Mitigating chemo-mechanical heterogeneity of Ni-rich layered cathodes through the regulated medium-range order by doping

Abstract

Structural heterogeneity-induced chemo-mechanical interplay has been identified as the primary reason for the capacity degradation of high-voltage Ni-rich layered cathodes. In the current study, two dopants known to have different site occupancies in Ni-rich layered cathodes, Mg and W, are revisited, revealing the relationship between dopant-induced local structure changes and electrochemical reversibility across a broad charge/discharge voltage range. Our results indicate that the introduced W induces Li–W–Li medium-range order (MRO), resulting in a lower electron concentration in O 2p orbitals. The MRO in a W-doped Ni-rich layered oxide (LNCW) triggers asymmetry in the NiO₆ octahedra and weakens the surrounding hybridization, leading to a highly covalent Ni⁴⁺–O bond and activating charge transfer from the ligand. Although the contribution of oxygen oxidation was somewhat reduced in LNCW, the distorted NiO₆ octahedra arising from deepened cationic oxidation worsened the electrochemical performance significantly. By contrast, Mg occupies the 3b site of Li and thereby forms MgO₄ tetrahedra, inducing Ni–Mg–Ni MRO and a resulting charge redistribution that shrinks the Ni 3d orbital band, leading to a reversible Ni-redox process and suppressing oxygen oxidation. Hence, the Mg-doped Ni-rich layered oxide (LNCMg) maintains superior structural integrity to LNCW, avoiding the parasitic distortion of NiO₆ octahedra and alleviating the chemo-mechanical heterogeneity. This work underscores the significance of medium-range order in the chemo-mechanical interplay in Ni-rich layered cathodes.