Bombyx mori silkworm cocoon separators for lithium-ion batteries with superior safety and sustainability

Abstract

In the quest for solutions to enhance the safety and sustainability of energy storage devices, Bombyx mori silkworm cocoons are proposed as separators for lithium (Li)‐ion batteries obtained literally from nature, and employed directly in a device. The suitability of the cocoons is evaluated in Li‐ion batteries using two different electrolytes: a mixture of lithium hexafluorophosphate (LiPF6), ethylene carbonate (EC), and dimethyl carbonate (DMC), and the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim]TFSI). Li/C–LiFePO4 half cells comprising the EC–DMC–LiPF6 electrolyte present excellent cycling performance, with a discharge capacity up to 86 mAh g−1 and a capacity retention up to 81% after 50 cycles at C rate. Values of 66 mAh g−1 at C/5 and 85% result with [Emim]TFSI, respectively. Separator/electrolyte interactions during battery cycling lead to a decrease of the degree of crystallinity of the cocoon silk fibers, and, in the presence of EC–DMC–LiPF6, to a concomitant increase of the concentration of free PF6− ions. The self‐extinguishing ability of silkworm cocoons represents an additional benefit to batteries, providing fire risk reduction or even suppression. This bio‐inspired work paves the way toward the fabrication of environmentally friendlier separators for next‐generation sustainable and safer Li‐ion batteries.