NMR study for electrochemically inserted Na in hard carbon electrode of sodium ion battery

• Published in: Journal of power sources 2013-03, Vol.225, p.137-140
• Main Authors: Gotoh, Kazuma, Ishikawa, Toru, Shimadzu, Saori, Yabuuchi, Naoaki, Komaba, Shinichi, Takeda, Kazuyuki, Goto, Atsushi, Deguchi, Kenzo, Ohki, Shinobu, Hashi, Kenjiro, Shimizu, Tadashi, Ishida, Hiroyuki
• Format: Article
• Language: English
• Subjects: 23Na; Anode; Applied sciences; Carbon; Charging; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Electrodes; Exact sciences and technology; Graphene; Hard carbon; Nuclear magnetic resonance; Porosity; Sodium; Sodium ion battery; Solid state NMR; Spectra
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2012.10.025

The 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. ► A hard carbon electrode in a sodium ion battery was investigated using 23Na NMR. ► Two peaks at 10 ppm and 5 ppm and a broad signal of Na in carbon were observed at room temperature. ► The signals were assigned to Na stored between disordered graphene sheets and in closed nanopores. ► The sodium doesn't have quasi-metallic property in hard carbon.