Combination of solid state NMR and DFT calculation to elucidate the state of sodium in hard carbon electrodes

We examined the state of sodium electrochemically inserted in HC prepared at 700-2000 °C using solid state Na magic angle spinning (MAS) NMR and multiple quantum (MQ) MAS NMR. The ²³Na MAS NMR spectra of Na-inserted HC samples showed signals only in the range between +30 and -60 ppm. Each observed spectrum was ascribed to combinations of Na⁺ ions from the electrolyte, reversible ionic Na components, irreversible Na components assigned to solid electrolyte interphase (SEI) or non-extractable sodium ions in HC, and decomposed Na compounds such as Na₂CO₃. No quasi-metallic sodium component was observed to be dissimilar to the case of Li inserted in HC. MQMAS NMR implies that heat treatment of HC higher than 1600 °C decreases defect sites in the carbon structure. To elucidate the difference in cluster formation between Na and Li in HC, the condensation mechanism and stability of Na and Li atoms on a carbon layer were also studied using DFT calculation. Na₃ triangle clusters standing perpendicular to the carbon surface were obtained as a stable structure of Na, whereas Li₂ linear and Li₄ square clusters, all with Li atoms being attached directly to the surface, were estimated by optimization. Models of Na and Li storage in HC, based on the calculated cluster structures were proposed, which elucidate why the adequate heat treatment temperature of HC for high-capacity sodium storage is higher than the temperature for lithium storage.