Design of Na3MnZr(PO4)3/Carbon Nanofiber Free-Standing Cathodes for Sodium-Ion Batteries with Enhanced Electrochemical Performances through Different Electrospinning Approaches

Abstract

The NASICON-structured Na₃MnZr(PO₄)₃ compound is a promising high-voltage cathode material for sodium-ion batteries (SIBs). In this study, an easy and scalable electrospinning approach was used to synthesize self-standing cathodes based on Na₃MnZr(PO₄)₃ loaded into carbon nanofibers (CNFs). Different strategies were applied to load the active material. All the employed characterization techniques (X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), thermal gravimetric analysis (TGA), and Raman spectroscopy) confirmed the successful loading. Compared to an appositely prepared tape-cast electrode, Na₃MnZr(PO₄)₃/CNF self-standing cathodes demonstrated an enhanced specific capacity, especially at high C-rates, thanks to the porous conducive carbon nanofiber matrix. Among the strategies applied to load Na₃MnZr(PO₄)₃ into the CNFs, the electrospinning (vertical setting) of the polymeric solution containing pre-synthesized Na₃MnZr(PO₄)₃ powders resulted effective in obtaining the quantitative loading of the active material and a homogeneous distribution through the sheet thickness. Notably, Na₃MnZr(PO₄)₃ aggregates connected to the CNFs, covered their surface, and were also embedded, as demonstrated by TEM and EDS. Compared to the self-standing cathodes prepared with the horizontal setting or dip-drop coating methods, the vertical binder-free electrode exhibited the highest capacity values of 78.2, 55.7, 38.8, 22.2, 16.2, 12.8, 10.3, 9.0, and 8.5 mAh/g at C-rates of 0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C, and 20C, respectively, with complete capacity retention at the end of the measurements. It also exhibited a good cycling life, compared to its tape-cast counterpart: it displayed higher capacity retention at 0.2C and 1C, and, after cycling 1000 cycles at 1C, it could be further cycled at 5C, 10C, and 20C.

Keywords: Na3MnZr(PO4)3; carbon nanofibers; cathode; electrospinning; free-standing; sodium-ion batteries.

Figure 1

A scheme of the samples’ synthesis.

Figure 2

XRPD patterns of CNFs, pristine Na₃MnZr(PO₄)₃ powder, and self-standing cathodes.

Figure 3

Raman spectra of p-MnZr, h-10%MnZr/CNF, h-30%MnZr/CNF, v-30%MnZr/CNF, and dd-MnZr/CNF samples.

Figure 4

SEM and TEM images of Na₃MnZr(PO₄)₃ powder: (a) 20.42 kX and (b) 100 kX.

Figure 5

SEM images of h-10%MnZr/CNF (a,b) surface and (c) cross-section; h-30%MnZr/CNF (d,e) surface and (f) cross-section; v-30%MnZr/CNF (g,h) surface and (i) cross-section; and dd-MnZr/CNF (j,k) surface and (l) cross-section.

Figure 6

TEM images at different magnifications (20 kX e 50 kX) of (a–c) h-10%MnZr/CNF, (d–f) h-30%MnZr/CNF, (g–i) v-30%MnZr/CNF, and (j–l) dd-MnZr/CNF samples.

Figure 7

SEM image and EDS maps of the different elements for the h-10%MnZr/CNF (a–e) surface and (f–j) its cross-section.

Figure 8

SEM image and EDS maps of the different elements for the h-30%MnZr/CNF (a–e) surface and (f–j) its cross-section. The yellow frame indicates the mapped portion.

Figure 9

SEM image and EDS maps of the different elements for the v-30%MnZr/CNF (a–e) surface and (f–j) its cross-section. The yellow frame indicates the mapped portion.

Figure 10

SEM image and EDS maps of the different elements for the dd-MnZr/CNF (a–e) surface and (f–j) its cross-section. The yellow frame indicates the mapped portion.

Figure 11

TGA analysis of p-MnZr (black), h-10%MnZr/CNF (blue), h-30%MnZr/CNF (red) v-30%MnZr/CNF (green), and dd-MnZr/CNF (purple) samples.

Figure 12

Charge/discharge cycles at different C-rates of (a) p-MnZr, (b) h-10%MnZr/CNF, (c) h-30%MnZr/CNF, (d) v-30%MnZr/CNF, and (e) dd-MnZr/CNF; (f) comparison of average discharge capacity values for all the samples at different C-rates.

Figure 13

Cycling performance of (a) p-MnZr slurry electrode and (b) v-30%MnZr/CNF self-standing cathode.