MOF-derived crystalline carbon with graphite-like crystal: A high initial coulombic efficiency, low potential and large capacity anode for Na-ion batteries

Carbonaceous materials, such as hard carbon and porous carbon, have gained great attentions as the anode materials for Na-ion batteries (NIBs). However, their defects and disordered structure cause the low initial coulombic efficiency (ICE), high-potential slope region and small charge capacity below 0.5 V. Herein, a structurally stable MOF contained 1,3,5-triazine-2,4,6-triamine moieties, belonging to the tetragonal space group I 4/m, is synthesized. On this basis, MOF-derived crystalline carbon with graphite-like crystal, instead of amorphous porous carbon, can be obtained due to slow pyrolysis of MOF framework along with timely and sufficient reassembly of the pyrolyzed carbon atoms into graphite-like crystal under graphite-induced condition. The graphite-like crystal has a geometrically similar unit cell to graphite crystal but with a magnification ratio of 1.25, yielding proportionally expanded d-spacing of (002) plane (4.2 Å), which enables the reversible Na+ intercalation/de-intercalation. When applied in NIBs, it exhibits a very high ICE of 96.0% and reversible charge capacity of 356.2 mAh g−1, reaching the advanced level of graphite applied in LIBs. More importantly, its charge capacity below 0.5 V can account for 90% of the total charge capacity, which is also the key parameter to effectively increase the energy density of full batteries.