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

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-<i>g</i>-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. <i>Operando</i> characterizations and electrochemical analyses reveal a 5-fold reduction in potential polarization, along with the suppression of rock-salt phase formation. Consequently, GM-<i>g</i>-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.