Unique Double-Interstitialcy Mechanism and Interfacial Storage Mechanism in the Graphene/Metal Oxide as the Anode for Sodium-Ion Batteries

Graphene/metal oxides (G/MO) composite materials have attracted much attention as the anode of sodium ion batteries (SIBs), because of the high theoretical capacity. However, most metal oxides operate based on the conversion mechanism and the alloying mechanism has changed to Na2O after the first cycle. The influence of G/Na2O (G/N) on the subsequent sodiation process has never been clearly elucidated. In this work, we report a systematic investigation on the G/N interface from both aspects of theoretical simulation and experiment characterization. By applied first-principles simulations, we find that the sluggish kinetics in the G/MO materials is mainly caused by the high diffusion barrier (0.51 eV) inside the Na2O bulk, while the G/N interface shows a much faster transport kinetics (0.25 eV) via unique double-interstitialcy mechanism. G/N interface possesses an interfacial storage of Na atom through the charge separation mechanism. The experimental evidence confirms that high interfacial ratio structure of G/N greatly improves the rate performance and endows G/MO materials the interfacial storage. Furthermore, the experimental investigation finds that the high interfacial ratio structure of G/N also benefits from the reversible reaction between SnO2 and Sn during cycling. Lastly, the effects of (N, O, S) doping in graphene systems at the G/N interface were also explored. This work provides a fundamental comprehension on the G/MO interface structure during the sodiation process, which is helpful to design energy storage materials with high rate performance and large capacity.