P2–Na_0.67Ni_0.33Mn_0.67–xFe_xO₂ with Superior Na⁺ Diffusion and Cycle Stability at High Voltage for Sodium-Ion Batteries

Abstract

Aiming at inhibiting the irreversible P2–O2 phase transition of conventional P2-type cathode materials at high voltage and enhancing the cycling stability of sodium-ion batteries, in this article, based on a strategy of adjusting the Na⁺ ion occupancy within the crystal structure, Na_0.67Ni_0.33Mn_0.67–xFe_xO₂ (NM–xFe, x = 0.10, 0.15, 0.20) cathode materials were synthesized by high shear mixer (HSM)-assisted co-precipitation method and evaluated the electrochemical performance at high voltage (4.35 V). The optimal sample NM–0.15Fe exhibits an initial discharge capacity of 130.8 mAh/g (0.1 C), with exceptional retention of 95.9% after 100 cycles (1 C). XRD analysis reveals that Fe intercalation promotes the more amount of Nae-similar occupation; the Nae/Naf ratio equals 1.93 for NM–0.15Fe versus 1.62 for NM, which enhances Na⁺ diffusion kinetics, as confirmed by GITT tests. Through characterizations of in situ XRD, XPS, HRTEM, CV, etc., it is illustrated that the Fe³⁺ intercalation can effectively disrupt the Na⁺/vacancy ordering and inhibit the harmful P2–O2 phase transition, and then improve the cycling stability of the cathode. DFT calculations disclose that intercalated Fe can reduce the electron densities of adjacent transition metallic elements, generating more repulsive forces impacted on sodium and consequently appearance of more Nae sites, leading to a lower Na⁺ diffusion energy barrier. Such strategy of modulating Na occupation sites in crystal structure is conducive to the development of low-cost and high-performance layered cathode materials for sodium-ion batteries.

Keywords

Na occupation site / P2 cathode / Sodium-ion battery / Diffusion energy barrier

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