AFM Publication: CIQTEK SEM Aids in Hard Carbon Morphology Study

Sodium-ion batteries (SIBs) are attracting attention as a cost-effective alternative to lithium-ion batteries, thanks to the abundant sodium content in Earth’s crust (2.6% vs. 0.0065% for lithium). Despite this, SIBs still lag in energy density, highlighting the need for high-capacity electrode materials. Hard carbon is a strong candidate for SIB anodes due to its low sodium storage potential and high capacity. However, factors like graphite microdomain distribution, closed pores, and defect concentration significantly impact initial Coulombic efficiency (ICE) and stability. Modification strategies face limits. Heteroatom doping can raise capacity but reduce ICE. Traditional CVD helps form closed pores but suffers from slow methane decomposition, long cycles, and defect buildup.

Professor Yan Yu’s team at the University of Science and Technology of China (USTC) utilized the CIQTEK Scanning Electron Microscope (SEM) to investigate the morphology of various hard carbon materials. The team developed a catalyst-assisted chemical vapor deposition (CVD) method to promote CH₄ decomposition and regulate the microstructure of hard carbon. Transition metal catalysts such as Fe, Co, and Ni effectively lowered the energy barrier for CH₄ decomposition, thereby improving efficiency and reducing deposition time.

However, Co and Ni tended to cause excessive graphitization of the deposited carbon, forming elongated graphite-like structures in both lateral and thickness directions, which hindered sodium-ion storage and transport. In contrast, Fe facilitated appropriate carbon rearrangement, resulting in an optimized microstructure with fewer defects and well-developed graphite domains. This optimization reduced irreversible sodium storage, enhanced initial Coulombic efficiency (ICE), and increased the availability of reversible Na⁺ storage sites.

As a result, the optimized hard carbon sample (HC-2) achieved an impressive reversible capacity of 457 mAh g⁻¹ and a high ICE of 90.6%. Moreover, in-situ X-ray diffraction (XRD) and in-situ Raman spectroscopy confirmed a sodium storage mechanism based on adsorption, intercalation, and pore filling. The study was published in Advanced Functional Materials under the title:
Catalyst-Assisted Chemical Vapor Deposition Engineering of Hard Carbon with Abundant Closed Pores for High-Performance Sodium-Ion Batteries.

As illustrated in Figure 1a, the hard carbon was synthesized via a catalyst-assisted chemical vapor deposition (CVD) method using commercial porous carbon as the precursor and methane (CH₄) as the feed gas. Figure 1d shows the adsorption energies of CH₄ and its dehydrogenated intermediates on metal catalysts (Fe, Co, Ni) and porous carbon surfaces, indicating that the introduction of metal catalysts lowers the energy barrier for CH₄ decomposition, with Fe being the most effective in promoting the breakdown of CH₄ and its intermediates.

High-resolution TEM (HRTEM) images under different catalyst conditions (Figures 1e–h) reveal that:

• Without a catalyst, the hard carbon exhibits a highly disordered structure rich in defects.

• With Fe as the catalyst, the resulting hard carbon features short-range ordered graphite-like microcrystals and closed pores embedded between graphite domains.

• Co promotes the expansion of graphite domains and increases the number of graphite layers.

• Ni leads to a graphitic structure and even the formation of carbon nanotubes, which, despite their high order, are unfavorable for sodium-ion storage and transport.

Figure 2 presents the structural characterization results of hard carbon materials prepared with varying concentrations of FeCl₃. The XRD patterns (Figure 2a) and Raman spectra (Figure 2b) indicate that as the FeCl₃ concentration in the impregnation solution increases, the graphite interlayer spacing gradually decreases (from 0.386 nm to 0.370 nm), the defect ratio (ID/IG) decreases, and the lateral crystallite size (La) increases. These changes confirm that Fe catalyzes the rearrangement of carbon atoms, enhancing the degree of graphitization.

X-ray photoelectron spectroscopy (XPS) results (Figures 2c and 2e) show that with increasing Fe catalyst concentration, the proportion of sp²-hybridized carbon in hard carbon increases, further indicating improved graphitization. At the same time, the oxygen content in the hard carbon decreases, which may be attributed to hydrogen (H₂) generated from CH₄ decomposition consuming oxygen during carbonization, thereby reducing surface oxygen-related defects.

Small-angle X-ray scattering (SAXS) analysis (Figure 2f) reveals average closed-pore diameters of 0.76, 0.83, 0.90, 0.79, and 0.78 nm, respectively. Larger closed pores are beneficial for stabilizing sodium clusters and improving Na⁺ transport kinetics.

