SnP2O7 Covered Carbon Nanosheets as a Long-Life and High-Rate Anode Material for Sodium-Ion Batteries

SnP2O7 attached to reduced graphene oxide (rGO) is synthesized by a solvothermal reaction, followed by a mild annealing in Ar/H-2. As an anode material for sodium-ion batteries, this composite is associated with the conversion reaction between Sn and SnP2O7 and the alloy reaction between Sn and NaxSn, as evidenced by ex situ techniques, such as high-resolution transmission electron microscope images, selected area electron diffraction patterns, and X-ray diffraction patterns. The close contact between SnP2O7 and rGO facilitates the charge transfer upon cycling and benefits the preservation of SnP2O7 on rGO even after pulverization. Therefore, this composite exhibits an extraordinary cycling stability. 99% of the initial capacity is remained after 200 cycles at 0.2 A g(-1) and also 99% is kept after 1000 cycles at 1.0 A g(-1). The similar results are also observed in full cells. Quantitative kinetic analysis confirms that sodium storage in this composite is governed by pseudocapacitance, especially at high rates. These results indicate the promising potential of metal pyrophosphates in sodium-ion batteries.