Polyhedral engineering in garnet solid electrolytes via equimolar co-doping for enhanced ionic conductivity

Garnet-type Li 7 La 3 Zr 2 O 12 (LLZO) is a leading solid-state electrolyte for Li-metal batteries, yet the structural origin by which substitutional doping governs Li-ion transport remains insufficiently understood. Here, we establish a polyhedral-engineering strategy to regulate Li-ion migration in LLZO by directly tuning the ZrO 6 host framework through Ta 5+ /Nb 5+ mono-doping and equimolar co-doping, combined with rational control of lithium stoichiometry and dual-stage sintering. Among Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 , Li 6.5 La 3 Zr 1.5 Ta 0.25 Nb 0.25 O 12 , and Li 6.5 La 3 Zr 1.5 Nb 0.5 O 12 , the co-doped composition exhibits the lowest impedance, lowest Li + migration barrier, and highest ionic conductivity (5.50 ± 0.153 × 10 −4 S·cm −1 ), together with 600-cycle (−hour) stable Li-metal symmetric-cell operation and quasi-solid-state cell validation. Synchrotron powder diffraction reveals that Ta/Nb co-doping minimizes the distortion index of ZrO 6 , while aberration-corrected scanning transmission electron microscopy confirms partial compensation of dopant-induced octahedral distortion and restoration of higher average octahedral symmetry. Raman spectroscopy further shows that co -doping selectively concentrates local disorder at dopant-centered polyhedra while preserving a more ordered ZrO 6 transport framework. This framework-level regulation flattens the Li-site energy landscape, reduces bottleneck fluctuation and lattice reorganization penalties, and thereby accelerates Li + hopping. These results identify host-framework polyhedral regulation as a transferable crystallographic design principle for fast-ion-conducting solid electrolytes, offering a new route toward safer, higher-energy-density, and longer-lived all-solid-state batteries.