Facile Synthesis of High Electrochemical Performance Na₂FePO₄F@CNT&GN Cathode Material as Sodium Ion Batteries

Abstract

Here, we report a novel material structure for sodium ion battery cathodes. The material has an enormously enhanced conductive framework of Na2FePO4F (NFPF), combining both 1D carbon nanotube (CNT) and 2D graphene nanosheets (GNs). Uniform Na2FePO4F nanorods were synthesized first via a one-step hydrothermal method. Then, the highly efficient conductive network of NFPF@CNT&GN was synthesized with a straightforward mechanical ball milling technique. The obtained NFPF@CNT&GN shows a reversible capacity of 67.5 mA h g–1 at 10C rate, which exceeds all of the published results for NFPF to date. Remarkably, the degradation rate is only 0.02% per cycle even after 2500 cycles at the 5C rate. The electrochemical impedance spectroscopy kinetic study demonstrates that the superior electrochemical properties for NFPF@CNT&GN should be attributed to the great improvement of the electronic conductivity and sodium-ion diffusion ability.

Scalable Synthesis of the Na2FePO4F Cathode Through an Economical and Reliable Approach for Sodium-Ion Batteries

“…A detailed calculation of a single titration step can be seen in Figure S6. The results show that the sodium diffusion coefficients D Na + of the NFPF/C/rGO electrodes vary from 10 –8 to 10 –11 cm 2 s –1 , which is at the same level of Na 2 FePO 4 F materials synthesized by other methods, ,, further confirming the reliability of the synthesis approach. The high sodium diffusion coefficients also explain the high specific capacity, flat platforms, good rate performance, and fast-charging properties of NFPF/C/rGO.…”

Monoclinic α-Na2FePO4F with Strong Antisite Disorder and Enhanced Na+ Diffusion

“…It is worth mentioning that the Na 2 FePO 4 F materials prepared by the soft chemistry approach without annealing at high temperature were characterized by PXRD before. Albeit the PXRD pattern of Na 2 FePO 4 F nanorods 32 was interpreted in the orthorhombic symmetry, it looks quite similar to that of monoclinic α-Na 2 FePO 4 F (Figure 3a), but due to the absence of the Rietveld or Le Bail profile fit, no information on the unit cell parameters is available. As well, the PXRD pattern of Na 2 FePO 4 F nanoparticles 33 prepared by polyol synthesis at 335 °C resembles that of α-Na 2 FePO 4 F, though the extreme peak broadening precludes retrieving reliable crystallographic information.…”

Iron‐Phosphate‐Based Cathode Materials for Cost‐Effective Sodium‐Ion Batteries: Development, Challenges, and Prospects

“…d) The SEM image of Na 2 FePO 4 F nanorods and e) corresponding long‐term electrochemical performance at 5 C. Reproduced with permission. [ 82 ] Copyright 2020, American Chemical Society. f) The digital photo of the soft pack battery of Na 2 FePO 4 F. Reproduced under the terms of the CC‐BY 4.0 license.…”

“…Xu et al. [ 82 ] constituted conductive network that consists of 1D CNTs and 2D graphene nanosheets (Figure 10d) to obtain excellent electrochemical properties. The cathode compounded with the conductive network exhibits an incredible discharge capacity of 67.5 mA h g –1 at 10 C and the capacity decay per cycle is only 0.02% after 2500 cycles of 5 C (Figure 10e).…”

“…[80] Due to the electron-conducting network with enhanced electron transportation, the cathode can provide high capacity of 77.8 mA h g -1 after 400 cycles at 0.4 C. Different carbon materials can be combined to form conductive network to further improve the conductivity of the material. Xu et al [82] Figure 10. a) The crystal structure diagram of Na 2 FePO 4 F along the [1 0 0] direction.…”