Meaning
Distance between adjacent graphene layers within the carbon microstructure determines the ease of ion transport and the overall capacity for sodium storage in hard carbon anodes. This interlayer spacing d002 is a fundamental structural parameter that differentiates hard carbon from crystalline graphite. It governs the mechanical strain experienced by the lattice during the insertion and extraction of ions.
The measurement is derived from the position of the (002) diffraction peak in an x-ray diffraction pattern.
Lattice Analysis
Bragg’s law is applied to the diffraction data to calculate the precise separation between the carbon planes. This interlayer spacing d002 is typically greater than zero point three seven nanometers in high performance hard carbons. A larger spacing is necessary to accommodate the relatively large ionic radius of sodium compared to lithium.
Graphite possesses a much tighter spacing of zero point three three five nanometers which limits its effectiveness in sodium ion systems. The width of the diffraction peak indicates the degree of crystallinity and the size of the carbon crystallites. Researchers use this information to tune the carbonization process for optimal results.
Capacity Determination
Storage of ions in the sloping region of the voltage profile is directly related to the availability of sites between the expanded layers. This interlayer spacing d002 facilitates the rapid diffusion of ions throughout the electrode material. A wider gap reduces the energy barrier for ion entry and improves the rate capability of the battery.
If the spacing is too narrow, the ions cannot penetrate the lattice and the capacity is significantly reduced. The expansion of the lattice during charging can lead to mechanical stress and eventual pulverization of the carbon particles. Maintaining a stable spacing over thousands of cycles is a primary requirement for long life batteries.
Battery Performance
Optimization of the structural parameters leads to improved energy density and faster charging times for the final cell. This interlayer spacing d002 is influenced by the selection of the precursor and the final carbonization temperature. Higher temperatures tend to shrink the spacing as the carbon layers become more ordered.
A balance must be found between the conductivity of the material and the space available for ion storage. The stability of the d002 value under different environmental conditions is essential for reliable operation. Engineers monitor this metric during the quality control process of material production.
The final performance of the sodium ion battery is a direct result of the atomic scale architecture of the carbon.
