<i>In Operando</i> Visualization of Polymerized Ionic Liquid Electrolyte Migration in Solid-State Lithium Batteries

Polymerized ionic liquid (PIL) electrolytes with high flaming resistance, wide electrochemical stability window, and high flexibility have been widely explored for high safety, high energy density, and long-cycle lithium metal batteries (LMBs). Great efforts have been made in inhibiting anion movement in the PIL to increase the lithium transference number (tLi+), which reduces polarization loss and improves rate performance. However, the effect of tLi+ on the cycle performance is often overlooked because of self-limiting parasitic interfacial reactions involving anions. Here, PIL migration induced by mobile anions was visualized in operando via cross-sectional atomic force microscopy. Intense migration of the PIL with a low tLi+ breaks the interphase on electrodes and exacerbates anion decomposition, resulting in escalating interfacial impedance. The increased tLi+ inhibits interfacial electromechanical degradation and enhances cycle performance. This work reveals the pivotal but often unnoticed role of interfacial electro-mechanical coupling in PIL-based LMBs by taking advantages of in operando scrutiny of the buried interface.