The production and electrochemical performance analysis of an O3-NaTi0.2Mn0.2Fe0.2Ni0.2Co0.2O2 high-entropy oxide cathode for Na-ion batteries

Abstract

In this study, we report the suppression of degradative transformations observed in Na-ion batteries by utilizing five different transition-metal cations at a single crystallographic site, which is expected to impart entropy-induced stabilization, resulting in enhanced structural stability. We successfully synthesized phase-pure O3-NaTi_0.2Mn_0.2Fe_0.2Ni_0.2Co_0.2O₂ through a facile solid-state sintering method. In this compound, Co³⁺, Fe³⁺, and Ni²⁺ provide charge compensation for capacity, Mn⁴⁺ acts as a structure former, and Ti⁴⁺ helps stabilize the overall structure. The synthesized material was successfully characterized using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX), inductively coupled plasma mass spectroscopy (ICP-MS), X-ray photo-electron spectroscopy (XPS), and transmission electron microscopy (TEM). The configurational entropy was calculated to be ∼1.6 R, which is consistent with a high-entropy oxide. The material revealed a single phase with Rm symmetry, which matches well with the O3-type layered structure. SEM revealed irregular 1–3 µm particles, while EDX mapping confirmed uniform elemental dispersion. ICP provided the composition as Na_0.94(Ti_0.18Mn_0.20Fe_0.20Ni_0.21Co_0.21)O₂. The XPS analysis showed mixed-valence chemistry for each component (Ti is predominantly Ti⁴⁺ with minor metallic Ti; Mn is consistent with Mn⁴⁺; Fe exhibits Fe²⁺/Fe³⁺ coexistence; Co exhibits Co²⁺/Co³⁺ coexistence; and Ni exhibits Ni²⁺/Ni³⁺ coexistence), which can be explained by a charge compensation mechanism. Transmission electron microscopy and selected area electron diffraction (TEM-SAEED) analysis confirmed the formation of an Rm structure, and the interlayer separation was also calculated. The electrochemical properties were systematically evaluated, and the capacity of the cells was found to be 120 mA h g⁻¹ at a C/3-rate, which demonstrates the promising potential of this composition for sodium-ion battery applications.