Bulk-Surface Coupled Regulation of Magnetic Frustration and Interphase Architecture in Co-Free LiNi0.6Mn0.4O2 Cathodes.

Co-free layered LiNi0.6Mn0.4O2 (NM64) is an attractive cost-effective cathode, yet its practical viability is limited by bulk Li/Ni antisite disorder and high-voltage interfacial degradation. Addressing these coupled limitations requires a bulk-surface design framework that balances magnetic-frustration-associated cation ordering with nanoscale interphase stability. Here, nominal 1 mol % high-valence, nonmagnetic d0 modifiers (V5+, Hf4+, Ta5+) are used as a chemically defined model series to probe the coupling between magnetic-frustration-associated cation ordering and nanoscale surface/near-surface interphase architecture. Within this series, 1%Hf-NM64 lowers the apparent frustration index from 9.562 to 7.366 and the refined Li/Ni mixing from 7.67 to 7.14%, whereas V increases both descriptors and Ta leaves them nearly unchanged. Low-temperature M-H loops further indicate that 1%Hf-NM64 exhibits the weakest hysteretic magnetic irreversibility. Surface/near-surface characterization reveals modifier-dependent nanoscale interphase architectures, with localized Li2HfO3-rich domains in 1%Hf-NM64 coinciding with the lowest polarization and the best cycling response among the series. Compared with pristine NM64, 1%Hf-NM64 raises the initial 1 C discharge capacity from 136.6 to 159.0 mAh g-1 and improves the 500-cycle capacity retention from 69.0 to 73.3%. 3%Hf-NM64 further shows that excessive Hf increases Li2HfO3-related secondary-phase content and apparent Li+ diffusivity but compromises long-term cycling stability. These findings support optimized bulk-surface coupled regulation of magnetic frustration, cation disorder, and interphase architecture as a design principle for Co-free layered oxide cathodes.