Stabilizing the Solid‐Electrolyte Interphase with Polyacrylamide for High‐Voltage Aqueous Lithium‐Ion Batteries

The introduction of “water‐in‐salt” electrolyte (WiSE) concept opens a new horizon to aqueous electrochemistry that is benefited from the formation of a solid‐electrolyte interphase (SEI). However, such SEI still faces multiple challenges, including dissolution, mechanical damaging, and incessant reforming, which result in poor cycling stability. Here, we report a polymeric additive, polyacrylamide (PAM) that effectively stabilizes the interphase in WiSE. With the addition of 5 molar % PAM to 21 mol kg−1 LiTFSI electrolyte, a LiMn2O4∥L‐TiO2 full cell exhibits enhanced cycling stability with 86 % capacity retention after 100 cycles at 1 C. The formation mechanism and evolution of PAM‐assisted SEI was investigated using operando small angle neutron scattering and density functional theory (DFT) calculations, which reveal that PAM minimizes the presence of free water molecules at the anode/electrolyte interface, accelerates the TFSI− anion decomposition, and densifies the SEI. Polyacrylamide (PAM) was introduced to “water‐in‐salt” electrolyte (21 mol kg−1 LiTFSI) to realize the formation of an effective solid‐electrolyte interphase (SEI) in LiMn2O4∥L‐TiO2 full cells by lowering the cathodic limit in the electrochemical stability window (ESW) and suppressing the hydrogen evolution.