Pores and surface functional groups are endowed on graphene nanosheets (GNSs) to improve their electrochemical Na+ storage properties. An optimal capacity of 220 mA g-1 is obtained (at a charge-discharge rate of 0.03 A g-1) in ethylene carbonate/diethyl carbonate mixed electrolyte containing 1 M NaClO4. Ex situ X-ray photoelectron spectroscopy and synchrotron X-ray diffraction techniques are employed to study the electrode charge storage mechanism. The results indicate that reversible surface redox reactions at the electrode surface are the predominant processes affecting charge storage in a potential range of 0.5-2.0 V (vs. Na/Na+), whereas Na+ intercalation/deintercalation between carbon layers occurs at lower potentials. When the charge-discharge rate was increased by >300 fold (to 10 A g-1), a capacity as high as 85 mA h g-1 is obtained, reflecting the excellent rate capability of the electrode. The physiochemical characteristics that affect the Na+ storage performance are explored. A highly promising GNS anode for sodium-ion batteries is proposed.