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

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.