Case study of an exploratory high voltage NASICON-based Na$_4$NiCr(PO$_4$)$_3$ cathode material for sodium-ion batteries

TL;DR

This paper evaluates $Na_4NiCr(PO_4)_3$ (NNCP) as a high-voltage NASICON-type cathode for sodium-ion batteries, addressing the challenge of achieving reversible multielectron storage at elevated voltages. The authors synthesize NNCP by a sol-gel route and confirm a rhombohedral NASICON structure with space group $R\bar{3}c$, while a bond-valence energy landscape reveals a connected 3D Na$^+$ diffusion network with a migration barrier of $0.468$ eV. Electrochemical testing shows Cr$^{3+}$/Cr$^{4+}$ activity around $4.5$ V but negligible Na intercalation during discharge, and carbon coating or argon-sintering only marginally improves capacity; cycling modifies the Na-site energetics to a barrier of $0.324$ eV but reversibility remains poor due to limited electronic conductivity and hole-polaron dynamics. The study highlights the need for targeted doping, structural modifications, and electrolyte optimization to realize practical, high-energy-density SIB cathodes based on Ni–Cr NASICONs.

Abstract

We examine a new NASICON-type Na$_4$NiCr(PO$_4$)$_3$ material designed for high-voltage and multi-electron reactions for the sodium-ion batteries (SIBs). The Rietveld refinement of the X-ray diffraction pattern, using the R$\bar{3}$c space group, confirmed the stabilization of the rhombohedral NASICON framework. Furthermore, the Raman and Fourier transform infrared spectroscopy are employed to probe the structure and chemical bonding. The core-level photoemission analysis reveals the Cr$^{3+}$ and mixed Ni$^{2+}$/Ni$^{3+}$ oxidation states in the sample. Moreover, the bond valence energy landscape (BVEL) analysis, based on the refined structure, revealed a three-dimensional network of well-connected sodium sites with a migration energy barrier of 0.468 eV. The material delivered a good charge capacity at around 4.5 V, but showed no sodium-ion intercalation during discharge, resulting in negligible discharge capacity. The post-mortem analysis confirmed that the crystal structure remained intact. The calculated energy barrier values indicated a reversal in sodium site stability after cycling, though the barriers can still permit feasible ion migration. This suggests that ion transport alone cannot explain the lack of reversibility, which likely arises from intrinsically poor electronic conductivity. These findings highlight key challenges in achieving stable, reversible capacity in this system and underscore the need for doping, structural modification, and electrolyte optimization to realize its full potential as a high-voltage SIB cathode.

Figures (5)

• Figure 1: Rietveld-refined XRD patterns of (a) NNCP-Air, (b) NNCP-Ar, and (c) NNCP-Air/AB samples, and (d) crystal structure of NNCP-Air showing calculated iso-surfaces (in grey colour) for 3D Na$^+$ migration channels.
• Figure 2: (a) The Raman, (b) FTIR spectra, (c1) the FE-SEM image, (c2-c6) the EDS elemental mappings, and (d) the DC polarization curve at a voltage of 1 V of the NNCP-Air sample.
• Figure 3: The core level photoemission spectra of (a) Na 1$s$, (b) O 1$s$ ,(c) P 2$p$, (d) Ni 2$p$, and (e) Cr 2$p$.
• Figure 4: (a1-a3) The EIS spectra with the equivalent circuit shown in the inset of (a3), (b1-b3) the cyclic voltammetry curves recorded at 0.05 mV/s, and (c1-c3) the galvanostatic charge-discharge profiles measured at 0.1 C for NNCP-Air, NNCP-Ar, and NNCP-Air/AB composite samples, respectively.
• Figure 5: The XRD pattern of (a) NNCP-Ar pristine sample and (b) NNCP-Ar electrode after cycling. A reaction pathway showing the 3D pathway network, in the (c) NNCP-Ar fresh sample and (d) NNCP cycled cathode, based on hopping between Na1 and Na2 sites.