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

► 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.