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• SiO 2 scavenges the Li 2 CO 3 on garnet LLZTO surface during densification process. • Higher relative density and Li+ conductivity are achieved after Li 2 CO 3 elimination. • Enhanced Li ion conduction benefits from Li-conducting Li 4 SiO 4 as reaction product. As promising next generation energy storage system, all solid state lithium batteries offer great potential for achieving high energy density and improved safety over lithium batteries with liquid counterpart. Dense microstructure with high ionic conductivity are prerequisites for solid electrolyte. The promising Li-garnet electrolyte suffers from poor sinterability, which results in porous structure and large grain boundary resistance. Herein, we report that the poor sinterability of garnet Li 7 La 3 Zr 2 O 12 is most likely related to the Li 2 CO 3 formed on particle surface, and SiO 2 is an efficient additive to scavenge the surface Li 2 CO 3 and so significantly improve the densification of electrolyte. The reaction product Li 4 SiO 4 of SiO 2 with Li 2 CO 3 is a Li-ion conductor, which exists at grain boundary and so can facilitate the grain boundary lithium ion diffusion. An addition of 1 mol.% SiO 2 , with respect to the total lithium content in Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO), significantly increases the relative density of LLZTO pellet by 28% and the conductivity by one order of magnitude, reaching 3.84 × 10−4 S cm−1 at room temperature by a conventional solid state sintering. A symmetric lithium cell assembled with such dense LLZTO electrolyte can cycle well for 260 h compared with unmodified LLZTO running only for 72 h before short-circuit. [ABSTRACT FROM AUTHOR]