Tailoring Mesopores and Nitrogen Groups of Carbon Nanofibers for Polysulfide Entrapment in Lithium-Sulfur Batteries

Abstract

In the current work, we combined different physical and chemical modifications of carbon nanofibers through the creation of micro-, meso-, and macro-pores as well as the incorporation of nitrogen groups in cyclic polyacrylonitrile (CPAN) using gas-assisted electrospinning and air-controlled electrospray processes. We incorporated them into electrode and interlayer in Li-Sulfur batteries. First, we controlled pore size and distributions in mesoporous carbon fibers (mpCNF) via adding polymethyl methacrylate as a sacrificial polymer to the polyacrylonitrile carbon precursor, followed by varying activation conditions. Secondly, nitrogen groups were introduced via cyclization of PAN on mesoporous carbon nanofibers (mpCPAN). We compared the synergistic effects of all these features in cathode substrate and interlayer on the performance Li-Sulfur batteries and used various characterization tools to understand them. Our results revealed that coating CPAN on both mesoporous carbon cathode and interlayer greatly enhanced the rate capability and capacity retention, leading to the capacity of 1000 mAh/g at 2 C and 1200 mAh/g at 0.5 C with the capability retention of 88% after 100 cycles. The presence of nitrogen groups and mesopores in both cathodes and interlayers resulted in more effective polysulfide confinement and also show more promise for higher loading systems.

Keywords: Lithium–Sulfur batteries; air-controlled electrospray; gas assisted electrospinning; mesoporous carbon nanofiber; nitrogen doping.

Figure 1

The schematic and photo images of multi-porous carbon nanofibers via ceramic plate load control.

Figure 2

SEM surface images of carbon nanofibers obtained with different ceramic pressure load at 59:41 weight ratio of PAN/PMMA: (a) 0 N, (b) 0.62 N, (c) 1.12 N, (d) 1.28 N.

Figure 3

The cross-section SEM images of carbon nanofibers obtained with different ceramic pressure load at 59:41 weight ratio of PAN/PMMA: (a) 0 N, (b) 0.62 N, (c) 1.12 N, (d) 1.28 N.

Figure 4

The cross-section SEM images of carbon nanofibers obtained with different weight ratio at ceramic pressure load of 1.28 N: (a) PAN/PMMA (56:44), (b) PAN/PMMA (77:23), (c) PAN/PMMA (80:20).

Figure 5

BJH analysis of nitrogen physisorption showing the pore volume distribution with ceramic pressure load: (a) 0 N; (b) 0.62 N; (c) 1.12 N; (d) 1.28 N.

Figure 6

BJH analysis of nitrogen physisorption showing the pore volume distribution with PAN/PMMA weight ratios: (a) 56:44; (b) 77:23; (c) 80:20.

Figure 7

(a) XPS survey scanning spectra for mpCNF and CPAN; (b) XPS survey scanning spectra for mpCPAN and CPAN; (c) High Resolution N 1s spectra of CPAN; (d) High Resolution N 1s spectra of mpCPAN.

Figure 8

Experimental FTIR spectra of mpCNF and mpCPAN.

Figure 9

SEM images of (a) mpCNF and (b) mpCPAN.

Figure 10

Comparison of pore-size distribution of mpCNF and CPAN assessed by the BET method.

Figure 11

(a) Pore-size distribution of CPAN and mpCPAN obtained by the BET surface area analysis; (b) Nitrogen adsorption isotherm for CPAN and mpCPAN.

Figure 12

(a) Cycling performance of four systems at 1.1 mg/cm² S-loading, 0.5 C; (b) Rate capability test comparison of systems using mpCPAN as interlayer/cathodes at 1.1 mg/cm² S-loading.

Figure 13

(a) EIS Spectra of fresh cells; (b) EIS Spectra of discharged cells after 100 cycles.

Figure 14

(a) High Resolution S 2p spectra of mpCPAN interlayer; (b) High Resolution N 1s spectra of mpCPAN interlayer; (c) High Resolution S 2p spectra of mpCPAN cathode; (d) High Resolution N 1s spectra of mpCPAN cathode.