Correlation of carbonization condition with metallic property of sodium clusters formed in hard carbon studied using ²³ Na nuclear magnetic resonance

The metallic property of sodium clusters stored in hard carbon (HC) was investigated by ²³ Na NMR using various HC samples prepared over a range of dehydration and carbonization temperatures. The fully sodiated sample prepared at higher carbonization temperature shows a higher ²³ Na NMR shift associated with Na clusters. Na in HC prepared at 2000 °C shows almost the same shift as sodium metal. A higher dehydration temperature of the precursor is favorable to form Na clusters with the higher shift, which implies that the HC pore structure is influenced by not only carbonization temperature but also the heat treatment process.