Layered Polymer Stacking for Stable Interfaces and Dendrite Growth Inhibition in All-Solid-State Lithium Batteries

To improve the ionic conductivity and cycling stability of solid-state lithium batteries based on poly(ethylene oxide) (PEO) electrolytes, we developed a sandwich-structured composite polymer electrolyte (sandwich-CPE) PEO-TiN/PEO-LiYF₄/PEO-TiN. The sandwich-CPE delivers a high ionic conductivity of 1.7 × 10^-4 S cm^-1 at 30 °C and a wide potential window of 0 to 5.0 V (vs Li/Li⁺). Adding PEO-TiN leads to the formation of Li₃N between Li and sandwich-CPE during cycling, which effectively reduces the level of Li dendrite formation. Additionally, PEO-TiN acts as a sacrificial layer to stop the entry of Li dendrites into the interlayer PEO-LiYF₄. Using the sandwich-CPE, LiFePO₄ retains a reversible capacity of 113.8 mA h g^-1 at 30 °C after 300 cycles under 0.5 C. For high-voltage cells, LiNi_0.5Co_0.2Mn_0.3O₂ retains a capacity retention of 71.4% at 45 °C after 300 cycles under 0.2 C among 3.0-4.3 V, while Li₃V₂(PO₄)₃ delivers an initial discharge capacity of 108.1 mA h g^-1 at 60 °C and retains 81.6% after 500 cycles under 1 C among 2.8-4.4 V. These results demonstrate the strong electrochemical compatibility of the sandwich-CPE, enabling high reversible capacity and good cycling stability for solid-state Li batteries with different cathodes at different temperatures and current rates.