Al3+/Zr4+ co-doped Li0.33La0.56TiO3 as a multi-functional filler enabling competitive composite solid electrolytes toward lithium-metal batteries

Composite solid-state electrolytes (CSEs) are highly promising for lithium-metal batteries (LMBs) but suffer from low ionic conductivity, limited mechanical strength, and unstable interfaces with lithium metal. A multi-functional perovskite filler, Li 0.33 La 0.56 Ti 0.95 Al 0.01 Zr 0.04 O 3 (AZ-LLTO), is incorporated into a solid polymer electrolyte. This filler is engineered via an Al 3+ /Zr 4+ co-doping strategy that synergistically optimizes bulk and grain boundary properties, enabling simultaneous enhancement of bulk ion transport and stabilization of the Li/electrolyte interface. Al 3+ doping generates oxygen vacancies facilitating Li  +  migration through the perovskite lattice, thereby enhancing bulk ionic conductivity. Concurrently, Zr 4+ doping suppresses La 3+ segregation at grain boundaries, reducing interfacial resistance and favoring Li + transport across grain boundaries. The resultant AZ-LLTO-based composite solid electrolyte achieves a high room-temperature ionic conductivity of 2.3 × 10 −4  S cm −1 , a Li + transference number of 0.42, and stable Li deposition/stripping in Li/CSE-AZ-LLTO/Li symmetric cells for over 3100 h at 0.1 mA cm −2 . Thus, LiFePO 4 /CSE-AZ-LLTO/Li battery delivers an initial capacity of 146.9 mAh g −1  at 0.2 C and retains 85% of its capacity after 200 cycles, demonstrating the practical viability of this designed co-doped perovskite filler. Our work establishes a rational co-doping paradigm for perovskite-based fillers to concurrently enhance bulk conduction and interfacial compatibility in CSEs for next-generation LMBs.