Understanding acid pretreatment of lotus leaves to prepare hard carbons as anodes for sodium ion batteries

Abstract

Lotus leaves are pretreated by exposure to hydrochloric acid (HCl) by several methods, including mild (PHT), hydrothermal (HHT), and mild and hydrothermal (PHHT) treatments, and subsequently pyrolyzed to yield hard carbon materials. The hydrothermal pretreatment produces the most substantial impact on the microstructures, surface compositions, and electrochemical performances of the resulting hard carbons. Materials prepared by the HHT pretreatment exhibit carbon microspheres with the largest graphene interlayer spacing, lowest quantities of oxygen functional groups, and no observable inorganic impurities. Moreover, such materials exhibit initial coulombic efficiencies and discharge capacities of 66% and 250 mA h g−1, respectively, at 0.2 C rate and after 100 cycles, which are significantly higher than those of non-treated hard carbons (53% and 162 mA hg−1). Surprisingly, the HHT pretreated materials show a maximum discharge capacity of 58 mA h g−1 at a current density of 20 C, while the non-treated materials show no detectable discharge capacity. The superior charging rates of the HHT-treated materials originate from surface properties that facilitate rapid charging processes. Specifically, these capabilities arise from a less reactive surface and larger pore volume that reduce the resistance of the solid electrolyte interphase (SEI) and enhance the diffusivity of sodium ions.