Sodium birnessite@graphene hierarchical structures for ultrafast sodium ion storage

Abstract

• The sodium birnessite@graphene hierarchical structures has been fabricated as the electrode material for sodium ion battery. • The Na + mobility reaches 5.38 × 10 -12 cm 2 s −1 , because the graphene layers work as electric double layer to accelerate sodium ion transmission. • At a rate of 0.5C, the discharge specific capacity of 1:1 MnO 2 /rGO is 167 mAh g −1 in the first cycle, and the capacity remains at 100 mAh g −1 after 200 cycles. The sodium ion batteries have received great attention in recent years. However, the radius of sodium ion is larger than that of lithium ion, which results in the limited performance of sodium ion battery. In this work, to decrease the the diffusion time of sodium ions and improve the cyclic stability of sodium ion battery, the stack of manganese layered transition metal oxides and reduced graphene oxide (rGO) sheets has been fabricated as the cathode material. The layered MnO 2 @rGO composites were synthesized via the flocculation of the delaminated MnO 2 nanosheets and rGO nanosheets with Na + ions. By the synergistic effect of the two-dimensional MnO 2 nanosheets and rGO, the sodium ion diffusion distance was decreased efficiently and abundant active sites were provided. By adopting the MnO 2 @rGO cathode, the sodium ion battery delivered specific capacity up to 167 mAh g −1 at 0.5C, and exhibited excellent cycling stability (100 mAh g -1 after 200 cycles). This work provides an efficient way to improve performance of sodium ion batteries.