Abstract Lithium garnets have been widely studied as promising electrolytes that could enable the next-generation all-solid-state lithium batteries. However, upon exposure to atmospheric moisture and carbon dioxide, insulating lithium carbonate forms on the surface and deteriorates the interfaces within electrodes. Here, we report a scalable solid sintering method, defined by lithium donor reaction that allows for complete decarbonation of Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 (LLZTO) and yields an active LiCoO 2 layer for each garnet particle. The obtained LiCoO 2 coated garnets composite is stable against air without any Li 2 CO 3 . Once working in a solid-state lithium battery, the LiCoO 2 -LLZTO@LiCoO 2 composite cathode maintains 81% of the initial capacity after 180 cycles at 0.1 C. Eliminating CO 2 evolution above 4.0 V is confirmed experimentally after transforming Li 2 CO 3 into LiCoO 2 . These results indicate that Li 2 CO 3 is no longer an obstacle, but a trigger of the intimate solid-solid interface. This strategy has been extended to develop a series of LLZTO@active layer materials.