<p>The Ga-doped Li₇La₃Zr₂O₁₂ (Ga-LLZO) system currently exhibits the highest ionic conductivity among garnet-type solid-state electrolytes and faces persistent challenges including pore formation—arising from high-temperature disproportionation reactions and elevated sintering activity—as well as lithium filament growth and short-circuit failure at pores and grain boundaries. Effectively addressing these issues while retaining high ionic conductivity remains a major obstacle. In this study, Ce was introduced into the Ga-LLZO lattice via an in situ tuning strategy, which significantly reduced internal pore retention and suppressed the reduction of migrating Li⁺ into dead lithium. The optimized Ce in situ tuning controlled Ga-LLZO achieved an ionic conductivity exceeding 1 mS cm⁻¹ at 25&#xa0;°C, while the critical current density (CCD) in lithium symmetric cells reached 0.7&#xa0;mA&#xa0;cm⁻²—twice that of unmodified Ga-LLZO. Remarkably, no short-circuiting occurred even after 2700&#xa0;h of cycling at 0.3&#xa0;mA&#xa0;cm⁻², in contrast to the unmodified Ga-LLZO, which failed after only 50&#xa0;h. The corresponding full cells also demonstrated excellent cycling stability and ultra-high capacity retention, significantly outperforming unmodified Ga-LLZO. Compared with recent electrolyte modification strategies, the Ce in situ tuning controlled Ga-LLZO delivers outstanding overall performance. Moreover, it holds strong potential for synergistic integration with other advanced modification techniques, offering broad prospects for further development and practical implementation in next-generation all-solid-state batteries (ASSBs).</p> Graphical abstract <p></p>