Protein-modified SEI formation and evolution in Li metal batteries

Despite numerous reported lithium metal batteries (LMBs) with excellent cycling performance achieved in labs, transferring the high performing LMBs from lab-scale to industrial-production remains challenging. Therefore, via imitating the stand-still process in battery production, a conventional but important procedure, to investigate the formation and evolution of a solid electrolyte interface (SEI) is particularly important for LMBs. Our previous studies indicate that zein (corn protein)-modified carbonate-ester electrolyte (the most commercialized) effectively improves the performance of LMBs through guiding Li-ions and repairing cracked SEI. Herein, we investigate the formation and evolution of the protein-modified SEIs on Li anodes by imitating the stand-still temperature and duration. A simulation study on the protein denaturation in the electrolyte under different temperatures demonstrates a highly unfolded configuration at elevated temperatures. The experiments show that this heat-treated-zein (H-zein) modified SEI forms quickly and becomes stable after a stand-still process of less than 100 min. Moreover, the H-zein SEI exhibits excellent wetting behavior with the electrolyte due to the highly unfolded protein structures with more functional groups exposed. The Li|Li cell with the H-zein SEI achieves prolonged cycling performance (>360 h vs. ∼260 h of the cell with the untreated-zein (U-zein) modified SEI). The LiFePO4|Li cell with the H-zein SEI shows much stable long-term cycling performance of capacity retention (70% vs. 42% of the cell with U-zein SEI) after 200 cycles. This study confirms that the appropriately treated protein is able to effectively improve the performance of LMBs, and will inspire future studies for the production process of LMBs toward their commercialization.