Nanostructured anode materials for high-energy Li-/Na-ion large-scale energy storage

Due to their high volumetric and gravimetric capacities, Li-ion batteries (LiBs) are currently considered as the only viable energy storage solution for portable electronics and hybrid vehicles. Nevertheless, concerns on the availability of Li resources and their uneven distribution across the world are growing. As an alternative, Na-ion batteries (NiBs) are highly attractive due to the abundance of Na as well as its electrochemical similarity with Li. Na is also environmentally benign, thus promising a more sustainable energy storage. Although NiBs are unlikely to outperform LiBs in energy density, they are being actively pursued for low-cost grid storage where size or weight is not the most critical factor. The identification of a high-performance anode is a key challenge, particularly for NiBs. Anode materials investigated so far for NiBs can be classified into three categories: (i) layered materials, such as titania or titanates and transition metal dichalcogenides with a specific capacity of 50-250 mAh g–1; (ii) carbon-based materials, such as hard carbons; and (iii) nanostructures like nanowires and graphene-based carbon nanosheets offering capacities exceeding 300 mAh g–1. Here, we present our recent results regarding novel nanostructured negative electrodes, comprising TiO2 nanotubes, Ga2O3 nanorods and graphene-based carbon nanowalls. The resulting Li-/Na-ion cells exploting the newly elaborated nanostructures as working electrodes provide very high capacity retention even upon highly stressful conditions and very long-term cycling (up to > 2000 cycles). High surface area, self-induced doping, short diffusion path and fast kinetics of the nonostructured arrays are intriguing features, which can account for the noticeable electrochemical performance. Moreover, the characteristics and electrochemical behaviour of the annealed samples in comparison with the non-annealed ones were thoroughly investigated. These results demonstrate that stable, long-term performing nanostructured anodes, which can be obtained by fast and low cost procedures, are suitable for Li-/Na-ion battery application.