Revealing the interfacial kinetic mechanisms in high-entropy doped Na$_3$V$_2$(PO$_4$)$_3$ through electrochemical investigation and distribution of relaxation times

TL;DR

This work demonstrates that high-entropy doping at the vanadium site in NASICON Na3V2(PO4)3 yields a cathode with activated V4+/V5+ redox at around $4.0$ V, enhanced diffusion, and exceptional high-rate stability for sodium-ion batteries. Using distribution of relaxation times applied to in-situ impedance data, the authors disentangle interfacial charge-transfer, SEI, and bulk diffusion contributions, linking microstructure to kinetics. The combination of CV, GITT, and EIS indicates Na+ diffusion coefficients in the range $10^{-11}$ to $10^{-13}$ cm$^2$ s$^{-1}$ and activation barriers around $0.25$–$0.68$ eV depending on process, while BVSE and post-mortem analyses confirms structural stability after extensive cycling. In full cells with hard carbon, the cathode delivers about $326$ Wh kg$^{-1}$ (based on cathode mass) with high capacity retention, suggesting strong potential for high-energy-density SIBs with long cycle life.

Abstract

We designed a high-entropy doped NASICON cathode, Na$_3$V$_{1.9}$(CrMoAlZrNi)$_{0.1}$(PO$_4$)$_3$ and investigate its electrochemical performance for sodium-ion batteries (SIBs) to understand the diffusion mechanism including distribution of relaxation times analysis of interfacial kinetics. This trace doping induces high-entropy mixing at the vanadium site, tuning the lattice and enhancing specific capacity, activating V$^{4+}$/V$^{5+}$ redox couple 3.95~V. Interestingly, it delivers a reversible capacity of 119~mAh~g$^{-1}$ at 0.1~C, and demonstrate excellent stability of 68\% after 1000 cycles at 10~C. The calculated diffusion coefficient values are found within the range of \(10^{-11}\)--\(10^{-13}~\mathrm{cm^2\,s^{-1}}\). The systematic investigation of temperature and voltage-dependent impedance data using the distribution of relaxation times provides deeper insights into the underlying charge-transfer and transport processes. The full cells with hard carbon delivers 326~Wh~kg$^{-1}$ (with respect to cathode mass) at $\approx$3.2~V and retained $\sim$79\% capacity after 100 cycles at 2~C. Our study opens new avenues for developing high-entropy doped cathodes for enhanced structural stability, extended redox activity, and optimized electrochemical kinetics for practical implementation of SIBs.

Figures (7)

• Figure 1: (a) The XRD pattern with Rietveld refinement profile of the NVP-HE sample; (b) an isosurface representation of Na$^+$ ion diffusion pathways in the NVP-HE structure, and (c) the corresponding energy profile of migration barriers along the minimum energy path Na1--Na2--Na1, calculated using the BVSE approach; (d) The Raman spectrum; (e-g) the HR-TEM images, (h) the SAED pattern; (i) the FESEM image, (j) overlay image of elemental mapping, and (k1-k10) individual elemental mapping of constituent elements of NVP-HE sample; the core-level XPS spectra of the (l) V 2$p$, (m) Mo 3$d$, (n) Cr 2$p$, (o) Al 2$p$, (p) Zr 3$d$, and (q) Ni 2$p$.
• Figure 2: (a) The CV curves at 0.1 mV s$^{-1}$, (b) the GCD profiles at 0.1 C for the second cycle of NVP and NVP-HE in 2.0--4.3 V; (c) the first five cycles of GCD profile of NVP-HE at 0.1C; (d) the rate performances; (e) the corresponding GCD profiles for second cycle at each current rate; (f) the cycling performances of NVP-HE at the current rates of 2 C, 5 C, and 10 C.
• Figure 3: (a) The CV curves of NVP-HE at various scan rates 0.05--1.0 mV s$^{-1}$; (b) a linear relationship between peak current (i$_p$) and the square root of the scan rate ($\upsilon$$^{1/2}$); (c) a linear fitting of log(i$_p$) versus log($\upsilon$), and (d) i$_p$/$\upsilon$$^{1/2}$ versus $\upsilon$$^{1/2}$ for anodic and cathodic peaks; (e) the capacitive contribution at 0.5 mV/s, highlighted by the shaded area; (f) the distribution of capacitive and diffusion-controlled current components at various scan rates; (g) the GITT profile and diffusion coefficient recorded with a pulse time of 10 mins and rest time of 60 mins at 0.1 C in 2.0--4.3 V range after formation cycles; (h) depiction of various parameters during a galvanostatic titration step involving a 10 mins current pulse followed by a 60 mins rest.
• Figure 4: (a) The GCD profile of NVP-HE cathode with points indicating where the EIS measurements are performed; the corresponding $in-situ$ EIS spectra in (b) recorded at various potential values during charging and discharging; (c) the resistance as a function of voltage obtained from the EIS fitting and (e) sodium-ion diffusion coefficient values extracted from the Warburg region of the corresponding EIS spectra; (e) the simulated DRT profiles derived from EIS measured at different voltages; (f1--f4) the fitted DRT profiles for selected voltages, the DRT profiles of symmetric cells of NVP-HE$\parallel$NVP-HE in (g) and Na$\parallel$Na in (h), (i, j) the resistance values extracted from peak deconvolution of DRT profiles, shown as a function of voltage.
• Figure 5: (a) The EIS spectra of NVP-HE cathode recorded at different temperatures from 28° C to 55° C, (b) the resistance values derived from equivalent circuit fitting of the EIS spectra, (c) the Arrhenius plot corresponds to interfacial and charge-transfer resistances, (d) the GCD profile of NVP-HE cathode at various temperatures measured at 0.3 C; (e) the simulated DRT profiles extracted from each EIS spectrum; (f--h) the fitted DRT profiles at 28° C, 35° C, and 50° C, respectively; (i) the resistance values extracted from peak deconvolution of DRT profiles corresponding to individual peaks at different temperatures.