以交聯磺酸化幾丁聚醣進行鋰離子電池矽碳負極活性顆粒之表面改質以提升穩定性與快速充放電表現

Abstract

本研究中，我們採用交聯磺酸化幾丁聚醣 (X-SCS)來作為鋰離子電池 (LIBs)的矽/碳複合材料電極(Si@G)活性顆粒表面上的保護塗層。此保護塗層兼具熱穩定和化學穩定性，能有效的傳導鋰離子並防止活性顆粒和液態電解液直接接觸。因而，在長時間充放電循環下可以抑制矽顆粒從Si@G表面剝離和不好的SEI生成，從而提高循環穩定性。 X-SCS是由磺酸化幾丁聚醣 (SCS)伴隨著不同磺酸化程度(DS)以及特定交聯劑(CA)含量組成。戊二醛(GA)作為第一部分的交聯劑，形成堅固且穩定的XGA-SCS保護層來抑制矽顆粒的掉落，盡而使得容量保持率提升。在經過0.5 C 室溫300次充放電循環下，Si@G披覆XGA-SCS負極可以表現出70%容量保持率，跟純Si@G電極只有30%容量保持率相比有明顯的差異性。相同條件之下，XGA-SCS 披覆的Si@G電極體積膨脹的變異性僅僅只有30%遠小於純Si@G的130%。值得注意的是XGA-SCS表面改質同時提升電極在高充放電速率的性能，並且隨著SCS的DS增加效果更加明顯。對於Si@G披覆XGA-SCS保護層的電極，5 C放電條件下擁有83%的容量保持率遠高於純Si@G的50%容量保持率. 第二部分中，我們將用低分子量的PEO進行雙邊官能基化後當作交聯劑，為了提升交聯的磺酸化幾丁聚醣(X-SCS)保護層的可伸縮性與柔軟度來容忍矽更多的體積變異性從而進一步改善電池循環後的保護層穩定性。XPEO-SCS保護層表現出最佳的循環穩定性，在0.5 C 室溫下充放電循環300次後擁有80%容量保持率和在5 C的測試條件下可以達到95%容量保持率的卓越表現。簡而言之，採用具有適當含量的X-SCS塗層可以有效改善電化學性能並抑制Si@G複合電極的體積膨脹以獲得更好的LIB效能。

In this study, we employed crosslinking sulfonated chitosan (denoted as X-SCS) to serve as protective coating on the active materials of silicon/graphite composite (Si@G) anode for lithium ion batteries (LIBs). The protective coating is thermally and chemically stable, and which would allow effective lithium ion transport as well as prevent direct contact between the active particles and the liquid electrolyte. As a consequence, detachment of Si from Si@G and undesirable SEI thickening could be suppressed after long term cycling, leading to improved cycling stability. X-SCS is comprised of sulfonated chitosan (SCS) with various degree of sulfonation (DS) and crosslinking agents (CA) in designated amounts. In the first part, glutaraldehyde (GA) is employed as CA, and the resulting XGA-SCS coating is rigid and robust to effective suppress the detachment of Si, thus to promote the capacity retention. A 70% capacity retention after 300 cycles at 0.5 C and room temperature is achieved for XGA-SCS coated Si@G anode, while only a 30% capacity retention is obtained for the pristine Si@G. The volumetric expansion is merely 30% for the XGA-SCS coated Si@G, much smaller than a value of 130% for the pristine Si@G. Notably, high C-rate performance is simultaneously improved, and the enhancement is more pronouncedly with increasing DS of SCS. A 83% capacity retention at 5C with respect to the discharge capacity at 0.1C is achieved for the XGA-SCS coated Si@G, much higher than 50% for the reference Si@G. In the second part, low molecular weight bi-end functionalized PEO with epoxide terminals is served as CA, in order to soften the X-SCS coating to accommodate more volume change of Si, thus to further improve the coating integrity after cycling. The resulting XPEO-SCS coating exhibits the best cycling stability with 80% capacity retention after 300 cycles at 0.5 C and superior C-rate performance with 95% capacity retention at 5 C. In brief, employing X-SCS coating with suitable composition could be a feasible approach to effectively improve electrochemical performance and suppress vigorously volume expansion of Si@G composite electrode for better LIBs.