Regulating the Two‐Stage Accumulation Mechanism of Inactive Lithium for Practical Composite Lithium Metal Anodes

Abstract The rapid formation and accumulation of inactive lithium (Li) are principally responsible for the limited lifespan of high‐energy‐density Li metal batteries. The construction of composite Li metal anode with hosts emerges as a promising strategy to mitigate and accommodate inactive Li. However, the mechanism of inactive Li accumulation in composite Li metal anodes remains unknown, severely plaguing the stability of composite Li metal anodes. Herein, the two‐stage accumulation mechanism of inactive Li in composite Li metal anodes and its correlation with the stability of composite Li metal anodes are comprehensively unveiled in pouch cells by nondestructive 3D X‐ray microscopy. First, inactive Li accumulates in the interior of the host and results in a slowly increased polarization. Second, inactive Li overflows the inside of the host and induces a dramatically increased polarization chiefly responsible for the fast decay of the composite Li metal anodes. The current density and external pressure are identified as key factors to regulate the turning point between the two stages for practical composite Li metal anodes. This work provides original fundamentals for the recognition of inactive Li accumulation in composite Li metal anodes and the design of practical Li metal batteries.