通過磺酸化幾丁聚醣的人工固態電解質介面層以提高天然石墨陽極對丙烯碳酸酯的耐受性

Abstract

石墨是當今鋰離子電池中最常使用的負極材料，但目前商用電池中的液態電解質主要成分為Ethylene Carbonate (EC)和Ethyl Methyl Carbonate（EMC），這些溶劑的熔點高於室溫，其在低溫下電導率表現不佳，而具有低於零度熔點的Propylene carbonate（PC）被認為是液態電解質良好的替代品之一，可以實現更好的低溫性能，然而PC在循環過程，會嵌入進天然石墨（NG）顆粒中，導致NG的剝落和活性顆粒的崩塌，最終造成電極的結構崩壞。 為了解決這一問題，我們開發了一種人工固態電解質界面（A-SEI）層，使用定制的磺酸化幾丁聚醣接枝聚乙二醇（OSCS-g-PEO），通過簡單的過程塗覆在NG顆粒上，A-SEI層具有良好的鋰離子導電性、高機械強度和一定的柔韌性，經實驗證明，在使用含PC的電解質時，A-SEI層能有效改善循環穩定性，電池在含有PC的電解質存在下，Li//LE//未修飾的NG電池顯示出顯著高於NG理論容量的初始容量，這表明由於PC的嵌入，過量的鋰離子嵌入。然而Li//LE//A-SEI修飾的NG電池則接近NG的理論容量，表明PC的嵌入被成功抑制，進行以0.5C/0.5C進行循環時，前者在160個循環後顯示出顯著的容量下降，而後者在200個循環後仍保持穩定運作。之後進一步分析微觀結構，包括SEM、XPS和XRD，表明A-SEI能有效抑制SEI的增厚並保持NG顆粒的完整性，因此能延長使用富含PC電解質電池的壽命，此外過度鋰化實驗證明，A-SEI層還可以抑制鋰枝晶的形成，提高電池安全性。 我們也評估了使用含有PC電解質的Li//LE//A-SEI修飾的NG電池在0°C下的性能，經過200個循環後，仍擁有70%的容量維持率和超過99.8%的庫倫效率；而在相同設定下，使用不含有PC電解質的參考電池僅顯示出20%的容量維持率和低於97%的庫倫效率，並在200個循環後出現嚴重的極化現象，這結果證實了由OSCS-g-PEO之 A-SEI修飾的NG負極與富含PC的電解質相結合在低溫運作中的實用性。

Graphite is the mostly used anode materials for lithium ion batteries nowadays; however, the commercial batteries made of generally suffer un-satisfactory performance at low temperature because of the low ion conductivity of liquid electrolytes adopting ethylene carbonate (EC) and ethyl methyl carbonate (EMC) with above room temperature melting points as the major ingredients. Propylene carbonate (PC) with a sub-zero melting point is considered as an strong alternative for the liquid electrolytes to achieve better lower temperature performance. However, PC would infiltrate into the natural graphite (NG) particles during cycling, causing exfoliation of NG and collapse of the active particles, eventually structural breakdown of the electrode To address this issue, we develop an artificial solid electrolyte interphase (A-SEI) layer using custom made sulfonated chitosan-graft-polyethylene oxide (OSCS-g-PEO), which could be coated on NG particles via a simple solution coating process. The A-SEI possesses good lithium ion conductivity, high mechanical strengths with certain flexibility, which are demonstrated to effective improve the cycling stability when PC-containing electrolytes are used. With the presence of PC-containing electrolytes, the Li//LE//pristine NG cell exhibits an initial capacity significantly higher than the theoretical capacity of NG, indicating the intercalation of excess lithium ion due to PC infiltration. In contrast, the Li//LE//A-SEI-modified NG cell approaches the theoretical capacity of NG, suggesting the penetration of PC is successfully suppressed. When cycling at 0.5C/0.5C, the former shows noticeable capacity rollover after 160 cycles, while the latter one maintained stable operation beyond 200 cycles. Further microstructure analysis, including SEM and XPS, indicates the A-SEI could effectively hamper undesired SEI thickening and retain the integrity of the NG particles, therefore to extend the lifespan of batteries using PC-rich electrolytes. Moreover, the A-SEI should also suppress lithium dendrite formation demonstrated by the over-lithiation experiments to improve the battery safety. We also evaluate the performance of a Li//LE//A-SEI-modified NG cell at 0°C using PC-containing electrolytes. A 70% capacity retention and over than 99.8% Columbic efficiency is achieved after 200 cycles; while the reference one using PC-free electrolytes shows only 20% capacity retention and less than 97% CE under the same setup with severe polarization after 200 cycles. This observation assure the practicality of the combination of OSCS-g-PEO A-SEI modified NG anode and PC-rich electrolytes for low-temperature operation.