Sustainable Separators for High‐Performance Lithium Ion Batteries Enabled by Chemical Modifications

• Published in: Advanced functional materials 2019-07, Vol.29 (28), p.n/a
• Main Authors: Zhang, Tian‐Wen, Chen, Jia‐Lu, Tian, Te, Shen, Bao, Peng, Yan‐De, Song, Yong‐Hui, Jiang, Bin, Lu, Lei‐Lei, Yao, Hong‐Bin, Yu, Shu‐Hong
• Format: Article
• Language: English
• Subjects: chemical modification; Chitin; Density functional theory; High temperature; Ion currents; Ion transport; Lithium; Lithium-ion batteries; Materials science; Nanofibers; natural polymer nanofibers; Organic chemistry; Polymers; Raw materials; Rechargeable batteries; Separators; sustainable separators
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201902023

Natural polymer nanofibers are attractive sustainable raw materials to fabricate separators for high‐performance lithium ion batteries (LIBs). Unfortunately, complicated pore‐forming processes, low ionic conductivity, and relatively low mechanical strength of previously reported natural polymer nanofiber‐based separators severely limit their performances and applications. Here, a chemical modification strategy to endow high performance to natural polymer nanofiber‐based separators is demonstrated by grafting cyanoethyl groups on the surface of chitin nanofibers. The fabricated cyanoethyl‐chitin nanofiber (CCN) separators not only exhibit much higher ionic conductivity but also retain excellent mechanical strength in comparison to unmodified chitin nanofiber separators. Through density function theory calculations, the mechanism of high Li+ ion transport in the CCN separator is unraveled as weakening of the binding of Li+ ions over that of PF6− ions with chitin, via the cyanoethyl modification. The LiFePO4/Li4Ti5O12 full cells using CCN separators show much better rate capability and enhanced capacity retention compared to the cell using commercial polypropylene (PP) separators. Beyond this, the CCN separator can work very well even at an elevated temperature of 120 °C in the LiFePO4/Li cell. The proposed strategy chemical modification of natural polymer nanofibers will open a new avenue to fabricate sustainable separators for LIBs with superior performance. A dense chitin nanofiber‐based separator with a high lithium ion transference number is achieved by a facile surface chemical modification method. This is a new type of separator that falls in between a traditional separator and gel electrolyte, and thus shows good comprehensive performance when compared with the liquid electrolyte‐based polyolefin separator and gel electrolyte.