Hierarchical graphene@TiO2 sponges for sodium-ion storage with high areal capacity and robust stability

Abstract The search for high mass-loaded, long-cycle-life and high-rate electrodes for sodium-ion batteries is an enormous challenge. Here, a free-standing hierarchical sponge composed of the upper-layer Graphene@TiO2 composites and under-layer graphene (G@TiO2/G) was prepared as a sodium-ion anode. Thereinto, layer-by-layer graphene sheets not only construct robust framework but also guarantee fast electron transport paths, and the interconnected macroporous structure facilitates rapid ion transport. Moreover, the ultrasmall TiO2 nanodots offer the short diffusion path and fast pseudocapacitive sodium storage capability. Therefore, such G@TiO2/G electrode exhibits an ultralong cycle life (retaining 101 mA h g−1 after 20,000 cycles at 2 A g−1), a superior capacity (260 mA h g−1 at 0.05 A g−1) and a high rate capability (53 mA h g−1 at 10 A g−1) under a mass loading of 2.5 mg cm−2. Impressively, the excellent cycling stability (retaining 1.37 mA h cm−2 after 300 cycles at 0.1 A g−1) and high areal capacity (2.27 mA h cm−2 at 0.53 mA cm−2) are also achieved even at a high mass loading of 10.5 mg cm−2.