Surface Gradient Engineering Relaxes Stress Concentration in Ultrahigh-Ni Cathodes, Enabling Superior Cyclability in...

Developing ultrahigh-Ni cathodes is critical for advancing next-generation high-energy Li-ion batteries (LIBs), yet reducing the Co content exacerbates mechanochemical degradation and lowers Li-ion transport. Herein, we design a surface low-Co-gradient-distributed and bulk Mg-doped Li(Ni_0.97Mn_0.03)_0.995Ge_0.005O₂ (GM-g-NMC) cathode, in which nonmagnetic Ge substitution for Co not only stabilizes the crystal structure synergistically with Mg but also promotes a near-surface Co gradient distribution by suppressing its diffusion. These features mitigate lattice contraction while enhancing surface mechanical robustness, thereby relaxing the stress concentration and preserving structural and interfacial stability. Operando characterizations and electrochemical analyses reveal a 5-fold reduction in potential polarization, along with the suppression of rock-salt phase formation. Consequently, GM-g-NMC delivers an ultrahigh reversible capacity of 229.0 mAh g^-1 at 0.1C and 151.7 mAh g^-1 at 10C. In pouch-type full cells, it retains 80.1% of its initial capacity after 1200 cycles at 1C, prolonging the service period relative to the previously reported ultrahigh-Ni cathodes.