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

TL;DR

This work tackles the cost and performance challenges of Mn-rich NASICON-type cathodes for sodium-ion batteries by introducing Mo-doping and sodium enrichment to stabilize a high Mn content within a rhombohedral NASICON framework. A sol-gel synthesis yields a carbon-coated, Mn-rich NASICON material, $Na_{3.3}Mn_{1.2}Ti_{0.75}Mo_{0.05}(PO_4)_3$/C, with nanocrystalline domains (~$18$ nm) embedded in a conductive carbon matrix, enabling efficient Na$^+$ transport. Electrochemical testing shows a strong initial capacity of $124$ mAh g$^{-1}$ at $0.1$ C, good rate capability up to $5$ C, and about $70 ext{%}$ capacity retention after $400$ cycles at $2$ C, supported by diffusion coefficients in the $10^{-9}$ to $10^{-11}$ cm$^2$/s range and a migration barrier of ~ $0.76$ eV from BVSE. The study further leverages Distribution of Relaxation Time (DRT) impedance analysis to robustly deconvolute interfacial and bulk processes, aligning with GITT-derived kinetics and confirming the material’s potential as a practical, low-cost SIB cathode.

Abstract

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.

Figures (7)

• Figure 1: The structural, morphological, and compositional analysis of Na$_{3.3}$Mn$_{1.2}$Ti$_{0.75}$Mo$_{0.05}$(PO$_4$)$_3$/C: (a) the Rietveld-refined XRD pattern, (b) the crystal structure, (c) the Raman spectrum, (d–f) room temperature Mn 2$p$, Ti 2$p$, and Mo 3$d$ XPS core-level spectra, (g1) the FESEM image, (g2--g7) the EDS elemental mapping, (h1–h3) the HR-TEM images at different magnifications, (h4) lattice fringes with a measured $d$-spacing and (h5) the SAED pattern.
• Figure 2: The electrochemical performance of Na$_{3.3}$Mn$_{1.2}$Ti$_{0.75}$Mo$_{0.05}$(PO$_4$)$_3$/C cathode material in the potential window of 1.5–4.3 V: (a) initial CV curves at 0.1 mV/s, (b) first five GCD profiles at a current rate of 0.1 C, (c) second GCD cycle at various current rates, (d) the rate capability performance at various C-rates, (e) the long-term cycling stability tests at 0.5, 1, 2 C. (f) The GCD profiles in the second cycle, measured at 0.5 C, shown at selected temperatures and (g) the cyclic performance.
• Figure 3: (a) The dQ/dV curves of the discharge profiles [shown in Fig. \ref{['CV_GCD']}(c)], (b) the GCD profiles measured at 1 C up to 30 cycles, and inset shows the zoomed view of the dashed region.
• Figure 4: The electrochemical characterization of Na$_{3.3}$Mn$_{1.2}$Ti$_{0.75}$Mo$_{0.05}$(PO$_4$)$_3$/C at various scan rates (0.05–1.00 mV/s) in the potential window of 1.5--4.3 V: (a) The CV curves at different scan rates, (b) the linear relationship between peak current (i$_p$) and the square root of the scan rate ($\nu^{1/2}$), (c) the logarithmic correlation between log(i$_p$) and log($\nu$), (d) a linear fitting of i$_p$/$\nu^{1/2}$ versus $\nu^{1/2}$ for both anodic and cathodic peaks, (e) the capacitive contribution at 0.2 mV/s, highlighted by the shaded area and (f) the contributions of the capacitive and diffusive contributions to the total current at varying scan rates.
• Figure 5: The GITT measurement at a current rate of 0.05 C in a voltage window of 1.5–4.3 V plotted vs. (a) time and (b) specific capacity, after 5 GCD cycles at 0.05 C, (c) the schematic labeling of different parameters of a single titration curve before, during and after application of a current pulse for 20 min, (d) the calculated iso-surfaces for sodium-ion 3D migration channels (iso-surface level = 0.7 eV), (e) the connectivity of the Na2-Na1-Na2 migration channel, and (f) the calculated activation energy of the sodium-ion migration.