Facile synthesis of hollow stalagmite-like N, S-doped C and its capacity attenuation mechanism as anodes in K-ion batteries

A new hollow stalagmite-like N, S-doped carbon anode for K-ion batteries is constructed through the polymerization of tannic acid at room temperature and subsequent pyrolysis, and its capacity attenuation mechanism is clarified. The electrochemical measurements show that the higher the current density used, the better the cycling stability and coulombic efficiency of the anode. At 0.1 A g −1 , it presents a second discharge capacity of 362 mAh g −1 with a capacity decay rate of 0.5% per cycle. However, at 1 A g −1 , a second discharge capacity of about 256 mAh g −1 can be achieved and the capacity decay rate is only 0.055% per cycle in the tested 1000 cycles. The investigation of the cycled electrodes reveals that the capacity decay is mainly caused by the failure of K + in the areas with poor electrical contact to be completely extracted in time rather than the structural damage of the material itself, resulting in continuous depletion of K + storage sites. Moreover, a larger current density is helpful to the extraction of K ions, thus improving the cycle stability and coulombic efficiency of the electrode. This work provides a new insight into the electrochemical performance of hard carbon as anodes in K-ion batteries. The higher the current density, the better the cycling stability and coulombic efficiency. The capacity decay came from the failure of K + in the areas with poor electrical contact to be completely extracted in time. • Fabricate a new hollow stalagmite-like N, S-doped C anode for K-ion batteries. • Find the capacity decay of hard carbon came from incomplete extraction of K ions. • Reveal the reason for a poor performance of hard carbon at a low current density.