Tailoring the surface chemistry of hard carbon towards high-efficiency sodium ion storage

Hard carbon (HC) is most likely to be a commercialized anode material for sodium-ion batteries (SIBs). However, its low initial coulombic efficiency (ICE) impedes its further large-scale industrialization. Since the ICE is greatly related to the side reactions of the electrolyte on the HC surface, herein, we focus on tailoring the surface chemistry of HC via a facile low-temperature oxygen plasma (LTOP) treatment technique. The modified HC after a suitable treatment time possesses a highly ordered and low defect surface without a negligible change in layer spacing, thus facilitating Na+ deinsertion/insertion and reducing the HC/electrolyte side reactions. Moreover, LTOP treatment also brings oxygen functional groups (CO) to the HC surface to enrich Na+ storage active sites. Consequently, the modified HC reveals a higher ICE of 80.9% compared to 60.6% in the bare HC. Also, the modified HC delivers an ultrahigh specific capacity of 331.0 mA h g-1 at 0.1 A g-1 and exhibits superior rate performance with a high specific capacity of 211.0 mA h g-1 at 5 A g-1. This work provides a feasible strategy to tailor the surface chemistry of HC for high-efficiency Na-storage and provides a novel avenue to construct high-efficiency SIBs.