Garnet-type oxide Li₇La₃Zr₂O₁₂ (LLZO) has attracted considerable attention as promising solid-state electrolyte (SSE) owing to the exceptional bulk ion conduction. However, the Li⁺ transfer capacity of most LLZO-based solid electrolytes remains inadequate, primarily due to the presence of extensive grain boundaries and excessive thickness. Herein, we report the fabrication of a LLZO-based lamellar composite inorganic solid electrolyte (LLZO/LIC LISE) by in-situ sintering Li₃InCl₆ (LIC) in LLZO lamellar framework. LIC can act as an effective Li⁺-conductive grain boundary modifier within the electrolyte, leading to a remarkable reduction in grain boundary resistance of LLZO/LIC LISE (<9.2 Ω cm²), which is much lower than that of LLZO lamellar framework (827.2 Ω cm²). Together with the reduced internal grain boundary of LLZO nanosheet, a high ionic conductivity of 3.22 × 10⁻⁴ S cm⁻¹ at 25 °C is obtained for LLZO/LIC LISE. In addition, combining with the thin thickness of 29 μm, LLZO/LIC LISE obtains high ionic conductance of 223 mS and exceptional energy density of 246 Wh kg⁻¹, surpassing most developed SSEs. As a result, the cell assembled with LLZO/LIC LISE achieves superior cycling performance of 144.2 mAh g⁻¹ after 200 cycles at 0.5C with a capacity decay of only 0.055% per cycle. This study may present a facile approach to effectively eliminate the grain boundary resistance of inorganic solid electrolytes toward high-performance all-solid-state lithium batteries.