Capacitive Sodium-Ion Storage Based on Double-Layered Mesoporous Graphene with High Capacity and Charging/Discharging Rate

Sodium-ion batteries (SIBs) are regarded as an ideal alternative to lithium-ion batteries, but the larger radius of Na⁺ compared with Li⁺ results in lower energy density, shorter cycle life, and sluggish kinetics of SIBs. Therefore, it is of significant importance to explore appropriate Na⁺ storage materials with high capacity and fast Na⁺ transport kinetics. Herein, doublelayered mesoporous graphene nanosheets codoped with oxygen and nitrogen (O,N-MGNSs) were developed as a new cathode material with high Na⁺ storage capacity and fast ion-transport kinetics for SIBs. The codoping of MGNSs with oxygen and nitrogen by in situ chemical vapor deposition endowed them with a hierarchical porous network, robust structures, good conductivity, and abundant functional groups. The O,N-MGNSs could host Na⁺ in two ways: surface adsorption and surface redox reaction, and this endowed them with high Na⁺ storage capacity and fast charging/discharging rates in SIBs. Electrochemical results revealed that the O,N-MGNSs delivered a reversible capacity of 156 mAh g^-1 at a current density of 0.5 A g^-1 (corresponding to a rate of 3 C) between 1.5 and 4.2 V and exhibited a high cycling stability (95 % capacity retention at 1 A g^-1 for more than 1000 cycles).