Electrochemical performance and diffusion kinetics of a NASICON type Na${}_{3.3}$Mn${}_{1.2}$Ti${}_{0.75}$Mo${}_{0.05}$(PO${}_4$)${}_3$/C cathode for low-cost sodium-ion batteries

We report the electrochemical performance and diffusion kinetics of a newly designed NASICON type Na${}_{3.3}$Mn${}_{1.2}$Ti${}_{0.75}$Mo${}_{0.05}$(PO${}_4$)${}_3$/C composite material 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 symmerty. The high-resolution microscopy analysis reveals nanocrystallites of around $\sim$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, we demonstrate a stable electrochemical behavior, 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 ${10}^{-11}$~cm${}^2$/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 $\sim$50 s, shows a consistent trend with the diffusion coefficients obtained from GITT and Warburg-based evaluations across the state of charge.