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_0.5Co_0.2Mn_0.3O₂ (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 LiO₄ 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_1.7NCM523 releases an additional charge capacity of ≈160 mAh g^-1 during the first charge. Consequently, the AFLMBs (Li_1.7NCM523||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.