Lattice-coherent epitaxial surface engineering in highly stable Co-free ultrahigh-Ni cathodes

Abstract

Mitigating surface-initiated structural degradation, which propagates inward and depletes all active particles is critical for improving the cycle lifespan of Co-free ultrahigh-Ni NMA (LiNi_1−x−yMn_xAl_yO₂, 1 − x − y − z ≥ 0.9) cathodes. Herein, an epitaxially grown lattice-coherent surface with stabilized low-valence Ni states and refined primary particles has been precisely implemented in LiNi_0.9Mn_0.05Al_0.05O₂ by designing the internal composition distribution within secondary particles, thereby simultaneously ameliorating surface chemistry and mechanical integrity. Key findings indicate that the Mn⁴⁺ species retained on the particle crust facilitate the formation of a localized layer of coherent Li/Ni anti-site defects on the periphery of the primary particles located on the surface of the secondary particle, which stabilizes the intrinsic nature of the surface lattice. Meanwhile, the thermally driven and inward diffusion of Mn, which acts as an oxygen carrier, during calcination ameliorates the local oxygen environment and suppresses interface-fusion intermediate phases (cation mixing) to refine the primary particles. The robust morphological integrity provided by refined primary particles prevents intergranular cracks, confining interfacial parasitic reactions to the surface region of the secondary particles protected by the lattice-coherent epitaxial surface. This universal approach optimally balances surface stabilization with charge transfer, significantly enhancing capacity retention and rate capability for the NMA cathode while opening a new avenue for next-generation Co-free ultrahigh-Ni cathodes.