Meaning
High-temperature sintering of non-graphitizable carbon creates isolated internal voids that are inaccessible to gas molecules. Closed pore clustering occurs during the final heat treatment of hard carbon anodes. These isolated pores act as storage sites for sodium ions, providing high reversible capacity without exposing the metal to the electrolyte.
Microstructural Generation
Adjusting the heating rate of the carbon precursor between one thousand and thirteen hundred degrees Celsius controls the formation of internal cavities. Through closed pore clustering, the anode material achieves a density that balances sodium storage and structural stability. This microstructural arrangement prevents the volume expansion that typically degrades alloy-based anodes.
Capacity Contribution
Electrochemical insertion of sodium into these internal voids occurs at a low potential close to zero volts. When closed pore clustering is optimized, the anode exhibits a long and flat voltage plateau. This characteristic is highly desirable for maximizing the operating voltage of the full cell.
Measurement Technique
Helium pycnometry offers a reliable method to quantify the volume of the internal cavities by comparing the skeletal density to the bulk density. In the analysis of closed pore clustering, standard nitrogen gas adsorption cannot measure these internal voids because the gas molecules cannot enter them. This difference in density measurement confirms the presence of the closed pores.
