Identifying interface evolutions for achieving stable solid-state Li metal batteries

Solid-solid interface contact and slow ion transport restrict solid-state polymer electrolytes practical application. The differences in interface structure design significantly influence the interfacial Li⁺ transport and diffusion as well as the Li atom nucleation, resulting in substantial variations in the macroscopic performance of polymer electrolytes-based solid-state Li metal batteries. Here, ceramic-polymer composite electrolytes (CPCEs) composed of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) polymer and Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) filler has been chosen as the demo to demonstrate that the interfacial electrochemistry between CPCEs and Li anode is not only affected by the physical interface contact but also associated with the internal/interfacial Li⁺ transport mechanism. This work shows that “point to point” Li⁺ diffusion, slow uneven interfacial Li⁺ transport in CPCEs with poor ionic conductivity and rough surface lead to uneven Li atom nucleation, leading to Li dendrites growth. While, the CPCEs with high ionic conductivity and smooth surface facilitate uniform and rapid ion transport, promoting uniform Li nucleation and transverse diffusion. This work highlights the importance of the interface structure design of polymer electrolytes for Li metal interface stability in polymer electrolytes-based quasi-solid-state batteries and provides valuable insights into the interfacial electrochemistry of solid-state batteries.