Garnet Electrolyte with an Ultralow Interfacial Resistance for Li-Metal Batteries

Garnet-structured Li ₇La ₃Zr ₂O ₁₂ is a promising solid Li-ion electrolyte for all-solid-state Li-metal batteries and Li-redox-flow batteries owing to its high Li-ion conductivity at room temperature and good electrochemical stability with Li metal. However, there are still three major challenges unsolved: (1) the controversial electrochemical window of garnet, (2) the impractically large resistance at a garnet/electrode interface and the fast lithium-dendrite growth along the grain boundaries of the garnet pellet, and (3) the fast degradation during storage. We have found that these challenges are closely related to a thick Li ₂CO ₃ layer and the Li-Al-O glass phase on the surface of garnet materials. Here we introduce a simple method to remove Li ₂CO ₃ and the protons in the garnet framework by reacting garnet with carbon at 700 °C; moreover, the amount of the Li-Al-O glass phase with a low Li-ion conductivity in the grain boundary on the garnet surface was also reduced. The surface of the carbon-treated garnet pellets is free of Li ₂CO ₃ and is wet by a metallic lithium anode, an organic electrolyte, and a solid composite cathode. The carbon post-treatment has reduced significantly the interfacial resistances to 28, 92 (at 65 °C), and 45 ω cm ² at Li/garnet, garnet/LiFePO ₄, and garnet/organic-liquid interfaces, respectively. A symmetric Li/garnet/Li, an all-solid-state Li/garnet/LiFePO ₄, and a hybrid Li-S cell show small overpotentials, high Coulombic efficiencies, and stable cycling performance.