Stabilizing the Interface between Lithium Metal Anode and Li_1.5Al_0.5Ti_1.5(PO₄)₃ Electrolyte Using Ion-Conductive Polymers

Solid-state electrolytes (SSEs) allow for pushing the limits of conventional lithium-ion batteries (LIBs) (e.g., low gravimetric/volumetric energy density, fundamental safety concerns) due to intrinsic properties such as superior electrochemical stability and nonflammability compared to liquid electrolytes. Li1.5Al0.5Ti1.5(PO4)3 (LATP), one of the promising SSEs, can be produced by a scalable, nontoxic, economic process. However, when LATPs come into contact with a lithium metal anode, the reduction of Ti4+ to Ti3+ by electron conduction forms a resistive layer that hinders lithium-ion migration, thereby causing a high level of polarization. In this study, we synthesized a polymer ionic liquid (PIL) with high ionic conductivity through anion exchange reaction and employed it as a coating layer on LATP by a simple dip-coating method. Li symmetric cell with PIL@LATP demonstrates stable cycling performance for over 300 cycles at 0.5 mA/cm2. The Li//PIL@LATP//LFP cells exhibit a stable voltage profile with low overpotential at various current densities and a high capacity retention of 83.73% over 200 charge–discharge cycles at 1 C. Also, the stacked bipolar cell demonstrated notable voltage performance with an operational voltage reaching approximately 6.7 V. This voltage profile suggests a robust electrical potential for energy storage applications. Consequently, the PIL layer coated on the solid electrolyte blocks electron conduction from the lithium metal anode at the interface, prevents Ti4+ reduction and reduces the interfacial resistance to ionic conduction, thereby improving the rate capability and cycle stability of the all-solid-state batteries.