<i>In Situ</i> Transmission Electron Microscopy Observation of Sodiation–Desodiation in a Long Cycle, High-Capacity Reduced Graphene Oxide Sodium-Ion Battery Anode

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 <i>in situ</i> 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