In Situ Lithium Compensation Driven Grain Boundary Engineering for Garnet Based Solid State Electrolytes

ABSTRACT Solid‐state lithium metal batteries (SSLMBs) have attracted significant attention due to their high safety and energy density. Garnet‐type electrolytes are among the most promising candidates, yet the poor sinterability of Li 7 La 3 Zr 2 O 12 (LLZO) often leads to internal dendrite growth. Moreover, conventional LLZO fabrication requires expensive sacrificial lithium to compensate for lithium loss during sintering. Here, we introduce lithium‐rich Li 4 SiO 4 (LSO) as a sintering additive into Ta‐doped LLZO (LLZTO) to enable densification without external sacrificial lithium, reducing cost while enhancing dendrite resistance. During sintering, LSO decomposes into Li 2 O and Li 2 SiO 3 /Li 2 Si 2 O 5 , providing a lithium‐rich atmosphere and in situ pinning at grain boundaries to suppress abnormal grain growth. Consequently, the bending strength is doubled, accompanied by an increase in the critical current density (CCD) from 0.4 to 0.9 mA cm −2 . Symmetric and full cells assembled with this electrolyte also demonstrated excellent long term cycling stability. This strategy offers a cost‐effective route to high performance SSLMBs and provides new insights for garnet electrolyte design and dendrite suppression.