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

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.