pH-Responsive Polymer as a New Stable Solid Electrolyte Interphase for Water-in-Salt Battery

Overcoming the cathodic limits of water-in-salt electrolyte (WiSE) and developing new active materials are crucial to producing aqueous Li-ion batteries (ALIBs) with higher voltage and energy. Stabilizing the solid-electrolyte interphase (SEI) in a WiSE is expected to improve battery performance. Here, a polymer is designed with pH-responsive properties under alkaline conditions and remains soluble in WiSE, which expands the electrochemical stability window of ALIBs. The polymer prevents water reduction and coprecipitates with the LiTFSI salt in alkaline conditions, resulting in the formation of a stable SEI layer. Moreover, this polymer promotes the crystallization of LiTFSI, leading to a change in the morphology of the SEI as observed by X-ray photoelectron spectroscopy and transmission electron microscopy. Finally, the addition of the polymer in Mo6S8//LiFePO4 full cells during charge/discharge cycling tests significantly improves cycling stability, achieving 87% discharge capacity after 100 cycles at 0.5 C.