High Cathode Loading and Low‐Temperature Operating Garnet‐Based All‐Solid‐State Lithium Batteries – Material/Process/Architecture Optimization and Understanding of Cell Failure

All‐solid‐state lithium batteries (ASSLBs) are prepared using garnet‐type solid electrolytes by quick liquid phase sintering (Q‐LPS) without applying high pressure during the sintering. The cathode layers are quickly sintered with a heating rate of 50–100 K min −1 and a dwell time of 10 min. The battery performance is dramatically improved by simultaneously optimizing materials, processes, and architectures, and the initial discharge capacity of the cell with a LiCoO 2 ‐loading of 8.1 mg reaches 1 mAh cm −2 and 130 mAh g −1 at 25 °C. The all‐solid‐state cell exhibits capacity at a reduced temperature (10 °C) or a relatively high rate (0.1 C) compared to the previous reports. The Q‐LPS would be suitable for large‐scale manufacturing of ASSLBs. The multiphysics analyses indicate that the internal stress reaches 1 GPa during charge/discharge, which would induce several mechanical failures of the cells: broken electron networks, broken ion networks, separation of interfaces, and delamination of layers. The experimental results also support these failures.