Structural and stability analysis of Na₄Mn₄Ti₅O₁₈ for energy storage application

We report a single-phase Na₄Mn₄Ti₅O₁₈ oxide prepared via ceramic processing route. Rietveld refinement of X-ray diffraction results indicated orthorhombic unit cell indexed with Pbam space group. Further, structural analysis using X-ray diffraction, transmission electron microscopy and Raman spectroscopy suggested crystal structure comprising two distinct tunnel pathway, polygonal and Z-shaped tunnels with void dimensions ~ 4.07 Å and ~ 3.01 Å, respectively, suitable for reversible cation transport desirable for energy storage applications. X-ray photoelectron spectroscopy confirmed presence of Mn²⁺, Mn³⁺, Mn⁴⁺ and Ti⁴⁺ in Na₄Mn₄Ti₅O₁₈. Surface morphology in field emission scanning electron microscopy exhibited inhomogenously distributed micro-bead grains separated by well-defined boundaries in good agreement with complex impedance spectrum results. Estimated dc conductivity and surface area is ~ 1.745 × 10^− 6 S cm^− 1 and ~ 0.66 m²g^− 1 respectively. Electrochemical energy storage performance with cell; Na || 1 M NaFSI (EC : DMC, 1:1 v/v%) || Na₄Mn₄Ti₅O₁₈ in voltage range 2.0–3.8 V indicated reversible redox action. Identical cell in voltage range 0.01–2.5 V, exhibited pseudo-capacitive behaviour. Results suggest energy storage possibility with Na₄Mn₄Ti₅O₁₈ electrode.