ROYAL SOC CHEMISTRYActa Medica Okayama2050-74884342016Combination of solid state NMR and DFT calculation to elucidate the state of sodium in hard carbon electrodes 1318313193 ENRyoheiMoritaGraduate School of Natural Science & Technology, Okayama UniversityKazumaGotohGraduate School of Natural Science & Technology, Okayama UniversityMikaFukunishiElements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto UniversityKeiKubotaElements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto UniversityShinichiKomabaElements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto UniversityNaotoNishimuraDepartment of Chemistry and Materials Technology, Kyoto Institute of TechnologyTakashiYumuraDepartment of Chemistry and Materials Technology, Kyoto Institute of TechnologyKenzoDeguchiNational Institute for Materials ScienceShinobuOhkiNational Institute for Materials ScienceTadashiShimizueNational Institute for Materials ScienceHiroyukiIshidaGraduate School of Natural Science & Technology, Okayama UniversityWe 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.No potential conflict of interest relevant to this article was reported.Acta Medica Okayama0378-77532252013NMR study for electrochemically inserted Na in hard carbon electrode of sodium ion battery137140ENKazumaGotohToruIshikawaSaoriShimadzuNaoakiYabuuchiShinichiKomabaKazuyukiTakedaAtsushiGotoKenzoDeguchiShinobuOhkiKenjiroHashiTadashiShimizuHiroyukiIshidaThe state of sodium inserted in the hard carbon electrode of a sodium ion battery having practical cyclability was investigated using solid state 23Na NMR. The spectra of carbon samples charged (reduced) above 50 mAh g|1 showed clear three components. Two peaks at 9.9 ppm and 5.2 ppm were ascribed to reversible sodium stored between disordered graphene sheets in hard carbon because the shift of the peaks was invariable with changing strength of external magnetic field. One broad signal at about |9 to |16 ppm was assigned to sodium in heterogeneously distributed closed nanopores in hard carbon. Low temperature 23Na static and magic angle spinning NMR spectra didn't split or shift whereas the spectral pattern of 7Li NMR for lithium-inserted hard carbon changes depending on the temperature. This strongly suggests that the exchange of sodium atoms between different sites in hard carbon is slow. These studies show that sodium doesn't form quasi-metallic clusters in closed nanopores of hard carbon although sodium assembles at nanopores while the cell is electrochemically charged.No potential conflict of interest relevant to this article was reported.