Unraveling Phase Evolution Pathways and Sequential Multi‐Metal Reduction Mechanisms in Over‐Lithiated NCM Cathodes

ABSTRACT

Layered ternary oxide cathodes (NCM) play a vital role in advancing lithium batteries, yet their accessible capacity is limited by conventional lithium stoichiometry. Driving NCM cathodes into the over‐lithiated regime Li _{1+ x} NCM enables excess lithium storage and higher energy density. However, the underlying mechanisms are poorly understood due to compositional complexity and coupled multi‐metal redox chemistry. Herein, we established a unified over‐lithiation model encompassing NCM cathodes across various compositions (NCM333/523/811, etc.), categorizing the over‐lithiation process into intercalation reaction region and conversion reaction region. In the intercalation region, excess lithium is accommodated within the layered structure through a composition‐dependent sequential reduction mechanism. Equimolar Ni‐Mn NCM333 initiates over‐lithiation via Co ³⁺ reduction at the first intercalation plateau, followed by dominant Mn ⁴⁺ reduction at the second plateau. In contrast, Ni‐Mn non‐equimolar compositions preferentially undergo sequential Ni ³⁺ and Co ³⁺ reduction, while Mn ⁴⁺ reduction is largely deferred. Upon deeper lithiation, all compositions converge to a universal conversion reaction region, characterized by the formation of unstable metallic species, severe lattice strain, and irreversible structural degradation. This work establishes a mechanistic framework for over‐lithiation in layered NCM cathodes, providing general principles for accessing capacity beyond conventional intercalation chemistry toward high‐energy lithium batteries.