Synergistic effect of multi-transition metal co-substitution in high cycle life performance of NaxCo0.5Fe0.25Mn0.25O2 cathode for sodium-ion batteries†

Abstract

In this work, multi-transition metal co-substituted Na_xCo_0.5Fe_0.25Mn_0.25O₂ is synthesized through a solid-state method using a two-step heating approach and its physicochemical and electrochemical features as a cathode material for sodium-ion batteries (SIBs) are studied. Various advanced physicochemical characterization studies reveal the P3 structure of the as-prepared Na_xCo_0.5Fe_0.25Mn_0.25O₂ possessing multiple crystal symmetries with high-order crystallinity, suitable for enhanced Na⁺-ion intercalation and deintercalation. Its electrochemical performances are investigated with the fabricated Na/1 M-NaClO₄/Na_xCo_0.5Fe_0.25Mn_0.25O₂ coin cells. The cyclic voltammetry study reveals that the redox process of the cathode material is due to the M³⁺/M⁴⁺ (where M = Co_0.5Fe_0.25Mn_0.25) redox couple with excellent structural reversibility during the charging/discharging process. The electrochemical impedance spectroscopy analysis suggests excellent compatibility of the electrolyte with the cathode, showing a good state of health, a low value of resistance offered to the cell, and a very negligible value of double-layer capacitance. The galvanostatic charge–discharge interpretations reveal that Na_xCo_0.5Fe_0.25Mn_0.25O₂ delivers significant rate capability and a high discharge capacity of 94.22 mA h g⁻¹ at 0.05C by maintaining stable performance across a range of C-rates. The material exhibits high coulombic efficiency and impressive energy densities, with a maximum discharge energy density of 279.82 W h kg⁻¹ at 0.05C. Notably, Na_xCo_0.5Fe_0.25Mn_0.25O₂ demonstrates excellent cycle life, retaining 92.2, 78.4, 53.9, 39.4, and 28.3% of the initial discharge capacity at the 100^th, 200^th, 300^th, 400^th, and 500^th cycles, respectively, owing to the synergistic effect of co-substituted multi-transition metals.