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 Li6.5La3Zr1.5Ta0.5O12 (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 Ω cm2 (100 ℃) 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 μA cm-2. Furthermore, we reveal that the 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 to the degradation of Li/SSE interface and a strategy to radically improve the electrochemical performance of garnet-based all-solid-state Li batteries.