An interlayer is usually employed to tackle the interfacial instability issue between solid electrolytes (SEs) and Li metal caused by the side reaction. However, the failure mechanism of the ionic conductor interlayers, especially the influence from electron penetration, remains largely unknown. Herein, using Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) as the model SE and LiF as the interlayer, we use metal semiconductor contact barrier theory to reveal the failure origin of Li/LiF@LATP interface based on the calculation results of density functional theory (DFT), in which electrons can easily tunnel through the LiF grain boundary with F vacancies due to its narrow barrier width against electron injection, followed by the reduction of LATP. Remarkably, an Al-LiF bilayer between Li/LATP is found to dramatically promote the interfacial stability, due to the highly increased barrier width and homogenized electric field at the interface. Consequently, the Li symmetric cells with Al-LiF bilayer can exhibit excellent cyclability of more than 2,000 h superior to that interlayered by LiF monolayer (~ 860 h). Moreover, the Li/Al-LiF@LATP/LiFePO₄ solid-state batteries deliver a capacity retention of 83.2% after 350 cycles at 0.5 C. Our findings emphasize the importance of tuning the electron transport behavior by optimizing the potential barrier for the interface design in high-performance solid-state batteries.