利用磺酸化幾丁聚醣及聚輪烷高分子網路結構形成人工固態電解質介面層以提高天然石墨負極之效能

Abstract

大多數鋰離子電池 ( LIBs ) 採用石墨作為負極材料，因其具有較高的安全性。然而石墨負極仍然有些缺點需要改進，例如較低的理論電容量、快速充電性能不佳、低溫表現不佳以及在高倍率充放電下容易產生鋰枝晶。LIB低溫表現不佳的原因與電解液的組成有關，目前商用電池電解液主要為鋰鹽溶解在高介電常數的ethylene carbonate ( EC ) 和 ethyl methyl carbonate ( EMC )中，這些溶劑的熔點高於室溫，在低溫環境下會結晶使電解液傳導度大幅下降導致電池效能的下降。在電解液中導入低熔點的propylene carbonate ( PC )作為共溶劑以提升鋰離子電池在低溫下的性能是一常見的策略。但 PC 會在充放電的過程中會嵌入天然石墨 ( natural graphite, NG ) 層間，將 NG 的結構破壞而讓電容量與循環壽命下降。為了解決這些問題，在本研究中，我們製備了含有氮位磺酸化幾丁聚醣 ( N-sulfonated chitosan, N-SCS ) 與聚輪烷 (polyrotaxane, PR )的聚合物人工固態電解質介面層 (artificial solid electrolyte interphase, A-SEI )，透過簡單的溶劑揮發製程使其包覆於天然石墨 ( NG ) 表面形成A-SEI@NG來提升循環穩定性與安全性。N-SCS 具有良好的剛性，提供較高的機械強度，並透過其中的磺酸根基團實現鋰離子的單離子導體特性。同時，PR的滑環特性賦予 A-SEI 額外的柔韌性與延展性，使其能在充放電過程中承受負極體積變化，並維持結構完整性。透過適當的交聯處理，我們希望此 A-SEI 形成一種互穿網路結構，並展現出 N-SCS 與 PR 的優點。 我們系統化改變A-SEI中N-SCS與PR的組成與PR的套環數以調控A-SEI的性質，在以不含PC的電解液組成的電池在室溫進行0.5C/0.5C充放電循環時，Li//LE//NG 電池充放電循環到大約 200 圈即開始衰退，而 Li//LE//A-SEI@NG 電池則都可以維持更長的圈數，最高達到 240 圈。在含PC的電解液組成的電池，其0.5C / 0.5C 循環時，Li//LE//NG 電池在大約 160 圈時即開始衰退，而 Li//LE//A-SEI@NG 電池則最高維持到 200 圈。透過交流阻抗分析、SEM形貌觀察、XRD結構分析和XPS成分分析，了解A-SEI提升電池效能循環穩定性的原因。

Most lithium-ion batteries ( LIBs ) employ graphite as the anode material due to its relatively high safety under normal operating conditions. However, graphite anode still has several challenges, such as a low theoretical capacity, poor fast-charging performance, unsatisfactory low-temperature behavior, and the formation of lithium dendrites under high-rate charging. In addition, the commercial electrolytes used in LIBs are primarily composed of ethylene carbonate ( EC ) and ethyl methyl carbonate ( EMC ), both of which have melting points above room temperature, resulting in poor performance under low-temperature conditions. To address this issue, propylene carbonate ( PC ) can be introduced into the electrolyte, leveraging its low melting point to improve LIB performance at low temperatures. However, a drawback of PC is its tendency to co-intercalate into natural graphite ( NG ) during charge / discharge process, disrupting the layered graphite structure and ultimately leading to anode disintegration. To solve these problems, previous studies have proposed constructing an artificial solid electrolyte interface ( A-SEI ) on the graphite surface as an effective strategy to improve cycling stability and safety. In this study, we develop a polymer A-SEI composed of nitrogen-sulfonated chitosan ( NSCS ) and polyrotaxane ( PR ), which is applied onto the NG surface via a simple solution casting process. NSCS provides good rigidity and mechanical strength, and its sulfonate groups offer single-ion conductivity for lithium ions. Meanwhile, the sliding-ring structure of PR imparts additional flexibility and ductility to the A-SEI, allowing it to accommodate volume changes during charge / discharge process and maintain structural integrity. Through appropriate crosslinking reaction, we aim to form an interpenetrating polymer network that combines the advantages of both NSCS and PR. In electrochemical testing with EC-series electrolytes under 0.5C / 0.5C cycling, Li//LE//NG cell began to degrade after approximately 200 cycles, whereas Li//LE//A-SEI@NG cells maintained a longer cycle life. Similarly, in PC-series electrolytes, Li//LE//NG cell began to degrade after approximately 125 cycles under 0.5C / 0.5C cycling, while Li//LE//A-SEI@NG cells showed longer cycle life. Further characterizations including electrochemical impedance spectroscopy ( EIS ), scanning electron microscopy ( SEM ), X-ray diffraction ( XRD ), and X-ray photoelectron spectroscopy ( XPS ) were conducted to investigate the differences between NG and A-SEI@NG anodes after cycling, and to explore the underlying mechanisms.