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.