Cobalt‐free LiNixMn1−xO2 (NM, x ≥ 0.5) layered oxides are considered to be promising cathode materials for next‐generation lithium‐ion batteries because of exceptionally high capacity and low cost, yet the fundamental role of manganese ions in the NM layered structure and rate performance has not been fully addressed to date. Herein, a series of Ni‐fixed LiNi0.6Co0.4−xMnxO2 (x = 0, 0.1, 0.2, 0.3, and 0.4) systems are employed as cathode materials to investigate the functionality of Mn ions on their structures and electrochemical properties. It is found that contrary to prior reports, the change in the c‐axis lattice parameter is not in close connection with the rate performance of NM cathodes. In particular, superconducting quantum interference device (SQUID) measurements are performed to verify the fact that Mn3+ and Mn4+ ions with high spin states cause severe magnetic frustration in the structures of cathode materials, which profoundly aggravates the Li/Ni ionic disorder and blocks Li+ migration, contributing to inferior rate performance. In addition, Li+ migration hindered by Li/Ni disorder, is theoretically demonstrated by ab initio calculation. This work not only provides fresh insight into the role of Mn in NM layered oxide cathodes but also proposes an effective strategy to resolve their inferior rate performance.

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