Conformal, nanoscale γ-Al2O3 coating of garnet conductors for solid-state lithium batteries

The use of garnet-type conductors as electrolytes for safe lithium solid-state batteries is impeded by the lack of critical processing strategy to stabilize and densify the cubic lithium garnets. The synthesis and processing of dense cubic lithium garnets are typically depending on solid state reaction as a result of unintentional Al contamination from the alumina crucibles at temperatures up to 1230 °C. High Al concentration is also believed to induce the sluggish migration of lithium ions . Here, we demonstrate an ultrathin, conformal γ-Al 2 O 3 surface coating on garnet solids by a hybrid microemulsion solid-state method for accomplishing rational and controllable synthesis and processing of dense cubic garnets with low Al concentration. The γ-Al 2 O 3 coating demonstrates fast-ion conducting Al-doped Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 phase with low Al concentration (~0.17 mol per formula unit), low tetragonal-to-cubic phase transformation temperature of 500 °C, and total conductivity of ~4.4 × 10 − 4  S cm − 1 . Specifically, the desired microstructures of the sintered pellets with uniform grains are revealed. As a proof-of-concept, we demonstrate that the nanoscale γ-Al 2 O 3 modification garnet conductor Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 is available to suppress lithium dendrites in symmetric Li/γ-Al 2 O 3 coating Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 /Li cells during lithium plating/stripping at room temperature, with current densities of 0.026 to 0.5 mA cm − 2 . The Li/γ-Al 2 O 3 coating Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 /LiNi 0.5 Co 0.2 Mn 0.3 O 2 cell delivered a high initial discharge capacity of ~164.2 mAh g − 1 , more importantly, an outstanding cycling performance with a capacity retention of 90.16% after 30 cycles at 0.1C and room temperature. This new synthesis route and the utilization of nanoscale γ-Al 2 O 3 surface modification could open up new avenue to lithium garnet conductors with reliable synthesis for all-solid-state lithium batteries application.