE被視為一種兼具環境友善與環境永續性的電解質。

Lithium metal batteries (LMBs) are top candidates for next-generation high-energy-density batteries due to their high theoretical specific capacity. However, non-uniform deposition/stripping of lithium ions on the lithium metal anode upon cycling often leads to lithium dendrite formation, causing internal short circuits and thus flammable problems. To relieve the safety issues of LMBs, quasi-solid polymer electrolytes (QSPEs), in which organic solvents are immobilized within a polymer matrix, are considered a highly potential solution to prevent solvent leakage and explosion while retaining good cell performance and mitigate lithium dendrite growth. In this study, we design and synthesize a graft-copolymer containing low molecular weight poly(ethylene glycol) side chains and photo-responsive coumarin groups. Upon 365 nm UV irradiation, the coumarin groups undergo photo-dimerization to produce mechanical robust crosslinked network while the PEG segments should help lithium ion transport. A composite membrane comprising a photocured copolymer layer sandwiching between two mesoporous cellulose composite separators (Nano Base 2®) is fabricated to serve as the matrix of the QSPE. The crosslinking density could be tuned by adjusting the composition of the copolymer, and which plays a critical role in the electrochemical properties of the QSPE. The optimized QSPE exhibited a 258 wt% electrolyte uptake and a conductivity of 1.49 × 10-3 S cm-1 at 20°C with a lithium transference number of 0.58. The corresponding lithium symmetric cell demonstrates stable cycling over 400 hours upon galvanostatic cycling at a current density of 0.2 mA cm-2 and 2h/2h charging/discharging, remarkably better than the performance of liquid electrolytes saturated within the Nano Base 2® separator, indicating the capability of the QSPE to mitigate lithium dendrite growth. It is noteworthy that the low temperature conductivity still reaches 9.30x10-4 S cm-1 at -20°C, suggesting good potential for low temperature operation. Furthermore, the coumarin moieties undergo de-dimerization upon 254 nm UV irradiation to dissemble the QSPEs, verified by macroscopic morphological changes, fluorescence response of the solution, and microscopic structural alterations, endowing the QSPE for potential recyclability and feasibility in waste battery treatment. Combining with the bio-regenerative nature of the cellulose fibers, this QSPE is considered as an eco-friendly and environment-sustainable electrolyte.