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 LiPF electrolyte. The ionic conductivities for SF-L and SF-E were 1.00 and 0.32 mS cm 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-LiFePO. The discharge capacities of SF-L and SF-E were 126 and 108 mA h g, respectively, at the C/2-rate and 99 and 54 mA h g, 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.

Polymer electrolytes, which are commonly used as separator materials in electrochemical devices, have ionic conductivity that is thought to be controlled by segmental mobility. Thus, any improvements made toward increasing ionic mobility come at the expense of mechanical integrity. However, selectively solvating the ionic domain, the region responsible for ion conduction, with water or polar organic solvents presents a potential opportunity to circumvent this physical constraint. Here, we explore the role of hydration on the transport properties of membranes formed from randomly sulfonated polystyrene (PS-r-sPS). We find that the water volume fraction underpins an intrinsic trade-off between separator permselectivity (Ψm) and ion conductivity (κ)thus, improvements in ion diffusion because of increased water content come at the expense of charge density in the membrane which yields a reduced Ψm. We provide a summary of the Ψm–κ trade-off for a suite of commercially available separators to elucidate structure–property relationships and present methodologies for improving both Ψm and κ.

Sulfurized polyacrylonitrile (SPAN) has been shown to be an attractive cathode material for Lithium-Sulfur (Li-S) batteries demonstrating high gravimetric capacity of up to 650mAh/g, and stable cycle performance due to the lack of a polysulfide shuttle in commercially viable carbonate electrolyte. While carbonate electrolyte works well with the cathode of Li-SPAN batteries, it is known to destabilize lithium metal, leading to inactive lithium on the anode and poor coulombic efficiencies (CE). Ether electrolytes in contrast have shown much more stable lithium metal behavior, with consistently high CE. While ether-based electrolytes offer advantages in their ability to stabilize the lithium metal anode, SPAN’s performance in ether-based electrolytes suffers from rapid capacity fade possibly due the dissolution of polysulfide species into the electrolyte during the charge discharge cycle. In this work, we investigated the redox behavior of SPAN in ether electrolytes using in-operando FTIR, and the role of lithium nitrate in cycling stability in SPAN batteries. Our cyclic voltammetry measurements have revealed that the presence of lithium nitrate in sufficient concentration (0.5M) prevents the appearance of redox peaks at 2.1V and 2.3V associated with soluble polysulfides seen in cells using ether electrolyte lacking lithium nitrate. High concentrations of lithium nitrate also prevented rapid capacity loss in cells made using ether electrolyte during long term cycling. XPS measurements showed higher concentrations of lithium fluoride in cells cycled with high concentrations of lithium nitrate, suggesting the formation of a robust cathode electrolyte interface. We developed an in-situ FTIR cell utilizing an attenuated total reflectance (ATR) accessory that can detect the major features of SPAN, such as the cyclized ring structure, the C-S bond, and the S-S bonds. For the in-operando cell, we fabricated a freestanding SPAN cathode and constructed a Li-SPAN cell on top of the ATR accessory. We examined the behavior of polysulfides to understand the role that lithium nitrate plays in preventing the shuttle effect by examining the S-S vibrational modes at 500 cm-1. Additionally, we studied the interactions between the lithium ion and the carbon backbone of SPAN by examining the cyclic behavior of vibrational modes associated with SPAN’s ring structure in the fingerprint region (1000-1500 cm-1), and ring breathing modes (800 cm-1). These peaks disappear during discharge from our in-situ spectra when the cell is at 1.7 V, which corresponds with the major redox peak of SPAN and reappears and returns to its initial position at 2.35V during charge. This indicates that the change in the ring structure is associated with a redox process, which we attribute to the intercalation of lithium into the SPAN structure.