Influence of the manganese and cobalt content on the electrochemical performance of P2-Na0.67MnxCo1−xO2 cathodes for sodium-ion batteries

Abstract

The resurgence of sodium-ion batteries in recent years is due to their potential ability to form intercalation compounds possessing a high specific capacity and energy density comparable to existing lithium systems. To comprehend the role of cobalt substitution in the structure and electrochemical performance of Na_0.67MnO₂, the solid solutions of P2-Na_0.67Mn_xCo_1−xO₂ (x = 0.25, 0.5, 0.75) are synthesized and characterized. The XRD-Rietveld analysis revealed that the Co-substitution in Na_0.67MnO₂ decreases lattice parameters ‘a’ and ‘c’ resulting in the contraction of MO₆ octahedra and the enlargement of inter-layer ‘d’ spacing. XPS indicates that the isovalent cobalt substitution in Na_0.67MnO₂ results in the partial/complete replacement of Jahn–Teller active trivalent manganese to form low-spin complexes of better structural stability. The Na-ion diffusion coefficient, D_Na⁺, derived from cyclic voltammetry and impedance spectroscopy, confirmed the enhanced mass transport in Co-rich phases compared to Mn-rich phases. Furthermore, higher diffusion coefficient values are observed for Co³⁺/Co⁴⁺ than for their Mn³⁺/Mn⁴⁺ redox processes. In addition, Co-rich phases exhibit a high structural stability and superior capacity retention, whereas Mn-rich phases discharge higher capacities.