Soft-Carbon-Coated, Free-Standing, Low-Defect, Hard-Carbon Anode To Achieve a 94% Initial Coulombic Efficiency for Sodium-Ion Batteries

Developing hard carbon with a high initial Coulombic efficiency (ICE) and very good cycling stability is of great importance for practical sodium-ion batteries (SIBs). Defects and oxygen-containing groups grown along either the carbon edges or the layers, however, are inevitable in hard carbon and can cause a tremendous density of irreversible Na+ sites, decreasing the efficiency and therefore causing failure of the battery. Thus, eliminating these unexpected defect structures is significant for enhancing the battery performance. Herein, we develop a strategy of applying a soft-carbon coating onto free-standing hard-carbon electrodes, which greatly hinders the formation of defects and oxygen-containing groups on hard carbon. The electrochemical results show that the soft-carbon-coated, free-standing hard-carbon electrodes can achieve an ultrahigh ICE of 94.1% and long cycling performance (99% capacity retention after 100 cycles at a current density of 20 mA g–1), demonstrating their great potential in practical sodium storage systems. The sodium storage mechanism was also investigated by operando Raman spectroscopy. Our sodium storage mechanism extends the "adsorption–intercalation–pore filling–deposition" model. We propose that the pore filling in the plateau area might be divided into two parts: (1) sodium could fill in the pores near the inner wall of the carbon layer; (2) when the sodium in the inner wall pores is close to saturation, the sodium could be further deposited onto the existing sodium.