Excess-lithium-free LLZO enabling fast ion conduction and ultra-low interfacial resistance for solid-state lithium metal batteries

Garnet-type La_6.25Ga_0.25La₃ZrO₁₂ (LLZO:Ga) is a leading solid electrolyte for next-generation solid-state lithium batteries (SSLBs), yet its widespread adoption is hindered by the routine need for excess lithium and protective powder coverings during synthesis, both of which increase costs and compromise interface stability. Here, we present a scalable, excess-lithium-free synthesis of LLZO:Ga that achieves ultrafast Li-ion conductivity of 1.64(3) × 10^–3 S/cm at 25 °C, surpassing many Li-rich counterparts. Molecular dynamics simulations reveal that excess lithium blocks diffusion pathways, hindering ion mobility and cooperative hopping. Remarkably, the zero-Li-excess LLZO:Ga delivers ultralow interfacial resistance (∼5 Ω·cm²) in symmetric Li|LLZO:Ga|Li cells, enabling stable cycling beyond 700 h at 0.2 mA/cm² with minimal overpotential (∼60 mV). Full cells paired with LiFePO₄ retain ∼96 % capacity over 80 cycles (at 0.1C), demonstrating practical viability. This work establishes a new benchmark in solid electrolyte design, achieving high conductivity, stable interfaces, and scalable processability without lithium excess.