Spin-regulated d-p hybridization enables high energy density and wide temperature operation of Na3V2(PO4)2O2F-type cathodes for sodium-ion batteries

Abstract

As a member of the sodium superionic conductor (NASICON) family, Na₃V₂(PO₄)₂O₂F (NVPOF) has attracted tremendous research interest owing to its high operating voltage and excellent structural stability. It is well established that NVPOF cathodes are electron-ion mixed conductors. However, improving electron or ionic conductivity via a single approach fails to effectively enhance the rapid sodium storage capability, which impedes their practical application in sodium-ion batteries. Herein, the electronic and ionic conductivities were enhanced through a transition-metal/fluorine dual-doping method. The introduction of transition metals with different spin states adjusted the spacing between the V 3d _xz -O 2p _x bond and the Fermi level, thereby improving the material's intrinsic conductivity. Meanwhile, the introduction of F atoms effectively optimized the diffusion kinetics of Na⁺. In particular, Na₃(VO)_1.9Fe_0.1(PO₄)₂F_1.1 (NVPOF-Fe) is obtained by dual-doping with high-spin Fe³⁺/F^-. It has considerable specific capacity and power density at 100C (80 mA h g^-1, 245 Wh kg^-1) and retains 90.6% capacity after 2500 cycles at 20C. The assembled NVPOF-Fe‖hard-carbon full cell exhibits excellent capacity and cycling stability at -20 °C, 25 °C, and 45 °C, respectively. This work provides a new paradigm for the development of advanced sodium-ion battery cathodes.

Scheme 1. Schematic diagrams of modification mechanisms for NVPOF–M (M = Sc, V, Cr, and Fe) cathodes.

Fig. 1. Theoretical and structural characterization of NVPOF–M (M = Sc, V, Cr, and Fe) materials. (a) The comparative electronic spin states. (b) Projection density of states for Sc, V, Cr, and Fe elements. (c) XRD and (d) EPR spectra. (e) 2D contour plots of the four-point probe conductivity results of NVPOF–M (M = Sc, V, Cr, and Fe) and NVPOF under a load of 2–10 MPa.

Fig. 2. Structural characterization of NVPOF–M (M = Sc, V, Cr, and Fe) materials. XRD Rietveld refinement patterns of NVPOF–Fe based on (a) P4₂/mnm and (b) I4/mmm crystal structures. (c) FTIR spectra. (d) High-resolution V 2p spectra, (e) high-resolution Sc, Cr, and Fe 2p spectra. (f) SEM image, (g) HRTEM image, (h) the FFT result of (g), and (i) EDS mapping of NVPOF–Fe.

Fig. 3. The electrochemical performance of NVPOF and NVPOF–M (M = Sc, V, Cr, and Fe) cathodes. (a) Capacity comparison between NVPOF and NVPOF–M (M = Sc, V, Cr, and Fe) at 0.5C. (b) Comparison of operating voltage, specific capacity, and energy density of reported SIB half cells. (c) Rate performance. (d) GCD curves of NVPOF–Fe at different rates. The cycling performances at (e) 5C and (g) 20C, respectively. (f) GCD curves of NVPOF–Fe cathode at 5C. (h) The cycling performance of NVPOF–Fe at 1C and −20 °C. (i) The cycling performance and (j) GCD curves of NVPOF–Fe at 2C and 45 °C. (k) The Ragone plots of various advanced cathodes for SIBs.

Fig. 4. (a) Voltage profiles of NVPOF and NVPOF–M (M = Sc, V, Cr, and Fe) cathodes at 5C (insets: the voltage hysteresis of the NVPOF cathode during the charge/discharge process). (b) GITT curves of NVPOF and NVPOF–Fe electrodes and the corresponding D_Na⁺ distributions. (c) The contour maps of CV curves of NVPOF–Fe at different scan rates. (d) Proportions of pseudocapacitive (orange area) and diffusion-controlled (green area) capacity of the NVPOF–Fe cathode at different scanning rates. (e) Distribution of relaxation time (DRT) obtained by deconvolution EIS. (f) The rate performance of NVPOF–Fe‖HC full cell (inset: the schematic diagram of the NVPOF–Fe‖HC full cell). (g) The cycling performance of the NVPOF–Fe‖HC full cell at 5C and room temperature. (h) The cycling performance of the NVPOF–Fe‖HC full cell at 2C and 45 °C. (i) The cycling performance of the NVPOF–Fe‖HC full cell at 1C and −20 °C.

Fig. 5. (a) The 3D contour plot of in situ X-ray diffraction spectra of the (111), (002), (220), (113), and (222) diffraction peaks of the NVPOF–Fe cathode during the charging/discharging process was selected. (b) The changes in lattice parameters of the NVPOF–Fe cathode during the complete charge/discharge process. (c) In situ XRD patterns of the (002) and (222) diffraction peaks of the NVPOF–Fe cathode during the charging/discharging process. (d) FTIR spectra and (e) vanadium K-edge XANES spectra of the NVPOF–Fe cathode at different charge/discharge states. (f) EXAFS spectra of the NVPOF–Fe cathode at different charging states of the vanadium K-edge.

Fig. 6. (a) Electron Localization Function (ELF) calculation results. The density of states of (b) NVPOF and (e) NVPOF–Fe. COHP of (c) the V–O bond in NVPOF and (f) the Fe–O bond in NVPOF–Fe. Na⁺ diffusion energy barriers for (d) P1 and (g) P2 pathways in NVPOF and NVPOF–Fe.