Surface-Conducting Lithium Superionic Conductors for Solid-State Batteries

Bulk Li⁺-conducting lithium superionic conductors are susceptible to disruption by grain boundaries and interparticle porosity, necessitating high densification and consequently limiting the gravimetric energy density of solid-state batteries. Here, we discovered a new class of surface-conducting lithium superionic conductors achieved through surface chemisorption. After bonding with ligands, surface atoms of inert substrates become binding sites for lithium salt dissociation and hopping sites for fast surface Li⁺ diffusion, transforming inert materials into surface-conducting Li⁺ conductors. Using two-dimensional TiO₂ nanosheets as a proof of concept, we show that ethylene glycolate-chemisorbed TiO₂ significantly enhances lithium salt dissociation and promotes fast Li⁺ hopping between surface oxygen atoms, achieving a high surface ion mobility of 3.61 × 10^-7 cm²·V^-1·s^-1─an improvement of 600% over the bulk Li⁺ mobility of Li₇La₃Zr₂O₁₂ solid oxide electrolytes. Benefiting from surface Li⁺ conduction, an ultralight oxide aerogel solid-state electrolyte was developed with an unprecedented low density of 0.29 g·cm^-3, which is only 25% of that of liquid electrolytes and 5.7% of garnet-type solid electrolytes. A LiFePO₄-based solid-state battery utilizing this new electrolyte exhibits a significantly high energy density of ∼295 Wh·kg^-1, achieving 160% of that of a Li₇La₃Zr₂O₁₂-based solid-state battery even with the same electrolyte thickness. Furthermore, this guideline for designing surface-conducting superionic conductors is generalizable and can be extended to diverse cations and substrates, promising lightweight, highly conductive solid-state electrolytes with broad implications beyond solid-state batteries.