Abstract
<jats:p>Interfacial instability at the Li|garnet interface remains a central obstacle to practical solid-state batteries. Prevailing design paradigms regard Ta doping as the most effective route to stabilizing garnet electrolytes, while reducible dopants such as Nb are typically avoided. Here, we demonstrate that this strategy fails under electrochemical operation: Ta-LLZO (Li6.75La3Zr1.75Ta0.25O12), undergoes deep Ta and Zr reduction, rapid interfacial degradation, and severe impedance growth (14 to 75 Ω cm2), leading to early short-circuiting with a critical current density of only 0.4 mA cm-2. In contrast, a high-entropy multi-dopant garnet (Li6La3Zr0.5Nb0.5Ta0.5Hf0.5O12, LLZNTH) exhibits fundamentally different interfacial behavior, characterized by self-limiting redox activity, suppressed interphase growth (16 to 20 Ω cm2), and a doubled critical current density of 0.8 mA cm-2. X-ray photoelectron spectroscopy and Raman spectroscopy reveal inhibited cation reduction and preserved garnet framework integrity. Molecular dynamics calculations establish that entropy-enabled multi-dopant interactions promote charge delocalization and raise oxygen-vacancy formation energies, thereby suppressing runaway interfacial reduction. These findings identify entropy-driven multi-dopant synergy, not Ta-only stabilization, as the governing mechanism of robust Li|garnet interfaces, establishing a general design principle for next-generation solid-state electrolytes.</jats:p>