Garnet-type solid electrolytes have gained considerable attention owing to their exceptional ionic conductivity and broad electrochemical stability window, making them highly promising for solid-state batteries (SSBs). However, this polycrystalline ceramic electrolyte contains an abundance of grain boundaries (GBs). During the repetitive electroplating and stripping of Li ions, uncontrolled growth and spreading of lithium dendrites often occur at GBs, posing safety concerns and resulting in a shortened cycle life. Reducing the formation and growth of lithium dendrites can be achieved by rational grain boundary design. Herein, the garnet-type solid electrolyte LLZTO was firstly coated with Al₂O₃ using the atomic layer deposition (ALD) technique. Subsequently, an annealing treatment was employed to introduce Al₂O₃ into grain boundaries, effectively modifying them. Compared with the Li/LLZTO/Li cells, the Li/LLZTO@Al₂O₃-annealed/Li symmetric batteries exhibit a more stable cycling performance with an extended period of 200 h at 1 mA cm⁻². After matching with the NMC811 cathode, the capacity retention rate of batteries can reach 96.8% after 50 cycles. The infusion of Al₂O₃ demonstrates its capability to react with LLZTO particles, creating an ion-conducting interfacial layer of Li-Al-O at the GBs. This interfacial layer effectively inhibits Li nucleation and filament growth within LLZTO, contributing to the suppression of lithium dendrites. Our work provides new suggestions for optimizing the synthesis of solid-state electrolytes, which can help facilitate the commercial application of solid-state batteries.