Dielectric polarization in MgFe2O4 coating and bulk doping to enhance high-voltage cycling stability of Na2/3Ni1/3Mn2/3O2 cathode material

Abstract

Charging P2-Na2/3Ni1/3Mn2/3O2 to 4.5 V for higher capacity is enticing. However, it leads to severe capacity fading, ascribing to the lattice oxygen evolution and the P2-O2 phase transformation. Here, the MgFe2O4 coating and Mg, Fe co-doping were constructed simultaneously by Mg, Fe surface treatment to suppress lattice oxygen evolution and P2-O2 phase transformation of P2-Na2/3Ni1/3Mn2/3O2 at deep charging. Through ex-situ X-ray diffraction (XRD) tests, we found that the Mg, Fe bulk co-doping could reduce the repulsion between transition metals and Na+/vacancies ordering, thus inhibiting the P2-O2 phase transition and significantly reducing the irreversible volume change of the material. Meanwhile, the internal electric field formed by the dielectric polarization of MgFe2O4 effectively inhibits the outward migration of oxidized Oα− (α < 2), thereby suppressing the lattice oxygen evolution at deep charging, confirmed by in situ Raman and ex situ XPS techniques. P2-NaNM@MF-3 shows enhanced high-voltage cycling performance with capacity retentions of 84.8% and 81.3% at 0.1 and 1 C after cycles. This work sheds light on regulating the surface chemistry for Na-layered oxide materials to enhance the high-voltage performance of Na-ion batteries.