▎ 摘　　要

Sodium-ion batteries (SIBs) have attracted a great deal of attention recently as an economic alternative to Li-ion batteries. Cost-efficient reduced graphene oxide (rGO) has been intensively studied as both an active material and a functional additive in SIBs. However, the sodiation-desodiation process in rGO is not fully understood. In this study, we investigate the interaction of the Na ion with rGO by in situ transmission electron microscopy (TEM). For the first time, we observe reversible Na metal cluster (with a diameter of >10 nm) deposition on a rGO surface, which we evidence with an atom-resolved high-resolution TEM image of Na metal. This discovery leads to a porous reduced graphene oxide SIB anode with record high reversible specific capacity around 450 mAh/g at 25 mA/g, a high rate performance of 200 mAh/g at 250 mA/g, and stable cycling performance up to 750 cycles. In addition, direct observation of irreversible formation of Na2O on rGO unveils the origin of the commonly observed low first Columbic efficiency of rGO-containing electrodes.