Simple preparation of carbon nanofibers with graphene layers perpendicular to the length direction and the excellent li-ion storage performance

X-Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances.

Facile Synthesis of Highly Graphitized Carbon via Reaction of CaC2 with Sulfur and Its Application for Lithium/Sodium-Ion Batteries

In the present work, we report, for the first time, a novel one-step approach to prepare highly graphitized carbon (HGC) material by selectively etching calcium from calcium carbide (CaC₂) using a sulfur-based thermo-chemical etching technique. Comprehensive analysis using X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and N₂ adsorption-desorption isotherms reveals a highly graphitized mesoporous structure for the CaC₂-derived carbon with a specific surface area of 159.5 m² g^-1. Microscopic analysis displays micron-scale mesoporous frameworks (4-20 μm) with a distinct layered structure along with agglomerates of highly graphitized nanosheets (about 10 nm in thickness and 1-10 μm lateral size). The as-prepared HGC is investigated for the role of an anode material for lithium- and sodium-ion batteries. We found that HGC exhibits good lithium storage performance in the 0.01-1.5 V range (reversible capacity of 272.4 mA h g^-1 at 50 mA g^-1 after 100 cycles and 214.2 mA h g^-1 at 500 mA g^-1 after 500 cycles), whereas, when sodium is considered, we observed a drop in the overall electrochemical performance owing to the high graphitization degree. More importantly, the present study provides a perspective approach to fabricate HGC via a simple, cost-effective, and efficient synthetic route using CaC₂ and sulfur as reactants.

Doping-template approach of porous-walled graphitic nanocages for superior performance anodes of lithium ion batteries

Removal of the N-doped template creates nanopores in the shells of nanocages. The created nanopores enhance fast ion diffusion.

Thin-walled graphitic nanocages with nitrogen-doping as superior performance anodes for lithium-ion batteries

N-Doped nanocages were prepared with thin-walled graphitic shells (∼1.2 nm), which enhanced electrochemical performance of the nanocages.

Low-temperature reduction–pyrolysis–catalysis synthesis of carbon nanospheres for lithium-ion batteries

Carbon nanospheres synthesized using a reduction–pyrolysis–catalysis process at low temperature (160 °C) show a high specific capacity of 719 mA h g⁻¹ at a rate of 0.2 A g⁻¹ (98.5% charge capacity retention) after 100 charge–discharge cycles.