Silk Fibroin Separators: A Step Toward Lithium-Ion Batteries with Enhanced Sustainability

Battery separators based on silk fibroin (SF) have been prepared aiming at improving the environmental issues of lithium-ion batteries. SF materials with three different morphologies were produced: membrane films (SF-F), sponges prepared by lyophilization (SF-L), and electrospun membranes (SF-E). The latter materials presented a suitable porous three-dimensional microstructure and were soaked with a 1 M LiPF₆ electrolyte. The ionic conductivities for SF-L and SF-E were 1.00 and 0.32 mS cm^-1 at 20 °C, respectively. A correlation between the fraction of β-sheet conformations and the ionic conductivity was observed. The electrochemical performance of the SF-based materials was evaluated by incorporating them in cathodic half-cells with C-LiFePO₄. The discharge capacities of SF-L and SF-E were 126 and 108 mA h g^-1, respectively, at the C/2-rate and 99 and 54 mA h g^-1, respectively, at the 2C-rate. Furthermore, the capacity retention and capacity fade of the SF-L membrane after 50 cycles at the 2C-rate were 72 and 5%, respectively. These electrochemical results show that a high percentage of β-sheet conformations were of prime importance to guarantee excellent cycling performance. This work demonstrates that SF-based membranes are appropriate separators for the production of environmentally friendlier lithium-ion batteries.