Mechanochemistry assisted surface chemical modification on hard carbon for sodium storage: Size effect

• The size effect for hard carbon was studied during mechanochemical process. • The size effect results in varying degrees of directional surface oxygen functionalization. • The electrochemical performance improvement of the large-sized CSs is 18.4%, higher than that of small-sized CSs (9.3%). Mechanochemical modification has been proved to be an effective and green method for carbon materials in the field of electrochemical energy storage. However, the research on the size effect during mechanochemical process seems still in blank for carbon materials. In this work, the size effect on the variations of the structural and electrochemical properties after mechanochemical treatments is studied through hard carbon spheres (CSs) with sizes of 170 nm, 492 nm and 0.9 μm. The size effect during the treatments affects the specific surface areas, pore structures, defects, functional groups, and the sodium storage properties. It comes as a surprise to find that parts of C-O groups are converted to C=O groups directionally. Significantly, the content variation of oxygen functional groups of large-sized CSs is more drastic, causing larger fluctuation in electrochemical performance. Interestingly, the performance improvement of the large-sized CSs is 18.4% which is higher than that of medium-sized CSs (10.7%) and small-sized CSs (9.3%). The present work has provided a new scientific significance for carbon materials from lab-scale to industrial-scale in the field of battery electrode.