Reinforced cathode-garnet interface for high-capacity all-solid-state batteries

Garnet-type solid-state electrolytes (SSEs) are particularly attractive in the construction of all-solid-state lithium (Li) batteries due to their high ionic conductivity, wide electrochemical window and remarkable (electro)chemical stability. However, the intractable issues of poor cathode/garnet interface and general low cathode loading hinder their practical application. Herein, we demonstrate the construction of a reinforced cathode/garnet interface by spark plasma sintering, via co-sintering Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) electrolyte powder and LiCoO₂/LLZTO composite cathode powder directly into a dense dual-layer with 5 wt% Li₃BO₃ as sintering additive. The bulk composite cathode with LiCoO₂/LLZTO cross-linked structure is firmly welded to the LLZTO layer, which optimizes both Li-ion and electron transport. Therefore, the one-step integrated sintering process implements an ultra-low cathode/garnet interfacial resistance of 3.9 cm² (100 C) and a high cathode loading up to 2.02 mAh cm^-2. Moreover, the Li₃BO₃ reinforced LiCoO₂/LLZTO interface also effectively mitigates the strain/stress of LiCoO₂, which facilitates the achieving of superior cycling stability. The bulk-type Li|LLZTO|LiCoO₂-LLZTO full cell with areal capacity of 0.73 mAh cm^-2 delivers capacity retention of 81.7% after 50 cycles at 100 A cm^-2. Furthermore, we reveal that non-uniform Li plating/stripping leads to the formation of gaps and finally results in the separation of Li and LLZTO electrolyte during long-term cycling, which becomes the dominant capacity decay mechanism in high-capacity full cells. This work provides insight into the degradation of Li/SSE interface and a strategy to radically improve the electrochemical performance of garnet-based all-solid-state Li batteries.