Electrochemical Performance and Diffusion Kinetics of a NASICON type Na_3.3Mn_1.2Ti_0.75Mo_0.05(PO₄)₃/C Cathode for...

The electrochemical performance and diffusion kinetics of a newly designed NASICON-type Na_3.3Mn_1.2Ti_0.75Mo_0.05(PO₄)₃/C composite material is reported as a cathode for cost-effective sodium-ion batteries. A novel strategy of small Mo doping successfully stabilizes the sample having high Mn content in single-phase rhombohedral symmetry. The high-resolution microscopy analysis reveals nanocrystallites of around ∼18 nm, uniformly embedded within the semi-graphitic carbon matrix, which enhances the surface electronic conductivity and effectively shortens the sodium-ion diffusion path. More importantly, a stable electrochemical behavior is demonstrated, with enhanced discharge capacity of 124 mAh/g at 0.1 C, having good reversibility and retaining 77% of its capacity after 300 cycles, and 70% even after 400 cycles at 2 C. The sodium-ion diffusion coefficients, estimated using both galvanostatic intermittent titration technique (GITT) and cyclic voltammetry, are found to lie within the range of 10^-9 to 10^-11 cm²/s. Additionally, the bond-valence site energy mapping predicted a sodium-ion migration energy barrier of 0.76 eV. A detailed distribution of relaxation times (DRT) analysis is used to deconvolute the electrochemical impedance spectra into distinct processes based on their characteristic relaxation times. Notably, the solid-state diffusion of sodium ions within the bulk electrode, with a relaxation time of ∼50 s, shows a consistent trend with the diffusion coefficients obtained from GITT and Warburg-based evaluations across the state of charge.