Enhancing ionic conductivity and interfacial stability of perovskite-type LLTO solid electrolytes via synergistic A-site Sr substitution and polymer layer engineering for safer lithium-ion batteries.

The development of safe, high-performance solid electrolytes is critical for advancing next-generation solid-state lithium-ion batteries (SSLIBs). Perovskite-type Li0.33La0.56TiO3 (LLTO) has emerged as a promising solid electrolyte for SSLIBs owing to its high thermal and mechanical stability; however, its moderate ionic conductivity and interfacial instability with lithium metal hinder its practical application. In this work, the ionic conductivity of LLTO was enhanced through A-site doping via partial substitution with Sr using two different Sr precursors (SrCO3 and SrF2). Sr-doped LLTO solid electrolytes were synthesised using a solid-state reaction method. Structural and microstructural analyses confirmed the successful incorporation of Sr into LLTO without the formation of secondary phases. Ionic conductivity measurements revealed that Sr-doped LLTO synthesised using the SrF2 precursor (LLTO-SrF2) exhibited higher ionic conductivity than that synthesised using the SrCO3 precursor (LLTO-SrCO3). The enhanced conductivity of LLTO-SrF2 relative to LLTO-SrCO3 and pristine LLTO was attributed to differences in their lattice parameters, densities, and microstructures. LLTO-SrF2 exhibited a high room-temperature bulk ionic conductivity of 2.86 × 10-3 S cm-1. To further address the interfacial instability with lithium metal, a PEO-based protective layer (PL) was placed around the LLTO-SrF2 pellet ((LLTO-SrF2)PL) to suppress parasitic reactions between Ti4+ and lithium metal while improving interfacial contact. The symmetric cell (Li/(LLTO-SrF2)PL/Li) exhibited stable galvanostatic cycling for over 540 h with highly reversible and symmetric lithium plating/stripping behaviour. These results highlight the synergistic effect of Sr substitution in enhancing ionic conductivity and polymer layer engineering in improving electrochemical stability against lithium metal.