HRTEM images (Figures 2g–i) show small graphite domains at low Fe loading, while excessive catalyst loading leads to long-range ordered structures with narrower interlayer spacing, which can hinder Na⁺ transport.

Figure 3 shows the effect of different Fe catalyst loadings on the electrochemical performance of hard carbon materials. Galvanostatic charge–discharge tests (Figure 3a) reveal that as the concentration of FeCl₃ in the impregnation solution increases, HC-2 (0.02 M FeCl₃) exhibits the best performance, with a reversible capacity of 457 mAh g⁻¹ and a high initial Coulombic efficiency (ICE) of 90.6%. The low-voltage plateau accounts for a significant portion of the capacity (around 350 mAh g⁻¹), indicating the advantage of closed pores in sodium storage.

Excessive catalyst loading (e.g., HC-4) leads to a decrease in capacity (377 mAh g⁻¹) due to the over-ordering of carbon layers, highlighting the need to balance graphite domain growth and sodium-ion transport pathways. After 100 cycles at a current density of 0.5 A g⁻¹, the capacity remains at 388 mAh g⁻¹, demonstrating that larger closed pores enhance the stability of Na clusters and improve Na⁺ transport kinetics.

Figure 4 shows the SEI structure on different hard carbon surfaces: (a) and (b) depict the depth profiles and distributions of NaF⁻, P, and CH₂ species in opt-HC and HC-2, respectively. (c) and (d) present TEM images of opt-HC and HC-2 after 10 cycles at 30 mA g⁻¹. (e) and (f) display the XPS spectra of opt-HC and HC-2 after 10 cycles at 30 mA g⁻¹. (g) shows the HRTEM image of HC-2 after 10 cycles at 30 mA g⁻¹. EPMA mapping images of the electrode cross-sections for (h) opt-HC and (i) HC-2 are shown after the first cycle.

As shown in Figure 5, the GITT curves (Figure 5a) reveal that the Na⁺ diffusion coefficient (DNa⁺) of HC-2 is higher than that of opt-HC, indicating that HC-2 exhibits faster kinetics and enables quicker Na⁺ diffusion.

The in situ Raman spectra (Figure 5b) show that during discharge from open-circuit voltage to approximately 0.7 V, the D-band gradually broadens while the G-band remains relatively unchanged, suggesting that sodium storage at this stage is dominated by surface adsorption. As discharge proceeds further, the D-band intensity weakens and the G-band redshifts, indicating that Na⁺ begins to intercalate into graphene layers. After reaching the plateau near 0.05 V, the G-band stabilizes, implying that Na⁺ fills into the closed pores.

In the in situ XRD patterns (Figure 5c), the (002) peak intensity of HC-2 significantly decreases at lower angles during discharge, confirming Na⁺ intercalation between graphene layers. Compared to opt-HC, the (002) peak shift in HC-2 is more pronounced, indicating a greater extent of Na⁺ intercalation into the carbon layers, contributing to its higher capacity.

Together, Figures 5b and 5c illustrate that the sodium storage mechanism involves: (1) Na⁺ adsorption, (2) Na⁺ interlayer adsorption/intercalation, and (3) Na⁺ pore filling and clustering.

Figure 6 illustrates the electrochemical performance of a full cell assembled using the HC-2 anode and an O3-type NaNi₁/₃Fe₁/₃Mn₁/₃O₂ cathode. The cell demonstrates excellent rate capability and long-term cycling stability under various current densities, confirming the potential of the HC-2 anode for practical battery applications.

Professor Yu Yan’s team proposed a novel catalyst-assisted chemical vapor deposition (CA-CVD) method that enables the precise synthesis of hard carbon anodes featuring abundant closed pores, well-developed graphitic domains, and controllable defects. The optimized HC-2 anode exhibits a high reversible capacity of 457 mAh g⁻¹ and an impressive initial Coulombic efficiency of 90.6%. When paired with an O3-type layered cathode in a soft-packed full cell, the battery retains 83% of its capacity after 100 cycles, maintaining a reversible capacity above 400 mAh g⁻¹.

This method not only offers a new route for the controlled fabrication of high-capacity and high-efficiency hard carbon anodes but also provides mechanistic insights into sodium storage behavior, supporting further optimization of material systems. It holds significant promise for advancing high-energy-density sodium-ion battery (SIB) technologies toward practical applications.