Unveiling the Over‐Lithiation Behavior of NCM523 Cathode Towards Long‐Life Anode‐Free Li Metal Batteries

Abstract Anode‐free lithium metal batteries (AFLMBs) offer the potential for significantly enhanced energy densities. However, their practical application is limited by a shortened cycling life due to inevitable Li loss from parasitic reactions. This study addresses this challenge by incorporating an over‐lithiated Li 1+ x Ni 0.5 Co 0.2 Mn 0.3 O 2 (Li 1+ x NCM523) cathode as an internal Li reservoir to compensate for lithium loss during extended cycling. A rigorous investigation of the deep discharge behavior of the Li 1+ x NCM523 cathode reveals a critical over‐lithiation threshold at x  = 0.7. At this threshold, excess Li + ions are safely accommodated within the crystal structure by a transformation from the LiO 4 octahedron to two tetrahedral sites. Beyond this threshold ( x ≥ 0.7), the structural stability of the cathode is significantly compromised due to the irreversible reduction of transition metal (TM) ions. The optimal Li‐rich Li 1.7 NCM523 releases an additional charge capacity of ≈160 mAh g −1 during the first charge. Consequently, the AFLMBs (Li 1.7 NCM523||Cu) achieve outstanding capacity retention of 93.3% after 100 cycles at 0.5 C and 78.5% after 200 cycles at 1 C. The findings establish a research paradigm for designing superior over‐lithiated transition metal oxide cathode materials and underscore the critical role of the lithium reservoir in extending the cycle life of AFLMBs.