Voltage Noise Failure Induced by Li Dendritic Micro‐Penetration in All‐Solid‐State Li‐Metal Battery with Composite Solid Electrolyte.

All‐solid‐state Li‐metal batteries (ASSLBs) are the most attractive next‐generation batteries due to intrinsic safety and high energy density. Particularly, composite solid electrolyte (CSE)‐based ASSLBs, highly compatible with conventional Li‐ion batteries, are nearing commercialization. However, the understanding of ASSLBs’ failure remains deficient, thereby considerably hindering their advancement. Herein, the unrecognized failing mode of ASSLBs, voltage noise failure (VNF), characterized by irregular charging voltage configuration, is identified using comprehensive techniques, including laser‐induced breakdown spectroscopy. The VNF originates from micro‐penetration of Li dendrites, which is demonstrated through direct observation of 3D Li concentration map in CSE. In this phenomenon, the transition metals, dissolved from the cathode, hop to the anode and serve as seeds for dendritic growth in VNF. Inspired by this mechanism and with the aid of DFT calculations, a transition metal scavenging layer is proposed using Prussian blue analogue at the cathode‐CSE interface. Consequently, ASSLBs with transition metal scavenging layer exhibit superior capacity (189 mAh g−1 at 0.5 C, NCM811) and stable cyclability (1200 cycles without failure). [ABSTRACT FROM AUTHOR]