Unveiling LiTFSI Precipitation as a Key Factor in Solid Electrolyte Interphase Formation in Li-Based Water-in-Salt Electrolytes

A water-in-salt electrolyte is a highly concentrated aqueous solution (i.e., 21 mol LiTFSI in 1 kg H2O) that reduces the number of water molecules surrounding salt ions, thereby decreasing the water activity responsible for decomposition. This electrolyte widens the electrochemical stability window via the formation of a solid electrolyte interphase (SEI) at the electrode surface. However, using high concentration electrolytes in Li-ion battery technology to enhance energy density and increase cycling stability remains challenging. A parasitic reaction, called the hydrogen evolution reaction, occurs when the reaction operates at a lower voltage. It is demonstrated here that a micrometric white layer is indeed a component of the SEI layer, not just on the nanoscale, through the utilization of an operando high-resolution optical microscope. The results indicate that LiTFSI precipitation is the primary species present in the SEI layer. Furthermore, the passivation layer is found to be dynamic since it dissolves back into the electrolyte during open circuit voltage. Using an operando optical microscope, it is shown that, in addition to the nanometric layer of SEI in WiSEs, there is a micrometric layer formed mainly by the precipitation of LiTFSI during the consumption of water molecules. This result is counterintuitive to the state of the art, which focuses on the nanoscale layer close to the electrode (LiF and LiOH).image